Hamster

Hamsters are rodents belonging to the subfamily Cricetinae. The subfamily contains about 25 species, classified in six or seven genera. They have become established as popular small house pets and partly because they are easy to breed in captivity, hamsters are often used as laboratory animals.

In the wild, hamsters are crepuscular and remain underground during the day to avoid being caught by predators. They feed primarily on seeds, fruits, and vegetation, and will occasionally eat burrowing insects. They have elongated cheek pouches extending to their shoulders in which they carry food back to their burrows.

Hamster behavior varies depending on their environment, genetics, and interaction with people.

>> History
Although the Syrian hamster or golden hamster (Mesocricetus auratus) was first described scientifically by George Robert Waterhouse in 1839, researchers were not able to successfully breed and domesticate hamsters until 1939.The entire laboratory and pet populations of Syrian hamsters appear to be descendants of a single brother-sister pairing. These littermates were captured and imported in 1930 from Aleppo  by Israel Aharoni, a zoologist of the University of Jerusalem. In Jerusalem, the hamsters bred very successfully. Years later, animals of this original breeding colony were exported to the USA, where Syrian hamsters became one of the most popular pets and laboratory animals. Comparative studies of domestic and wild Syrian hamsters have shown reduced genetic variability in the domestic strain. However, the differences in behavioral, chronobiological, morphometrical, hematological, and biochemical parameters are relatively small and fall into the expected range of interstrain variations in other laboratory animals.

>> Early literature
In 1774, Friedrich Gabriel Sulzer, a companion of Johann-Wolfgang von Goethe, devoted a whole academic monograph in the domain of social sciences and natural history to hamsters, entitled "An approach to a natural history of the hamster" ("Versuch einer Naturgeschichte des Hamsters"). In several instances, he used the hamster to document the equal rights of all beings, including Homo sapiens.

>> Etymology
The name "hamster" is a loanword from the German, which itself derives from earlier Old High German hamustro. It is possibly related to Old Russian choměstrǔ, which is either a blend of the root of Russian khomiak "hamster" and a Baltic word (cf. Lithuanian staras "hamster") or of Persian origin (cf. Av hamaēstar "oppressor").

>> Description
Hamsters are typically stout-bodied, with tails shorter than body length, and have small, furry ears, short, stocky legs, and wide feet. They have thick, silky fur, which can be long or short, colored black, grey, honey, white, brown, yellow, red, or a mix, depending on the species. Two species of hamster belonging to the genus Phodopus, Campbell's dwarf hamster (P. campbelli) and the Djungarian hamster (P. sungorus), and two of the genus Cricetulus, the Chinese striped hamster (C. barabensis) and the Chinese hamster (C. griseus) have a dark stripe down their heads to their tails. The species of genus Phodopus are the smallest, with bodies 5.5 to 10.5 cm (2.2 to 4.1 in) long; the largest is the European hamster (Cricetus cricetus), measuring up to 34 cm (13.4 in) long, not including a short tail of up to 6 cm (2.4 in). The Angora hamster, also known as the long-haired or teddy bear hamster, which is a type of the golden hamster is the second-largest hamster breed, measuring up to 18 cm (7.1 in) long.
The hamster tail can be difficult to see, as it is usually not very long (about 1/6 the length of the body), with the exception of the Chinese dwarf hamster, which has a tail the same length as the body. One rodent characteristic that can be highly visible in hamsters is their sharp incisors; they have an upper pair and lower pair which grow continuously throughout life, so must be regularly worn down. Hamsters are very flexible, but their bones are somewhat fragile. They are extremely susceptible to rapid temperature changes and drafts, as well as extreme heat or cold.

Senses
Hamsters have poor eyesight; they are nearsighted and colorblind. Hamsters have scent glands on their flanks (and abdomens in Chinese and dwarf hamsters) which they rub against the substrate, leaving a scent trail.Hamsters also use their sense of smell to distinguish between the sexes, and to locate food. They are also particularly sensitive to high-pitched noises and can hear and communicate in the ultrasonic range.

Diet
Hamsters are omnivores. Although pet hamsters can survive on a diet of exclusively commercial hamster food, other items, such as vegetables, fruits, seeds, and nuts, can be given. Hamsters in the Middle East have been known to hunt in packs to find insects for food. Hamsters are hindgut fermenters and eat their own feces (coprophagy) to recover nutrients digested in the hindgut, but not absorbed.

>> Behavior
A behavioral characteristic of hamsters is food hoarding. They carry food in their spacious cheek pouches to their underground storage chambers. When full, the cheeks can make their heads double, or even triple in size.

>> Social behavior
Most hamsters are strictly solitary. If housed together, acute and chronic stress may occur,and they may fight fiercely, sometimes fatally. Some dwarf hamster species may tolerate conspecifics. Russian hamsters form close, monogamous bonds with their mates, and if separated, they may become very depressed. This happens especially in males. Males will become inactive, eat more, and even show some behavioral changes similar to some types of depression in humans. This can even cause obesity in the hamster.

Chronobiology
Evidence conflicts as to whether hamsters are crepuscular or nocturnal. Khunen writes, "Hamsters are nocturnal rodents who  are active during the night...",but others have written that because hamsters live underground during most of the day, only leaving their burrows about an hour before sundown and then returning when it gets dark, their behavior is primarily crepuscular. Fritzsche indicated although some species have been observed to show more nocturnal activity than others, they are all primarily crepuscular.

Wild Syrian hamsters are true hibernators and allow their body temperature to fall close to ambient temperature (but not below 20 °C). This kind of thermoregulation diminishes the metabolic rate to about 5% and helps the animal to considerably reduce the need for food during the winter. Hamsters may not hibernate per se, but instead reduce the rate of a number of physiological systems, such as breathing and heart rate, for short periods of time. These periods of torpor (defined as "a state of mental or physical inactivity or insensibility") can last up 10 days.

Burrowing behavior
All hamsters are excellent diggers, constructing burrows with one or more entrances, with galleries connected to chambers for nesting, food storage, and other activities. They use their fore- and hindlegs, as well as their snouts and teeth, for digging. In the wild, the burrow buffers extreme ambient temperatures, offers relatively stable climatic conditions, and protects against predators. Syrian hamsters dig their burrows generally at a depth of 0.7 m. A burrow includes a steep entrance pipe (4–5 cm in diameter), a nesting and a hoarding chamber and a blind-ending branch for urination. Laboratory hamsters have not lost their ability to dig burrows; in fact, they will do this with great vigor and skill if they are provided with the appropriate substrate.

Wild hamsters will also appropriate tunnels made by other mammals; the Djungarian hamster, for instance, uses paths and burrows of the pika.

>> Reproduction
> Fertility
Hamsters become fertile at different ages depending on their species. Both Syrian and Russian hamsters mature quickly and can begin reproducing at a young age (4–5 weeks), whereas Chinese hamsters will usually begin reproducing at two to three months of age, and Roborovskis at three to four months of age. The female's reproductive life lasts about 18 months, but male hamsters remain fertile much longer. Females are in estrus about every four days, which is indicated by a reddening of genital areas, a musky smell, and a hissing, squeaking vocalisation she will emit if she believes a male is nearby.

When seen from above, a sexually mature female hamster has a trim tail line; a male's tail line bulges on both sides. This might not be very visible in all species. Male hamsters typically have very large testes in relation to their body size. Before sexual maturity occurs, it is more difficult to determine a young hamster's sex. When examined, female hamsters have their anal and genital openings close together, whereas males have these two holes farther apart (the penis is usually withdrawn into the coat and thus appears as a hole or pink pimple).

> Gestation and fecundity
Hamsters are seasonal breeders and will produce several litters a year with several pups in each litter. The breeding season is from April to October in the Northern Hemisphere, with two to five litters of one to 13 young being born after a gestation period of 16 to 23 days. Gestation lasts 16 to 18 days for Syrian hamsters, 18 to 21 days for Russian hamsters, 21 to 23 days for Chinese hamsters and 23 to 30 for Roborovski hamsters. The average litter size for Syrian hamsters is about seven pups, but can be as great as 24, which is the maximum number of pups that can be contained in the uterus. Campbell's dwarf hamsters tend to have four to eight pups in a litter, but can have up to 13. Djungarian hamsters tend to have slightly smaller litters, as do Chinese and Roborovski hamsters.

> Intersexual aggression and cannibalism
Female Chinese and Syrian hamsters are known for being aggressive toward the male if kept together for too long after mating. In some cases, male hamsters can die after being attacked by the female. If breeding hamsters, separation of the pair after mating is recommended, or they will attack each other.

Female hamsters are also particularly sensitive to disturbances while giving birth, and may even eat their own young if they think they are in danger, although sometimes they are just carrying the pups in their cheek pouches.If captive female hamsters are left for extended periods (three weeks or more) with their litter, they may cannibalize the litter, so the litter must be removed by the time the young can feed and drink independently.

> Weaning
Hamsters are born hairless and blind in a nest the mother will have prepared in advance. After one week, they begin to explore outside the nest. They are completely weaned after three weeks, or four for Roborovski hamsters. Most breeders will sell the hamsters to shops when they are three to nine weeks old.

Longevity
Syrian hamsters typically live no more than two to three years in captivity, and less in the wild. Russian hamsters (Campbell's and Djungarian) live about two to four years in captivity, and Chinese hamsters 21⁄2–3 years. The smaller Roborovski hamster often lives to three years in captivity.

As pets
The best-known species of hamster is the golden or Syrian hamster (Mesocricetus auratus), which is the type most commonly kept as pets. It is also sometimes called a "fancy" hamster. The pet trade and fanciers have given names to several color variations, including "honey bear", "panda bear", "black bear", "European black bear", "polar bear", "teddy bear", and "Dalmatian".Several variations, including long-haired varieties, grow hair several centimeters long and often require special care. British zoologist Leonard Goodwin claimed most hamsters kept in the United Kingdom were descended from the colony he introduced for medical research purposes during the Second World War.

Other hamsters kept as pets are the various species of "dwarf hamster". Campbell's dwarf hamster (Phodopus campbelli) is the most common—they are also sometimes called "Russian dwarfs"; however, many hamsters are from Russia, so this ambiguous name does not distinguish them from other species appropriately. The coat of the Djungarian or winter-white Russian dwarf hamster (Phodopus sungorus) turns almost white during winter (when the hours of daylight decrease).The Roborovski hamster (Phodopus roborovskii) is extremely small and fast, making it difficult to keep as a pet. The Chinese hamster (Cricetulus griseus), although not technically a true "dwarf hamster", is the only hamster with a prehensile tail (about 4 cm long)—most hamsters have very short, nonprehensile tails.

Many breeders also show their hamsters, so breed towards producing a good, healthy, show hamster with a view to keeping one or two themselves, so quality and temperament are of vital importance when planning the breeding.

Fish

A fish is any member of a paraphyletic group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups. Most fish are ectothermic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature.Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish). With 33,100 described species, fish exhibit greater species diversity than any other group of vertebrates.

Fish are an important resource for humans worldwide, especially as food. Commercial and subsistence fishers hunt fish in wild fisheries (see fishing) or farm them in ponds or in cages in the ocean (see aquaculture). They are also caught by recreational fishers, kept as pets, raised by fishkeepers, and exhibited in public aquaria. Fish have had a role in culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

Because the term "fish" is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds and mammals) which descend from within the same ancestry, it is paraphyletic, and is not considered a proper grouping in systematic biology. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification.

The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just the prey of arthropods.

>> Evolution
Fish do not represent a monophyletic group, and therefore the "evolution of fish" is not studied as a single event.

Early fish from the fossil record are represented by a group of small, jawless, armored fish known as Ostracoderms. Jawless fish lineages are mostly extinct. An extant clade, the Lampreys may approximate ancient pre-jawed fish. The first jaws are found in Placodermi fossils. The diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors.

Fish may have evolved from a creature similar to a coral-like Sea squirt, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today), although perhaps the reverse is the case.

>> Taxonomy
Fish are a paraphyletic group: that is, any clade containing all fish also contains the tetrapods, which are not fish. For this reason, groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications.

Traditional classification divide fish into three extant classes, and with extinct forms sometimes classified within the tree, sometimes as their own classes:
Class Agnatha (jawless fish)
Subclass Cyclostomata (hagfish and lampreys)
Subclass Ostracodermi (armoured jawless fish) †
Class Chondrichthyes (cartilaginous fish)
Subclass Elasmobranchii (sharks and rays)
Subclass Holocephali (chimaeras and extinct relatives)
Class Placodermi (armoured fish) †
Class Acanthodii ("spiny sharks", sometimes classified under bony fishes)†
Class Osteichthyes (bony fish)
Subclass Actinopterygii (ray finned fishes)
Subclass Sarcopterygii (fleshy finned fishes, ancestors of tetrapods)
The above scheme is the one most commonly encountered in non-specialist and general works. Many of the above groups are paraphyletic, in that they have given rise to successive groups: Agnathans are ancestral to Chondrichthyes, who again have given rise to Acanthodiians, the ancestors of Osteichthyes. With the arrival of phylogenetic nomenclature, the fishes has been split up into a more detailed scheme, with the following major groups:

Class Myxini (hagfish)
Class Pteraspidomorphi † (early jawless fish)
Class Thelodonti †
Class Anaspida †
Class Petromyzontida or Hyperoartia
Petromyzontidae (lampreys)
Class Conodonta (conodonts) †
Class Cephalaspidomorphi † (early jawless fish)
(unranked) Galeaspida †
(unranked) Pituriaspida †
(unranked) Osteostraci †
Infraphylum Gnathostomata (jawed vertebrates)
Class Placodermi † (armoured fish)
Class Chondrichthyes (cartilaginous fish)
Class Acanthodii † (spiny sharks)
Superclass Osteichthyes (bony fish)
Class Actinopterygii (ray-finned fish)
Subclass Chondrostei
Order Acipenseriformes (sturgeons and paddlefishes)
Order Polypteriformes (reedfishes and bichirs).
Subclass Neopterygii
Infraclass Holostei (gars and bowfins)
Infraclass Teleostei (many orders of common fish)
Class Sarcopterygii (lobe-finned fish)
Subclass Actinistia (coelacanths)
Subclass Dipnoi (lungfish)
† – indicates extinct taxon
Some palaeontologists contend that because Conodonta are chordates, they are primitive fish. For a fuller treatment of this taxonomy, see the vertebrate article.

The position of hagfish in the phylum chordata is not settled. Phylogenetic research in 1998 and 1999 supported the idea that the hagfish and the lampreys form a natural group, the Cyclostomata, that is a sister group of the Gnathostomata.

The various fish groups account for more than half of vertebrate species. There are almost 28,000 known extant species, of which almost 27,000 are bony fish, with 970 sharks, rays, and chimeras and about 108 hagfish and lampreys.A third of these species fall within the nine largest families; from largest to smallest, these families are Cyprinidae, Gobiidae, Cichlidae, Characidae, Loricariidae, Balitoridae, Serranidae, Labridae, and Scorpaenidae. About 64 families are monotypic, containing only one species. The final total of extant species may grow to exceed 32,500

>> Diversity
The term "fish" most precisely describes any non-tetrapod craniate (i.e. an animal with a skull and in most cases a backbone) that has gills throughout life and whose limbs, if any, are in the shape of fins. Unlike groupings such as birds or mammals, fish are not a single clade but a paraphyletic collection of taxa, including hagfishes, lampreys, sharks and rays, ray-finned fish, coelacanths, and lungfish. Indeed, lungfish and coelacanths are closer relatives of tetrapods (such as mammals, birds, amphibians, etc.) than of other fish such as ray-finned fish or sharks, so the last common ancestor of all fish is also an ancestor to tetrapods. As paraphyletic groups are no longer recognised in modern systematic biology, the use of the term "fish" as a biological group must be avoided.

Many types of aquatic animals commonly referred to as "fish" are not fish in the sense given above; examples include shellfish, cuttlefish, starfish, crayfish and jellyfish. In earlier times, even biologists did not make a distinction – sixteenth century natural historians classified also seals, whales, amphibians, crocodiles, even hippopotamuses, as well as a host of aquatic invertebrates, as fish.However, according the definition above, all mammals, including cetaceans like whales and dolphins, are not fish. In some contexts, especially in aquaculture, the true fish are referred to as finfish (or fin fish) to distinguish them from these other animals.

A typical fish is ectothermic, has a streamlined body for rapid swimming, extracts oxygen from water using gills or uses an accessory breathing organ to breathe atmospheric oxygen, has two sets of paired fins, usually one or two (rarely three) dorsal fins, an anal fin, and a tail fin, has jaws, has skin that is usually covered with scales, and lays eggs.

Each criterion has exceptions. Tuna, swordfish, and some species of sharks show some warm-blooded adaptations—they can heat their bodies significantly above ambient water temperature.Streamlining and swimming performance varies from fish such as tuna, salmon, and jacks that can cover 10–20 body-lengths per second to species such as eels and rays that swim no more than 0.5 body-lengths per second. Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. Lungfish have paired lungs similar to those of tetrapods, gouramis have a structure called the labyrinth organ that performs a similar function, while many catfish, such as Corydoras extract oxygen via the intestine or stomach. Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as seahorses, pufferfish, anglerfish, and gulpers. Similarly, the surface of the skin may be naked (as in moray eels), or covered with scales of a variety of different types usually defined as placoid (typical of sharks and rays), cosmoid (fossil lungfish and coelacanths), ganoid (various fossil fish but also living gars and bichirs), cycloid, and ctenoid (these last two are found on most bony fish). There are even fish that live mostly on land. Mudskippers feed and interact with one another on mudflats and go underwater to hide in their burrows. The catfish Phreatobius cisternarum lives in underground, phreatic habitats, and a relative lives in waterlogged leaf litter.

Fish range in size from the huge 16-metre (52 ft) whale shark to the tiny 8-millimetre (0.3 in) stout infantfish.

Fish species diversity is roughly divided equally between marine (oceanic) and freshwater ecosystems. Coral reefs in the Indo-Pacific constitute the center of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests, especially the Amazon, Congo, and Mekong basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth. Exceptionally rich sites in the Amazon basin, such as Cantão State Park, can contain more freshwater fish species than occur in all of Europe.

>> Anatomy
Respiration
Gills
Most fish exchange gases using gills on either side of the pharynx. Gills consist of threadlike structures called filaments. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys, possess multiple gill openings. However, bony fish have a single gill opening on each side. This opening is hidden beneath a protective bony cover called an operculum.

Juvenile bichirs have external gills, a very primitive feature that they share with larval amphibians.

Air breathing
Fish from multiple groups can live out of the water for extended periods. Amphibious fish such as the mudskipper can live and move about on land for up to several days,or live in stagnant or otherwise oxygen depleted water. Many such fish can breathe air via a variety of mechanisms. The skin of anguillid eels may absorb oxygen directly. The buccal cavity of the electric eel may breathe air. Catfish of the families Loricariidae, Callichthyidae, and Scoloplacidae absorb air through their digestive tracts. Lungfish, with the exception of the Australian lungfish, and bichirs have paired lungs similar to those of tetrapods and must surface to gulp fresh air through the mouth and pass spent air out through the gills. Gar and bowfin have a vascularized swim bladder that functions in the same way. Loaches, trahiras, and many catfish breathe by passing air through the gut. Mudskippers breathe by absorbing oxygen across the skin (similar to frogs). A number of fish have evolved so-called accessory breathing organs that extract oxygen from the air. Labyrinth fish (such as gouramis and bettas) have a labyrinth organ above the gills that performs this function. A few other fish have structures resembling labyrinth organs in form and function, most notably snakeheads, pikeheads, and the Clariidae catfish family.

Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. Fish dependent solely on dissolved oxygen, such as perch and cichlids, quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of aestivation (summertime hibernation) until water returns
Air breathing fish can be divided into obligate air breathers and facultative air breathers. Obligate air breathers, such as the African lungfish, must breathe air periodically or they suffocate. Facultative air breathers, such as the catfish Hypostomus plecostomus, only breathe air if they need to and will otherwise rely on their gills for oxygen. Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators.

Circulation
Fish have a closed-loop circulatory system. The heart pumps the blood in a single loop throughout the body. In most fish, the heart consists of four parts, including two chambers and an entrance and exit. The first part is the sinus venosus, a thin-walled sac that collects blood from the fish's veins before allowing it to flow to the second part, the atrium, which is a large muscular chamber. The atrium serves as a one-way antechamber, sends blood to the third part, ventricle. The ventricle is another thick-walled, muscular chamber and it pumps the blood, first to the fourth part, bulbus arteriosus, a large tube, and then out of the heart. The bulbus arteriosus connects to the aorta, through which blood flows to the gills for oxygenation.

Digestion
Jaws allow fish to eat a wide variety of food, including plants and other organisms. Fish ingest food through the mouth and break it down in the esophagus. In the stomach, food is further digested and, in many fish, processed in finger-shaped pouches called pyloric caeca, which secrete digestive enzymes and absorb nutrients. Organs such as the liver and pancreas add enzymes and various chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption.

Excretion
As with many aquatic animals, most fish release their nitrogenous wastes as ammonia. Some of the wastes diffuse through the gills. Blood wastes are filtered by the kidneys.

Saltwater fish tend to lose water because of osmosis. Their kidneys return water to the body. The reverse happens in freshwater fish: they tend to gain water osmotically. Their kidneys produce dilute urine for excretion. Some fish have specially adapted kidneys that vary in function, allowing them to move from freshwater to saltwater.

Scales
Main article: Fish scale
The scales of fish originate from the mesoderm (skin); they may be similar in structure to teeth.

>> Sensory and nervous system
Central nervous system
Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. However, some fish have relatively large brains, most notably mormyrids and sharks, which have brains about as massive relative to body weight as birds and marsupials.

Fish brains are divided into several regions. At the front are the olfactory lobes, a pair of structures that receive and process signals from the nostrils via the two olfactory nerves.The olfactory lobes are very large in fish that hunt primarily by smell, such as hagfish, sharks, and catfish. Behind the olfactory lobes is the two-lobed telencephalon, the structural equivalent to the cerebrum in higher vertebrates. In fish the telencephalon is concerned mostly with olfaction. Together these structures form the forebrain.

Connecting the forebrain to the midbrain is the diencephalon (in the diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs functions associated with hormones and homeostasis. The pineal body lies just above the diencephalon. This structure detects light, maintains circadian rhythms, and controls color changes.

The midbrain or mesencephalon contains the two optic lobes. These are very large in species that hunt by sight, such as rainbow trout and cichlids.

The hindbrain or metencephalon is particularly involved in swimming and balance. The cerebellum is a single-lobed structure that is typically the biggest part of the brain.Hagfish and lampreys have relatively small cerebellae, while the mormyrid cerebellum is massive and apparently involved in their electrical sense.

The brain stem or myelencephalon is the brain's posterior.As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs respiration and osmoregulation.

Sense organs
Most fish possess highly developed sense organs. Nearly all daylight fish have color vision that is at least as good as a human's (see vision in fishes). Many fish also have chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears, many fish may not hear very well. Most fish have sensitive receptors that form the lateral line system, which detects gentle currents and vibrations, and senses the motion of nearby fish and prey.Some fish, such as catfish and sharks, have organs that detect weak electric currents on the order of millivolt. Other fish, like the South American electric fishes Gymnotiformes, can produce weak electric currents, which they use in navigation and social communication.

Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols. Fish behavior in mazes reveals that they possess spatial memory and visual discrimination.

Vision
Main article: Vision in fishes
Vision is an important sensory system for most species of fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rod cells and cone cells (for scotopic and photopic vision), and most species have colour vision. Some fish can see ultraviolet and some can see polarized light. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots.Fish vision shows adaptation to their visual environment, for example deep sea fishes have eyes suited to the dark environment.

Hearing
See also: Sensory systems in fish § Hearing
Hearing is an important sensory system for most species of fish. Fish sense sound using their lateral lines and their ears.

Capacity for pain
Further information: Pain in fish
Experiments done by William Tavolga provide evidence that fish have pain and fear responses. For instance, in Tavolga's experiments, toadfish grunted when electrically shocked and over time they came to grunt at the mere sight of an electrode.

In 2003, Scottish scientists at the University of Edinburgh and the Roslin Institute concluded that rainbow trout exhibit behaviors often associated with pain in other animals. Bee venom and acetic acid injected into the lips resulted in fish rocking their bodies and rubbing their lips along the sides and floors of their tanks, which the researchers concluded were attempts to relieve pain, similar to what mammals would do.Neurons fired in a pattern resembling human neuronal patterns.

Professor James D. Rose of the University of Wyoming claimed the study was flawed since it did not provide proof that fish possess "conscious awareness, particularly a kind of awareness that is meaningfully like ours". Rose argues that since fish brains are so different from human brains, fish are probably not conscious in the manner humans are, so that reactions similar to human reactions to pain instead have other causes. Rose had published a study a year earlier arguing that fish cannot feel pain because their brains lack a neocortex. However, animal behaviorist Temple Grandin argues that fish could still have consciousness without a neocortex because "different species can use different brain structures and systems to handle the same functions."

Animal welfare advocates raise concerns about the possible suffering of fish caused by angling. Some countries, such as Germany have banned specific types of fishing, and the British RSPCA now formally prosecutes individuals who are cruel to fish.

>> Muscular system
Most fish move by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the back fin, backward force is applied to the water, and in conjunction with the fins, moves the fish forward. The fish's fins function like an airplane's flaps. Fins also increase the tail's surface area, increasing speed. The streamlined body of the fish decreases the amount of friction from the water. Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a swim bladder that adjusts their buoyancy through manipulation of gases.

>> Homeothermy
Although most fish are exclusively ectothermic, there are exceptions.

Certain species of fish maintain elevated body temperatures. Endothermic teleosts (bony fish) are all in the suborder Scombroidei and include the billfishes, tunas, and one species of "primitive" mackerel (Gasterochisma melampus). All sharks in the family Lamnidae – shortfin mako, long fin mako, white, porbeagle, and salmon shark – are endothermic, and evidence suggests the trait exists in family Alopiidae (thresher sharks). The degree of endothermy varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks who maintain body temperatures elevated in excess of 20 °C above ambient water temperatures. See also gigantothermy. Endothermy, though metabolically costly, is thought to provide advantages such as increased muscle strength, higher rates of central nervous system processing, and higher rates of digestion.


>> Reproductive system
Fish reproductive organs include testes and ovaries. In most species, gonads are paired organs of similar size, which can be partially or totally fused. There may also be a range of secondary organs that increase reproductive fitness.

In terms of spermatogonia distribution, the structure of teleosts testes has two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in Atherinomorph fish they are confined to the distal portion of these structures. Fish can present cystic or semi-cystic spermatogenesis in relation to the release phase of germ cells in cysts to the seminiferous tubules lumen.

Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then enter the ostium, then through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the coelom from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct. Gymnovaries are the primitive condition found in lungfish, sturgeon, and bowfin. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct.Secondary gymnovaries are found in salmonids and a few other teleosts.

Oogonia development in teleosts fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilization processes. Changes in the nucleus, ooplasm, and the surrounding layers characterize the oocyte maturation process.

Postovulatory follicles are structures formed after oocyte release; they do not have endocrine function, present a wide irregular lumen, and are rapidly reabsorbed in a process involving the apoptosis of follicular cells. A degenerative process called follicular atresia reabsorbs vitellogenic oocytes not spawned. This process can also occur, but less frequently, in oocytes in other development stages.

Some fish, like the California sheephead, are hermaphrodites, having both testes and ovaries either at different phases in their life cycle or, as in hamlets, have them simultaneously.

Over 97% of all known fish are oviparous, that is, the eggs develop outside the mother's body. Examples of oviparous fish include salmon, goldfish, cichlids, tuna, and eels. In the majority of these species, fertilisation takes place outside the mother's body, with the male and female fish shedding their gametes into the surrounding water. However, a few oviparous fish practice internal fertilization, with the male using some sort of intromittent organ to deliver sperm into the genital opening of the female, most notably the oviparous sharks, such as the horn shark, and oviparous rays, such as skates. In these cases, the male is equipped with a pair of modified pelvic fins known as claspers.

Marine fish can produce high numbers of eggs which are often released into the open water column. The eggs have an average diameter of 1 millimetre (0.039 in).
The newly hatched young of oviparous fish are called larvae. They are usually poorly formed, carry a large yolk sac (for nourishment) and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed metamorphosis) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on zooplankton prey, a process which depends on typically inadequate zooplankton density, starving many larvae.

In ovoviviparous fish the eggs develop inside the mother's body after internal fertilization but receive little or no nourishment directly from the mother, depending instead on the yolk. Each embryo develops in its own egg. Familiar examples of ovoviviparous fish include guppies, angel sharks, and coelacanths.

Some species of fish are viviparous. In such species the mother retains the eggs and nourishes the embryos. Typically, viviparous fish have a structure analogous to the placenta seen in mammals connecting the mother's blood supply with that of the embryo. Examples of viviparous fish include the surf-perches, splitfins, and lemon shark. Some viviparous fish exhibit oophagy, in which the developing embryos eat other eggs produced by the mother. This has been observed primarily among sharks, such as the shortfin mako and porbeagle, but is known for a few bony fish as well, such as the halfbeak Nomorhamphus ebrardtii. Intrauterine cannibalism is an even more unusual mode of vivipary, in which the largest embryos eat weaker and smaller siblings. This behavior is also most commonly found among sharks, such as the grey nurse shark, but has also been reported for Nomorhamphus ebrardtii.

Aquarists commonly refer to ovoviviparous and viviparous fish as livebearers.

Bird

Birds (class Aves) are a group of endothermic vertebrates, characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds or, less accurately, as songbirds.

Scientific consensus is that birds are the last surviving lineage of dinosaurs, having evolved from feathered dinosaur ancestors within the theropod group of saurischian dinosaurs. The fossil record indicates that true birds first appeared during the Cretaceous period, around 100 million years ago. However, primitive bird-like "stem-birds" that lie outside class Aves proper, in the group Avialae, have been found dating back to the mid-Jurassic period.Many of these early stem-birds, such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks and long bony tails.

Birds have more or less developed wings; the only known groups without wings are the extinct moa and elephant birds. Bird wings, which evolved from forelimbs, enabled birds the ability of bird flight. The digestive and respiratory systems of birds are also uniquely adapted for flight, although further speciation has led to some flightless birds, including ratites, penguins, and diverse endemic island species of birds. Some bird species of aquatic environments, particularly the aforementioned flightless penguins, and also members of the duck family, have also evolved for swimming. Birds, specifically Darwin's finches, played an important part in the inception of Darwin's theory of evolution by natural selection.

Some birds, especially corvids and parrots, are among the most intelligent animals; several bird species make and use tools, and many social species pass on knowledge across generations, which is considered a form of culture. Many species annually migrate great distances. Birds are social, communicating with visual signals, calls, and bird songs, and participating in such social behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have polygynous ("many females") or, rarely, polyandrous ("many males") breeding systems. Birds produce offspring by laying eggs which are fertilized through sexual reproduction. They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as hens, lay eggs even when not fertilized, though unfertilized eggs do not produce offspring.

Many species of birds are economically important. Domesticated and undomesticated birds (poultry and game) are important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano (bird excrement) is harvested for use as a fertilizer. Birds prominently figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry.

>> Evolution and classification
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae. Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.

>Definition
Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica. However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents of the Phylocode system. Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae, in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

Gauthieridentified four conflicting ways of defining the term "Aves", which is a problem because the same biological name is being used four different ways. Gauthier proposed a solution, number 4 below, which is to reserve the term Aves only for the crown group, the last common ancestor of all living birds and all of its descendants. He assigned other names to the other groups.
<Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
<Aves can mean those that fly (alternately Avialae)
<Aves can mean all reptiles closer to birds than to crocodiles (alternately Avemetatarsalia )
<Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "crown group"). (alternately Neornithes)
Under the fourth definition Archaeopteryx is an avialan, and not a member of Aves. Gauthier's proposals have been adopted by many researchers in the field of paleontology and bird evolution, though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.

Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all theropods closer to birds than to Deinonychus." Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight, and the birds that descended from them.

>Dinosaurs and the origin of birds
Based on fossil and biological evidence, most scientists accept that birds are a specialized subgroup of theropod dinosaurs, and more specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod feathered dinosaurs, contribute to this ambiguity.

The consensus view in contemporary paleontology is that the flying theropods, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids.Together, these form a group called Paraves. Some basal members of this group, such as Microraptor, have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both. Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, recent studies suggest that the first avialans were omnivores
The Late Jurassic Archaeopteryx is well known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. Archaeopteryx was the first fossil to display both clearly traditional reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, as well as wings with flight feathers similar to those of modern birds. It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.

> Early evolution
The earliest known avialan fossils come from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago.The avialan species from this time period include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui. The well-known early avialan, Archaeopteryx, dates from slightly later Jurassic rocks (about 155 million years old) from Germany. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds, but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering.

Avialans diversified into a wide variety of forms during the Cretaceous Period. Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors, the tails of more advanced avialans were shortened with the advent of the pygostyle bone in the group Pygostylia. In the late Cretaceous, around 95 million years ago, the ancestor of all modern birds also evolved a better sense of smell.

> Early diversity of bird ancestors
The first large, diverse lineage of short-tailed avialans to evolve were the enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. While they were the dominant group of avialans during the Cretaceous period, enantiornithes became extinct along with many other dinosaur groups at the end of the Mesozoic era.

Many species of the second major avialan lineage to diversify, the Euornithes (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialized in eating fish and other small aquatic organisms. Unlike the enantiornithes, which dominated land-based and arboreal habitats, most early euornithes lacked perching adaptations and seem to have included shorebird-like species, waders, and swimming and diving species. The later included the superficially gull-like Ichthyornis, the Hesperornithiformes, which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. The early euornithes also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithes retained teeth in other parts of the jaws).Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers, which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.

> Diversification of modern birds
All modern birds lie within the crown group Aves (alternately Neornithes), which has two subdivisions: the Palaeognathae, which includes the flightless ratites (such as the ostriches) and the weak-flying tinamous, and the extremely diverse Neognathae, containing all other birds. These two subdivisions are often given the rank of superorder, although Livezey and Zusi assigned them "cohort" rank.Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800 to 10,050.

Due largely to the discovery of Vegavis, a late Cretaceous neognath member of the duck lineage, Aves is known to have split into several modern lineages by the end of the Mesozoic era. Studies using a "morphological clock" analysis have estimated that the actual origin of modern birds probably occurred slightly earlier than the earliest known fossils, during the mid-Cretaceous period

The earliest divergence within the Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers) and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders and the guans and their allies). The earliest fossil remains of true birds come from the possible galliform Austinornis lentus, dated to about 85 million years ago,but the dates for the actual splits are much debated by scientists. The Aves are agreed to have evolved in the Cretaceous, and the split between the Galloanseri from other Neognathes occurred before the Cretaceous–Paleogene extinction event, but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs.This disagreement is in part caused by a divergence in the evidence; molecular dating suggests a Cretaceous radiation, while fossil evidence supports a Cenozoic radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.

>> Behaviour
Most birds are diurnal, but some birds, such as many species of owls and nightjars, are nocturnal or crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night.
>> Diet and feeding
Birds' diets are varied and often include nectar, fruit, plants, seeds, carrion, and various small animals, including other birds.Because birds have no teeth, their digestive system is adapted to process unmasticated food items that are swallowed whole.

Birds that employ many strategies to obtain food or feed on a variety of food items are called generalists, while others that concentrate time and effort on specific food items or have a single strategy to obtain food are considered specialists.Birds' feeding strategies vary by species. Many birds glean for insects, invertebrates, fruit, or seeds. Some hunt insects by suddenly attacking from a branch. Those species that seek pest insects are considered beneficial 'biological control agents' and their presence encouraged in biological pest control programs.Nectar feeders such as hummingbirds, sunbirds, lories, and lorikeets amongst others have specially adapted brushy tongues and in many cases bills designed to fit co-adapted flowers. Kiwis and shorebirds with long bills probe for invertebrates; shorebirds' varied bill lengths and feeding methods result in the separation of ecological niches. Loons, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion, while aerial predators such as sulids, kingfishers and terns plunge dive after their prey. Flamingos, three species of prion, and some ducks are filter feeders. Geese and dabbling ducks are primarily grazers.

Some species, including frigatebirds, gulls,and skuas,engage in kleptoparasitism, stealing food items from other birds. Kleptoparasitism is thought to be a supplement to food obtained by hunting, rather than a significant part of any species' diet; a study of great frigatebirds stealing from masked boobies estimated that the frigatebirds stole at most 40% of their food and on average stole only 5%. Other birds are scavengers; some of these, like vultures, are specialised carrion eaters, while others, like gulls, corvids, or other birds of prey, are opportunists.
>> Water and drinking
Water is needed by many birds although their mode of excretion and lack of sweat glands reduces the physiological demands. Some desert birds can obtain their water needs entirely from moisture in their food. They may also have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting.Seabirds can drink seawater and have salt glands inside the head that eliminate excess salt out of the nostrils.

Most birds scoop water in their beaks and raise their head to let water run down the throat. Some species, especially of arid zones, belonging to the pigeon, finch, mousebird, button-quail and bustard families are capable of sucking up water without the need to tilt back their heads. Some desert birds depend on water sources and sandgrouse are particularly well known for their daily congregations at waterholes. Nesting sandgrouse and many plovers carry water to their young by wetting their belly feathers. Some birds carry water for chicks at the nest in their crop or regurgitate it along with food. The pigeon family, flamingos and penguins have adaptations to produce a nutritive fluid called crop milk that they provide to their chicks.

>> Feather care
Feathers being critical to the survival of a bird, require maintenance. Apart from physical wear and tear, feathers face the onslaught of fungi, ectoparasitic feather mites and birdlice. The physical condition of feathers are maintained by preening often with the application of secretions from the preen gland. Birds also bathe in water or dust themselves. While some birds dip into shallow water, more aerial species may make aerial dips into water and arboreal species often make use of dew or rain that collect on leaves. Birds of arid regions make use of loose soil to dust-bathe. A behaviour termed as anting in which the bird encourages ants to run through their plumage is also thought to help them reduce the ectoparasite load in feathers. Many species will spread out their wings and expose them to direct sunlight and this too is thought to help in reducing fungal and ectoparasitic activity that may lead to feather damage.

>> Migration
Main article: Bird migration
Many bird species migrate to take advantage of global differences of seasonal temperatures, therefore optimising availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds, shorebirds, and waterbirds undertake annual long distance migrations, usually triggered by the length of daylight as well as weather conditions. These birds are characterised by a breeding season spent in the temperate or polar regions and a non-breeding season in the tropical regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs.Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Landbirds have a flight range of around 2,500 km (1,600 mi) and shorebirds can fly up to 4,000 km (2,500 mi), although the bar-tailed godwit is capable of non-stop flights of up to 10,200 km (6,300 mi).Seabirds also undertake long migrations, the longest annual migration being those of sooty shearwaters, which nest in New Zealand and Chile and spend the northern summer feeding in the North Pacific off Japan, Alaska and California, an annual round trip of 64,000 km (39,800 mi). Other seabirds disperse after breeding, travelling widely but having no set migration route. Albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons.
Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. Irruptive species such as the boreal finches are one such group and can commonly be found at a location in one year and absent the next. This type of migration is normally associated with food availability. Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics; others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates.Partial migration can form a large percentage of the migration behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory. Altitudinal migration is a form of short distance migration in which birds spend the breeding season at higher altitudes elevations and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when the normal territories also become inhospitable due to lack of food. Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. Parrots as a family are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations.

The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s a Manx shearwater released in Boston returned to its colony in Skomer, Wales, within 13 days, a distance of 5,150 km (3,200 mi). Birds navigate during migration using a variety of methods. For diurnal migrants, the sun is used to navigate by day, and a stellar compass is used at night. Birds that use the sun compensate for the changing position of the sun during the day by the use of an internal clock.Orientation with the stellar compass depends on the position of the constellations surrounding Polaris.These are backed up in some species by their ability to sense the Earth's geomagnetism through specialised photoreceptors.

>> Communication
Birds communicate using primarily visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).

Birds sometimes use plumage to assess and assert social dominance, to display breeding condition in sexually selected species, or to make threatening displays, as in the sunbittern's mimicry of a large predator to ward off hawks and protect young chicks. Variation in plumage also allows for the identification of birds, particularly between species. Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression or submission or may contribute to the formation of pair-bonds. The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements;males' breeding success may depend on the quality of such displays.
Bird calls and songs, which are produced in the syrinx, are the major means by which birds communicate with sound. This communication can be very complex; some species can operate the two sides of the syrinx independently, allowing the simultaneous production of two different songs.

Calls are used for a variety of purposes, including mate attraction, evaluation of potential mates, bond formation, the claiming and maintenance of territories, the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season),and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat. Some birds also use mechanical sounds for auditory communication. The Coenocorypha snipes of New Zealand drive air through their feathers,woodpeckers drum territorially,and palm cockatoos use tools to drum

Rabbit

Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world. There are eight different genera in the family classified as rabbits, including the European rabbit (Oryctolagus cuniculus), cottontail rabbits (genus Sylvilagus; 13 species), and the Amami rabbit (Pentalagus furnessi, an endangered species on Amami Ōshima, Japan). There are many other species of rabbit, and these, along with pikas and hares, make up the order Lagomorpha. The male is called a buck and the female is a doe; a young rabbit is a kitten or kit

>> Habitat and range
Rabbit habitats include meadows, woods, forests, grasslands, deserts and wetlands. Rabbits live in groups, and the best known species, the European rabbit, lives in underground burrows, or rabbit holes. A group of burrows is called a warren.

More than half the world's rabbit population resides in North America. They are also native to southwestern Europe, Southeast Asia, Sumatra, some islands of Japan, and in parts of Africa and South America. They are not naturally found in most of Eurasia, where a number of species of hares are present. Rabbits first entered South America relatively recently, as part of the Great American Interchange. Much of the continent has just one species of rabbit, the tapeti, while most of South America's southern cone is without rabbits.

The European rabbit has been introduced to many places around the world.

>> Biology
Evolution
Because the rabbit's epiglottis is engaged over the soft palate except when swallowing, the rabbit is an obligate nasal breather. Rabbits have two sets of incisor teeth, one behind the other. This way they can be distinguished from rodents, with which they are often confused. Carl Linnaeus originally grouped rabbits and rodents under the class Glires; later, they were separated as the scientific consensus is that many of their similarities were a result of convergent evolution. However, recent DNA analysis and the discovery of a common ancestor has supported the view that they share a common lineage, and thus rabbits and rodents are now often referred to together as members of the superorder Glires.

>> Morphology
The rabbit's long ears, which can be more than 10 cm (4 in) long, are probably an adaptation for detecting predators. They have large, powerful hind legs. The two front paws have 5 toes, the extra called the dewclaw. The hind feet have 4 toes. They are plantigrade animals while at rest; however, they move around on their toes while running, assuming a more digitigrade form. Unlike some other paw structures of quadruped mammals, especially those of domesticated pets, rabbit paws lack pads. Their nails are strong and are used for digging; along with their teeth, they are also used for defense.

Wild rabbits do not differ much in their body proportions or stance, with full, egg-shaped bodies. Their size can range anywhere from 20 cm (8 in) in length and 0.4 kg in weight to 50 cm (20 in) and more than 2 kg. The fur is most commonly long and soft, with colors such as shades of brown, gray, and buff. The tail is a little plume of brownish fur (white on top for cottontails). Rabbits can see nearly 360 degrees, with a small blind spot at the bridge of the nose.

>> Ecology
Rabbits are hindgut digesters. This means that most of their digestion takes place in their large intestine and cecum. In rabbits, the cecum is about 10 times bigger than the stomach and it along with the large intestine makes up roughly 40% of the rabbit's digestive tract.The unique musculature of the cecum allows the intestinal tract of the rabbit to separate fibrous material from more digestible material; the fibrous material is passed as feces, while the more nutritious material is encased in a mucous lining as a cecotrope. Cecotropes, sometimes called "night feces", are high in minerals, vitamins and proteins that are necessary to the rabbit's health. Rabbits eat these to meet their nutritional requirements; the mucous coating allows the nutrients to pass through the acidic stomach for digestion in the intestines. This process allows rabbits to extract the necessary nutrients from their food.
Rabbits are prey animals and are therefore constantly aware of their surroundings. For instances, in Mediterranean Europe, rabbits are the main prey of red foxes, badgers, and Iberian lynxes. If confronted by a potential threat, a rabbit may freeze and observe then warn others in the warren with powerful thumps on the ground. Rabbits have a remarkably wide field of vision, and a good deal of it is devoted to overhead scanning. They survive predation by burrowing, hopping away in a zig-zag motion, and, if captured, delivering powerful kicks with their hind legs. Their strong teeth allow them to eat and to bite in order to escape a struggle.The expected wild rabbit lifespan is about 3 years.

>> Sleep
Rabbits are crepuscular, most active at dawn and dusk. The average sleep time of a rabbit in captivity is said to be 8.4 hours. As with other prey animals, rabbits often sleep with their eyes open so sudden movements will wake the rabbit and alert it to dangers.

>> Diet and eating habits
Rabbits are herbivores that feed by grazing on grass, forbs, and leafy weeds. In consequence, their diet contains large amounts of cellulose, which is hard to digest. Rabbits solve this problem via a form of hindgut fermentation. They pass two distinct types of feces: hard droppings and soft black viscous pellets, the latter of which are known as caecotrophs and are immediately eaten (a behaviour known as coprophagy). Rabbits reingest their own droppings (rather than chewing the cud as do cows and many other herbivores) to digest their food further and extract sufficient nutrients.
Rabbits graze heavily and rapidly for roughly the first half hour of a grazing period (usually in the late afternoon), followed by about half an hour of more selective feeding. In this time, the rabbit will also excrete many hard fecal pellets, being waste pellets that will not be reingested. If the environment is relatively non-threatening, the rabbit will remain outdoors for many hours, grazing at intervals. While out of the burrow, the rabbit will occasionally reingest its soft, partially digested pellets; this is rarely observed, since the pellets are reingested as they are produced. Reingestion is most common within the burrow between 8 o'clock in the morning and 5 o'clock in the evening, being carried out intermittently within that period.

Hard pellets are made up of hay-like fragments of plant cuticle and stalk, being the final waste product after redigestion of soft pellets. These are only released outside the burrow and are not reingested. Soft pellets are usually produced several hours after grazing, after the hard pellets have all been excreted. They are made up of micro-organisms and undigested plant cell walls.

The chewed plant material collects in the large cecum, a secondary chamber between the large and small intestine containing large quantities of symbiotic bacteria that help with the digestion of cellulose and also produce certain B vitamins. The pellets are about 56% bacteria by dry weight, largely accounting for the pellets being 24.4% protein on average. The soft feces form here and contain up to five times the vitamins of hard feces. After being excreted, they are eaten whole by the rabbit and redigested in a special part of the stomach. The pellets remain intact for up to six hours in the stomach; the bacteria within continue to digest the plant carbohydrates. This double-digestion process enables rabbits to use nutrients that they may have missed during the first passage through the gut, as well as the nutrients formed by the microbial activity and thus ensures that maximum nutrition is derived from the food they eat. This process serves the same purpose within the rabbit as rumination does in cattle and sheep.

Rabbits are incapable of vomiting.

>> Rabbit diseases
For a more comprehensive list, see Category:Rabbit diseases.
Rabbits can be affected by a number of diseases. These include pathogens that also affect other animals and/or humans, such as Bordetella bronchiseptica and Escherichia coli, as well as diseases unique to rabbits such as rabbit haemorrhagic disease: a form of calicivirus, and myxomatosis.

Rabbits and hares are almost never found to be infected with rabies and have not been known to transmit rabies to humans.

Among the parasites that infect rabbits are tapeworms such as Taenia serialis, external parasites like fleas and mites, coccidia species, and Toxoplasma gondii.

>> Differences from hares
Main article: Hare
The most obvious difference between rabbits and hares is how their kits are born. Rabbits are altricial, having young that are born blind and hairless. In contrast, hares are precocial, born with hair and good vision. All rabbits except cottontail rabbits live underground in burrows or warrens, while hares live in simple nests above the ground (as do cottontail rabbits), and usually do not live in groups. Hares are generally larger than rabbits, with longer ears, larger and longer hind legs and have black markings on their fur. Hares have not been domesticated, while European rabbits are both raised for meat and kept as pets.

Dog

The domestic dog (Canis lupus familiaris or Canis familiaris) is a domesticated canid which has been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes.

Although initially thought to have originated as a manmade variant of an extant canid species (variously supposed as being the dhole, golden jackal,or gray wolf), extensive genetic studies undertaken during the 2010s indicate that dogs diverged from other wolf-like canids in Eurasia 40,000 years ago. Being the oldest domesticated animals, their long association with people has allowed dogs to be uniquely attuned to human behavior, as well as thrive on a starch-rich diet which would be inadequate for other canid species.

Dogs perform many roles for people, such as hunting, herding, pulling loads, protection, assisting police and military, companionship, and, more recently, aiding handicapped individuals. This impact on human society has given them the nickname "man's best friend" in the Western world. In some cultures, however, dogs are a source of meat.

>> Etymology
The term "domestic dog" is generally used for both of the domesticated and feral varieties. The English word dog comes from Middle English dogge, from Old English docga, a "powerful dog breed". The term may possibly derive from Proto-Germanic *dukkōn, represented in Old English finger-docce ("finger-muscle"). The word also shows the familiar petname diminutive -ga also seen in frogga "frog", picga "pig", stagga "stag", wicga "beetle, worm", among others. The term dog may ultimately derive from the earliest layer of Proto-Indo-European vocabulary, reflecting the role of the dog as the earliest domesticated animal.

In 14th-century England, hound (from Old English: hund) was the general word for all domestic canines, and dog referred to a subtype of hound, a group including the mastiff. It is believed this "dog" type was so common, it eventually became the prototype of the category "hound". By the 16th century, dog had become the general word, and hound had begun to refer only to types used for hunting. The word Hound is ultimately derived from the Proto-Indo-European word *kwon- "dog"

In breeding circles, a male canine is referred to as a dog, while a female is called a bitch (Middle English bicche, from Old English bicce, ultimately from Old Norse bikkja). A group of offspring is a litter. The father of a litter is called the sire, and the mother is called the dam. Offspring are, in general, called pups or puppies, from French poupée, until they are about a year old. The process of birth is whelping, from the Old English word hwelp.

>> Taxonomy
In 1753, Carl Linnaeus listed among the types of quadrupeds familiar to him the Latin word for dog, canis. Among the species within this genus, Linnaeus listed the red fox (as Canis vulpes), wolves (Canis lupus), and the domestic dog (Canis canis). In later editions, Linnaeus dropped Canis canis and greatly expanded his list of the Canis genus of quadrupeds and, by 1758, included alongside the foxes, wolves, and jackals and many more terms that are now listed as synonyms for domestic dog, including aegyptius (hairless dog), aquaticus, (water dog), and mustelinus (literally "badger dog"). Among these were two that later experts have been widely used for domestic dogs as a species: Canis domesticus and, most predominantly, Canis familiaris, the "common" or "familiar" dog.

By 1993, with advancements in molecular biology, the mitochondrial DNA mtDNA analysis of extant (i.e. living today) Canidae species indicated that "The domestic dog is an extremely close relative of the gray wolf, differing from it by at most 0.2% of mtDNA sequence.... In comparison, the gray wolf differs from its closest wild relative, the coyote, by about 4% of mitochondrial DNA sequence." In the same year, the domestic dog Canis familiaris was reclassified as Canis lupus familiaris, a subspecies of the gray wolf Canis lupus in Mammal Species of the World. By 1999, further genetic analysis indicated that the domestic dog may have emerged from multiple wolf populations. Based on these latest two pieces of research and the reference reclassification, canis lupus familiaris is the name for the taxon listed by ITIS. However, canis familiaris is also accepted due to a nomenclature debate regarding the naming of wild and domestic sub-species.

In 2014, a mtDNA study of extant and extinct dogs and wolves questioned this classification, the study posited a common ancestor to wolves and dogs, rather than a speciation event. (See Origin and Gray wolf.)

>> Origin
Main article: Origin of the domestic dog
The origin of the domestic dog (Canis lupus familiaris or Canis familiaris) is not clear. Whole genome sequencing indicates that the dog, the gray wolf and the extinct Taymyr wolf diverged at around the same time 27,000–40,000 years ago. These dates imply that the earliest dogs arose in the time of human hunter-gatherers and not agriculturists. Modern dogs are most closely related to ancient wolf fossils that have been found in Europe than they are to modern gray wolves. Nearly all dog breeds' genetic closeness to the gray wolf are due to admixture, except several Arctic dog breeds are close to the Taimyr wolf of North Asia due to admixture.

>> Biology

- Anatomy
Main article: Dog anatomy
Domestic dogs have been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes. Modern dog breeds show more variation in size, appearance, and behavior than any other domestic animal. Dogs are predators and scavengers, and like many other predatory mammals, the dog has powerful muscles, fused wrist bones, a cardiovascular system that supports both sprinting and endurance, and teeth for catching and tearing.

- Size and weight
Dogs are highly variable in height and weight. The smallest known adult dog was a Yorkshire Terrier, that stood only 6.3 cm (2.5 in) at the shoulder, 9.5 cm (3.7 in) in length along the head-and-body, and weighed only 113 grams (4.0 oz). The largest known dog was an English Mastiff which weighed 155.6 kg (343 lb) and was 250 cm (98 in) from the snout to the tail. The tallest dog is a Great Dane that stands 106.7 cm (42.0 in) at the shoulder.

- Senses
The dog's senses include vision, hearing, sense of smell, sense of taste, touch and sensitivity to the earth's magnetic field.

- Coat
The coats of domestic dogs are of two varieties: "double" being common with dogs (as well as wolves) originating from colder climates, made up of a coarse guard hair and a soft down hair, or "single", with the topcoat only.
Domestic dogs often display the remnants of countershading, a common natural camouflage pattern. A countershaded animal will have dark coloring on its upper surfaces and light coloring below, which reduces its general visibility. Thus, many breeds will have an occasional "blaze", stripe, or "star" of white fur on their chest or underside.

- Tail
There are many different shapes for dog tails: straight, straight up, sickle, curled, or cork-screw. As with many canids, one of the primary functions of a dog's tail is to communicate their emotional state, which can be important in getting along with others. In some hunting dogs, however, the tail is traditionally docked to avoid injuries. In some breeds, such as the Braque du Bourbonnais, puppies can be born with a short tail or no tail at all.

Persian Cat

             

The Persian cat is a long-haired breed of cat characterized by its round face and short muzzle. In Britain, it is sometimes called the Longhair or Persian Longhair. It is also known as the Shiraz or Shirazi, particularly in the Middle East. The first documented ancestors of the Persian were imported into Europe from Persia around 1620. Recognized by the cat fancy since the late 19th century, it was developed first by the English, and then mainly by American breeders after the Second World War. Some cat fancier organizations' breed standards subsume the Himalayan and Exotic Shorthair as variants of this breed, while others treat them as separate breeds.

The selective breeding carried out by breeders has allowed the development of a wide variety of coat colors, but has also led to the creation of increasingly flat-faced Persians. Favored by fanciers, this head structure can bring with it a number of health problems. As is the case with the Siamese breed, there have been efforts by some breeders to preserve the older type of cat, the traditional breed, having a more pronounced muzzle, which is more popular with the general public. Hereditary polycystic kidney disease is prevalent in the breed, affecting almost half the population in some countries.

The placid and unpretentious nature of the Persian evinces a propensity for apartment living. It has been the most popular breed in the United States for many years but its popularity has seen a decline in Britain and France.

>> Origin
It is not clear when longhaired cats first appeared, as there are no known long-haired specimens of the African wildcat, the ancestor of the domestic subspecies. There were claims in the 19th century that the gene responsible for long hair was introduced through hybridization with the Pallas cat, but research in the early 20th century refutes this theory.
The first documented ancestors of the Persian were imported from Khorasan, Persia, into Italy in 1620 by Pietro della Valle, and from Angora (now Ankara), Turkey, into France by Nicholas-Claude Fabri de Peiresc at around the same time. The Khorasan cats were grey coated while those from Angora were white. From France, they soon reached Britain.Longhaired cats were also imported to Europe from Afghanistan, Burma, China and Russia. Interbreeding of the various types was common, especially between Angoras and Persians.

Recent genetic research indicates that present day Persians are related not to cats from the Near East but to cats from Western Europe. The researchers stated, "Even though the early Persian cat may have in fact originated from ancient Persia, the modern Persian cat has lost its phylogeographical signature.

>> Development
>> Persians and Angoras
The first Persian cat was presented at the first organized cat show, in 1871 in the Crystal Palace in London, England, organized by Harrison Weir. As specimens closer to the later established Persian conformation became the more popular types, attempts were made to differentiate it from the Angora. The first breed standard (then called a points of excellence list) was issued in 1889 by cat show promoter Weir. He stated that the Persian differed from the Angora in the tail being longer, hair more full and coarse at the end and head larger, with less pointed ears. Not all cat fanciers agreed with the distinction of the two types, and in the 1903 work The Book of the Cat, Francis Simpson states that "the distinctions, apparently with hardly any difference, between Angoras and Persians are of so fine a nature that I must be pardoned if I ignore the class of cat commonly called Angora".

Dorothy Bevill Champion lays out the difference between the two types in the 1909 Everybody's Cat Book

Our pedigree imported long-hairs of to-day are undoubtedly a cross of the Angora and Persian ; the latter possesses a rounder head than the former, also the coat is of quite a different quality. The coat of the Persian consists of a woolly under coat and a long, hairy outer coat. In summer it loses all the thick underwool, and only the long hair remains. The hair is also somewhat shorter on the shoulders and upper part of the hind legs.

Now, the Angora has a very different coat, consisting of long, soft hair, hanging in locks, inclining to a slight curl or wave on the under parts of the body. The hair is also much longer on the shoulders and hind legs than the Persian, this being a great improvement; but the Angora fails to the Persian in head, the former having a more wedge-shaped head, whereas that of the modern Persian excels in roundness.

Of course. Angoras and Persians have been constantly crossed, with a decided improvement to each breed; but the long-haired cat of to-day is decidedly more Persian-bred than Angora.

Champion lamented the lack of distinction among various long-haired types by English fanciers, who in 1887, decided to group them under the umbrella term "Long-haired Cats"

>> Traditional Persian
The traditional Persian, or doll-face Persian,are somewhat recent names for what is essentially the original breed of Persian cat, without the development of extreme features.

As many breeders in the United States, Germany, Italy, and other parts of the world started to interpret the Persian standard differently, they developed the flat-nosed "peke-face" or "ultra" type (see next section) over time, as the result of two genetic mutations, without changing the name of the breed from "Persian". Some organizations, including the Cat Fanciers' Association (CFA), today consider the peke-face type as their modern standard for the Persian breed. Thus the retronym Traditional Persian was created to refer to the original type, which is still bred today, mirroring the renaming of the original-style Siamese cat as the Traditional Siamese, to distinguish it from long-faced modern development which has taken over as simply "the Siamese".

Not all cat fancier groups recognize the Traditional Persian (at all, or as distinct), or give it that specific name. TICA has a very general standard, that does not specify a flattened face.

Peke-face and ultra-typing
In the late 1950s a spontaneous mutation in red and red tabby Persians gave rise to the "peke-faced" Persian, named after the flat-faced Pekingese dog. It was registered as a distinct breed in the CFA, but fell out of favor by the mid-1990s due to serious health issues; only 98 were registered between 1958 and 1995. Despite this, breeders took a liking to the look and started breeding towards the peke-face look. The over-accentuation of the breed's characteristics by selective breeding (called extreme- or ultra-typing) produced results similar to the peke-faced Persians. The term peke-face has been used to refer to the ultra-typed Persian but it is properly used only to refer to red and red tabby Persians bearing the mutation. Many fanciers and CFA judges considered the shift in look "a contribution to the breed. The traditional Persian, or doll-face Persian, are somewhat recent names for what is essentially the original breed of Persian cat, without the development of extreme features.

As many breeders in the United States, Germany, Italy, and other parts of the world started to interpret the Persian standard differently, they developed the flat-nosed "peke-face" or "ultra" type (see next section) over time, as the result of two genetic mutations, without changing the name of the breed from "Persian". Some organizations, including the Cat Fanciers' Association (CFA), today consider the peke-face type as their modern standard for the Persian breed. Thus the retronym Traditional Persian was created to refer to the original type, which is still bred today, mirroring the renaming of the original-style Siamese cat as the Traditional Siamese, to distinguish it from long-faced modern development which has taken over as simply "the Siamese".

Not all cat fancier groups recognize the Traditional Persian (at all, or as distinct), or give it that specific name. TICA has a very general standard, that does not specify a flattened face.

Peke-face and ultra-typing
In the late 1950s a spontaneous mutation in red and red tabby Persians gave rise to the "peke-faced" Persian, named after the flat-faced Pekingese dog. It was registered as a distinct breed in the CFA, but fell out of favor by the mid-1990s due to serious health issues; only 98 were registered between 1958 and 1995. Despite this, breeders took a liking to the look and started breeding towards the peke-face look. The over-accentuation of the breed's characteristics by selective breeding (called extreme- or ultra-typing) produced results similar to the peke-faced Persians. The term peke-face has been used to refer to the ultra-typed Persian but it is properly used only to refer to red and red tabby Persians bearing the mutation. Many fanciers and CFA judges considered the shift in look "a contribution to the breed.The traditional Persian, or doll-face Persian, are somewhat recent names for what is essentially the original breed of Persian cat, without the development of extreme features.

As many breeders in the United States, Germany, Italy, and other parts of the world started to interpret the Persian standard differently, they developed the flat-nosed "peke-face" or "ultra" type (see next section) over time, as the result of two genetic mutations, without changing the name of the breed from "Persian". Some organizations, including the Cat Fanciers' Association (CFA), today consider the peke-face type as their modern standard for the Persian breed. Thus the retronym Traditional Persian was created to refer to the original type, which is still bred today, mirroring the renaming of the original-style Siamese cat as the Traditional Siamese, to distinguish it from long-faced modern development which has taken over as simply "the Siamese".

Not all cat fancier groups recognize the Traditional Persian (at all, or as distinct), or give it that specific name. TICA has a very general standard, that does not specify a flattened face.

Peke-face and ultra-typing
In the late 1950s a spontaneous mutation in red and red tabby Persians gave rise to the "peke-faced" Persian, named after the flat-faced Pekingese dog. It was registered as a distinct breed in the CFA, but fell out of favor by the mid-1990s due to serious health issues; only 98 were registered between 1958 and 1995. Despite this, breeders took a liking to the look and started breeding towards the peke-face look. The over-accentuation of the breed's characteristics by selective breeding (called extreme- or ultra-typing) produced results similar to the peke-faced Persians. The term peke-face has been used to refer to the ultra-typed Persian but it is properly used only to refer to red and red tabby Persians bearing the mutation. Many fanciers and CFA judges considered the shift in look "a contribution to the breed. In 1958, breeder and author P. M. Soderberg wrote in Pedigree Cats, Their Varieties, breeding and Exhibition

"Perhaps in recent times there has been a tendency to over-accentuate this type of short face, with the result that a few of the cats seen at shows have faces which present a peke-like appearance. This is a type of face which is definitely recognized in the United States, and helps to form a special group within the show classification for the  breed. There are certainly disadvantages when the face has become too short, for this exaggeration of type is inclined to produce a deformity of the tear ducts, and running eyes may be the result. A cat with running eyes will never look at its best because in time the fur on each side of the nose becomes stained, and thus detracts from the general appearance  The nose should be short, but perhaps a plea may be made here that the nose is better if it is not too short and at the same time uptilted. A nose of this type creates an impression of grotesqueness which is not really attractive, and there is always a danger of running eyes."

While the looks of the Persian changed, the Persian Breed Council's standard for the Persian had remained basically the same. The Persian breed standard is, by its nature, somewhat open-ended and focused on a rounded head, large, wide-spaced round eyes with the top of the nose leather placed no lower than the bottom of the eyes. The standard calls for a short, cobby body with short, well-boned legs, a broad chest, and a round appearance, everything about the ideal Persian cat being "round". It was not until the late 1980s that standards were changed to limit the development of the extreme appearance.In 2004, the statement that muzzles should not be overly pronounced was added to the breed standard. The standards were altered yet again in 2007, this time to reflect the flat face, and it now states that the forehead, nose, and chin should be in vertical alignment.
In the UK, the standard was changed by the Governing Council of the Cat Fancy (GCCF) in the 1990s to disqualify Persians with the "upper edge of the nose leather above the lower edge of the eye" from Certificates or First Prizes in Kitten Open Classes.

While ultra-typed cats do better in the show ring, the public seems to prefer the less extreme, older "doll-face" types.

>> Variants
> Himalayan
In 1950, the Siamese was crossed with the Persian to create a breed with the body type of the Persian but colorpoint pattern of the Siamese. It was named Himalayan, after other colorpoint animals such as the Himalayan rabbit. In the UK, the breed was recognized as the Colorpoint Longhair. The Himalayan stood as a separate breed in the US until 1984, when the CFA merged it with the Persian, to the objection of the breed councils of both breeds. Some Persian breeders were unhappy with the introduction of this crossbreed into their "pure" Persian lines.

The CFA set up the registration for Himalayans in a way that breeders would be able to discern a Persian with Himalayan ancestry just by looking at the pedigree registration number. This was to make it easy for breeders who do not want Himalayan blood in their breeding lines to avoid individuals who, while not necessarily exhibiting the colorpoint pattern, may be carrying the point coloration gene recessively. Persians with Himalayan ancestry has registration numbers starting with 3 and are commonly referred to by breeders as colorpoint carriers (CPC) or 3000-series cats, although not all will actually carry the recessive gene. The Siamese is also the source for the chocolate and lilac color in solid Persians.

> Exotic Shorthair
The Persian was used as an outcross secretly by some American Shorthair (ASH) breeders in the late 1950s to "improve" their breed. The crossbreed look gained recognition in the show ring but other breeders unhappy with the changes successfully pushed for new breed standards that would disqualify ASH that showed signs of crossbreeding.

One ASH breeder who saw the potential of the Persian/ASH cross proposed and eventually managed to get the CFA to recognize them as a new breed in 1966, under the name Exotic Shorthair. Regular outcrossing to the Persian has made present day Exotic Shorthair similar to the Persian in every way, including temperament and conformation, with the exception of the short dense coat. It has even inherited much of the Persian's health problems. The easier to manage coat has made some label the Exotic Shorthair the lazy person's Persian.

Because of the regular use of Persians as outcrosses, some Exotics may carry a copy of the recessive longhair gene. When two such cats mate, there is a one in four chance of each offspring being longhaired. Ironically, longhaired Exotics are not considered Persians by CFA, although The International Cat Association accepts them as Persians. Other associations register them as a separate Exotic Longhair breed.

>> Popularity

The Persian is the most popular breed of pedigree cats in the United States. In the UK, registration numbers have dwindled since the early 1990s and the Persian lost its top spot to the British Shorthair in 2001. As of 2012, it was the 6th most popular breed, behind the British Shorthair, Ragdoll, Siamese, Maine Coon and Burmese. In France, the Persian is the only breed whose registration declined between 2003 and 2007, dropping by more than a quarter.

The most color popular varieties according to CFA registration data are seal point, blue point, flame point and tortie point Himalayan, followed by black-white, shaded silvers and calico