Rabbits

Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world. There are seven different genera in the family classified as rabbits, including the European rabbit (Oryctolagus cuniculus), Cottontail rabbit (genus Sylvilagus; 13 species), and the Amami rabbit (Pentalagus furnessi, 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 rabbit lives in many areas around the world. 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. Meadows, woods, forests, thickets, and grasslands are areas in which rabbits live. They also inhabit deserts and wetlands. More than half the world's rabbit population resides in North America. They also live in Europe, India, Sumatra, Japan, and parts of Africa. The European rabbit has been introduced to many places around the world. 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. Each foot has five toes, with one greatly reduced in size. They are digitigrade animals; they move around on the tips of their toes. 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 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. Cecotropes, sometimes called "night feces", come from the
cecum and are high in minerals, vitamins and proteins that are necessary to the rabbit's health. Rabbits eat these to meet their nutritional requirements. This process allows rabbits to extract the necessary nutrients from their food.

Rabbits have two sets of incisor teeth, one behind the other. This way they can be distinguished from rodents, with which they are often mistaken.

Rabbits, being prey animals, tend to be exploratory in new spaces and when confronted with a threat, they tend to freeze and observe. Rabbit vision has a remarkably wide field of vision, and a good deal of it is devoted to overhead scanning. Even indoors, rabbits will scan for overhead threats.

Rabbits have a complex social structure, and like dogs, will attempt to establish a hierarchy and dominance.

Female rabbits do not actually ovulate until after breeding. They have a bifurcated uterus and often, breeding can involve multiple acts that can result in multiple impregnations from different bucks (male rabbits). Males are commonly sterile during the heat of summer months.
A litter of rabbit kits (baby rabbits) can be as small as a single kit, ranging up to 12 or 13; however there have been litters as big as 18. The gestation period is 30–32 days.

Rabbits are herbivores who 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 by passing two distinct types of feces: hard droppings and soft black viscous pellets, the latter of which are immediately eaten. 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.

Pet rabbits kept indoors are referred to as house rabbits. House rabbits typically have an indoor pen or cage and a rabbit-safe place to run and exercise, such as an exercise pen, living room or family room. Rabbits can be trained to use a litter box and some can learn to come when called. Domestic rabbits that do not live indoors can also often serve as companions for their owners, typically living in an easily accessible hutch outside the home. Some pet rabbits live in outside hutches during the day for the benefit of fresh air and natural daylight and are brought inside at night.

Whether indoor or outdoor, pet rabbits' pens are often equipped with enrichment activities such as shelves, tunnels, balls, and other toys. Pet rabbits are often provided additional space in which to get exercise, simulating the open space a rabbit would traverse in the wild. Exercise pens or lawn pens are often used to provide a safe place for rabbits to run.

A pet rabbit's diet typically consists of unlimited Timothy hay, a small amount of pellets, and a small portion of fresh vegetables. Rabbits are social animals. Rabbits as pets can find their companionship with a variety of creatures, including humans, other rabbits, guinea pigs, and sometimes even cats and dogs. Animal welfare organisations such as the House Rabbit Society recommend that rabits do not make good pets for small children because children generally do not know how to stay quiet, calm, and gentle around rabbits. As prey animals, rabbits are alert, timid creatures that startle easily. They have fragile bones, especially in their backs, that require support on the belly and bottom when picked up. Children 7 years old and older usually have the maturity required to care for a rabbit.

Crocodile

The largest captive crocodile alive in the US is located in South Carolina. In June 2002, Alligator Adventure introduced Utan. At 20 feet (6.1 m) long and weighing in at more than a ton, "Utan", the largest crocodile to ever be exhibited in the United States, made his new home in Myrtle Beach.

The largest recorded crocodile is a giant saltwater crocodile measured at 8.6 metres (28 ft) and 1,352 kilograms (2,980 lb) shot in Australia, Queensland in 1957. A replica of this crocodile has been made as a tourist attraction. The largest living crocodile known is a 7.1 metres (23 ft) long saltwater crocodile, in Orissa, India. It lives in Bhitarkanika Wildlife Sanctuary and in June 2006, was entered in the Guinness Book of World Records.

There is no reliable way of measuring crocodile age, although several techniques are used to derive a reasonable guess. The most common method is to measure lamellar growth rings in bones and teeth—each ring corresponds to a change in growth rate which typically occurs once a year between dry and wet seasons. Bearing these inaccuracies in mind, the oldest crocodilians appear to be the largest species. C. porosus is estimated to live around 70 years on average, and there is limited evidence that some individuals may exceed 100 years. One of the oldest crocodiles recorded died in a zoo in Russia. A male freshwater crocodile at the Australia Zoo is estimated to be 130 years old. He was rescued from the wild by Bob Irwin and Steve Irwin after being shot twice by hunters. As a result of the shootings, this crocodile (known affectionately as "Mr. Freshy") has lost his right eye.

The larger species of crocodiles are very dangerous to humans. The main danger that crocodiles pose is not their ability to run after a person but their ability to strike before the person can react. The Saltwater and Nile Crocodiles are the most dangerous, killing hundreds of people each year in parts of south-east Asia and Africa. Mugger crocodiles and possibly the endangered Black Caiman are also very dangerous to humans. American alligators are less aggressive and rarely assault humans without provocation.

A crocodile is any species belonging to the family Crocodylidae (sometimes classified instead as the subfamily Crocodylinae). The term can also be used more loosely to include all members of the order Crocodilia: i.e. the true crocodiles, the alligators and caimans (family Alligatoridae) and the gharials (family Gavialidae), or even the Crocodylomorpha which includes prehistoric crocodile relatives and ancestors. Crocodiles are large aquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. Crocodiles tend to congregate in freshwater habitats like rivers, lakes, wetlands and sometimes in brackish water. They feed mostly on vertebrates like fish, reptiles, and mammals, sometimes on invertebrates like mollusks and crustaceans, depending on species. They are an ancient lineage, and are believed to have changed little since the time of the dinosaurs. They are believed to be 200 million years old whereas dinosaurs became extinct 65 million years ago; crocodiles survived great extinction events.

Crocodiles are among the more biologically complex reptiles despite their prehistoric look.     Unlike other reptiles, they incorporate muscles used for aquatic locomotion into respiration (e.g. M. diaphragmaticus), giving them the functional equivalent of a diaphragm; a cerebral cortex; and a four-chambered heart.     Their externalmorphology on the other hand is a sign of their aquatic and predatory lifestyle.     A crocodile’s physical traits allow it to be a successful predator.     They have a streamlined body that enables them to swim swiftly. Crocodiles also tuck their feet to their sides while swimming, which makes them faster by decreasing water resistance.     They have webbed feet which, although not used to propel the animal through the water, allow it to make fast turns and sudden moves in the water or initiate swimming.     Webbed feet are an advantage in shallower water where the animals sometimes move around by walking.

Crocodiles have a palatal flap, a rigid tissue at the back of the mouth that blocks the entry of water. The palate has a special path from the nostril to the glottis that bypasses the mouth. The nostrils are closed during submergence. Like other archosaurs, crocodilians are diapsid, although their post-temporal fenestrae are reduced. The walls of the braincase are bony but they lack supratemporal and postfrontal bones. Their tongues are not free but held in place by a membrane which limits movement; as a result, crocodiles are unable to stick out their tongues.

Crocodilian scales have pores that are believed to be sensory, analogous to the lateral line in fishes. They are particularly seen on their upper and lower jaws. Another possibility is that they are secretory, as they produce an oily substance that appears to flush mud off.

Crocodiles are very fast over short distances, even out of water. Since crocodiles feed by grabbing and holding onto their prey, they have evolved sharp teeth for tearing and holding onto flesh, and powerful muscles that close the jaws and hold them shut. These jaws can bite down with immense force, by far the strongest bite of any animal. The crocodile's bite force is more than 5,000 pounds per square inch (340 atm), compared to just 335 pounds per square inch (22.8 atm) for a rottweiler, 400 pounds per square inch (27 atm) for a large great white shark, or 800 pounds per square inch (54 atm) to 1,000 pounds per square inch (68 atm) for a hyena. The jaws are opened, however, by a very weak set of muscles. Crocodiles can thus be subdued for study or transport by taping their jaws or holding their jaws shut with large rubber bands cut from automobile inner tubes. They have limited lateral
(side-to-side) movement in their neck.

Crocodiles are ambush hunters, waiting for fish or land animals to come close, then rushing out to attack. As cold-blooded predators, they have a very slow metabolism, and thus can survive long periods without food. Despite their appearance of being slow, crocodiles are top predators in their environment, and various species have been observed attacking and killing sharks. A famous exception is the Egyptian Plover which is said to enjoy a symbiotic relationship with the crocodile. According to unauthenticated reports, the plover feeds on parasites that infest the crocodile's mouth and the reptile will open its jaws and allow the bird to enter to clean out the mouth.

Many large crocodilians swallow stones (called gastroliths or stomach stones) which may act as ballast to balance their body or assist in crushing food, similar to grit in birds.

Salt glands are present in the tongues of most crocodylids and they have a pore opening on the surface of the tongue.      They appear to be similar to those in marine turtles; they seem to be absent in Alligatoridae.

Crocodilians can produce sounds during distress and in aggressive displays. They can also hear well and the tympanic membranes are concealed by flat flaps that may be raised or lowered by muscles.     Crocodiles eat fish, birds, mammals and occasionally smaller crocodiles.

Crocodiles are protected in many parts of the world, but they also are farmed commercially. Their hide is tanned and used to make leather goods such as shoes and handbags, whilst crocodile meat is also considered a delicacy.

The most commonly farmed species are the Saltwater and Nile crocodiles, while a hybrid of the Saltwater and the rare Siamese Crocodile is also bred in Asian farms. Farming has resulted in an increase in the Saltwater crocodile population in Australia, as eggs are usually harvested from the wild, so landowners have an incentive to conserve crocodile habitat.

Crocodiles are more closely related to birds and dinosaurs than to most animals classified as reptiles, the three being included in the group Archosauria ('ruling reptiles').

Crocodile embryos do not have sex chromosomes, and unlike humans sex is not determined genetically. Sex is determined by temperature, with males produced at around 31.6 °C, and females produced at slightly lower and higher temperatures.

The average incubation period is around 80 days, and also is dependent upon temperature.Crocodiles may possess a form of homing instinct. Three rogue saltwater crocodiles were relocated 400 kilometres by helicopter in northern Australia but had returned to their original locations within three weeks, based on data obtained from tracking devices attached to the reptiles.

The land speed record for a crocodile is 17 km/h (11 mph) measured in a galloping Australian freshwater crocodile. Maximum speed varies from species to species. Certain types of crocodiles can indeed gallop, including Cuban crocodiles, New Guinea crocodiles, African dwarf crocodiles, and even small Nile crocodiles. The fastest means by which most species can move is a kind of "belly run", where the body moves in a snake-like fashion, limbs splayed out to either side paddling away frantically while the tail whips to and fro. Crocodiles can reach speeds of 10 or 11 km/h (around 7 mph) when they "belly run", and often faster if they're slipping down muddy riverbanks. Another form of locomotion is the "high walk" where the body is raised clear off the ground.

Size greatly varies between species, from the dwarf crocodile to the saltwater crocodile. Species of Palaeosuchus and Osteolaemus grow to an adult size of just 1 metre (3.3 ft) to 1.5 metres (4.9 ft). Larger species can reach over 4.85 metres (15.9 ft) long and weigh well over 1,200 kilograms (2,600 lb). Crocodilians show pronounced sexual dimorphism with males growing much larger and more rapidly than females. Despite their large adult size, crocodiles start their life at around 20 centimetres (7.9 in) long.

Antelope

Antelope is a term referring to many even-toed ungulate species found in the family Bovidae. The term does not refer to a monophyletic group, as not all members of Bovidae are considered antelope. Instead, the term refers to a ‘miscellaneous’ group within the family encompassing the species which are not cattle, sheep, buffalo, bison, or goats. A group of antelope is called a herd.

Confusingly, the pronghorn antelope of North America is not an antelope, and no antelope species are native to the Americas. The pronghorn is not a member of the family Bovidae, but the family Antilocapridae. True antelope have horns which are unbranched and never shed, while Pronghorns have branching horns, shed annually.

Antelope are even-toed ruminant mammals. There are almost a hundred species of antelope, which vary widely in appearance, strategy, habitat, and range.

Antelope are not a cladistic or taxonomically defined group. The term is used loosely to describe all members of the family Bovidae which do not fall under the category of sheep, cattle, or goat. Usually all species of the Alcelaphinae, Antilopinae, Hippotraginae, Reduncinae, Cephalophinae, many Bovinae, the Grey Rhebok and the Impala are called antelopes.

Most species of antelope are native to Africa, but some originate in Asia as well. The Arabian peninsula is home to the Arabian Oryx and Dorcas gazelle. India is home to the Nilgai, Chinkara and Blackbuck, while Russia and Southeast Asia have the Four-horned Antelope, Tibetan antelope, and Saiga antelope.

Many species of antelope have been imported to other parts of the world, especially the United States, for exotic game hunting. Due to the spectacular leaping and evasive skills of some species, escaped individuals are not unheard of. Texas in particular has high numbers of gaming ranches, as well as habitat and climate that are very hospitable to African plains antelope species. As such, wild populations of Blackbuck Antelope, Gemsbok, and Nilgai have established in Texas, though they are not native to the area.

Antelope exist in a wide range of habitats. Numerically, most exist in the African savannahs. However, there are many species of more secluded forest antelope, as well as the extreme-cold living saiga, desert adapted Arabian oryx, the rocky kopje-living klipspringer and semi-aquatic sitatunga.

Species of forest, woodland or bush tend to be sedentary, but many of the plains species undertake huge migrations. These migrations enable grass-eating species to follow the rains and therefore their food supply. The gnus and gazelles of East Africa perform some of the most impressive mass migratory circuits of all mammals.

All bovids have even-toed hooves, horizontal pupils, ruminating guts, and (in at least the males) bony horns. These basic characteristics, however, mask huge differences in appearance between antelopes, cattle, goats and sheep, and among the antelopes themselves. For example, a male Common Eland can measure 178 cm at the shoulder and weigh almost 950 kg, whereas an adult Royal Antelope may stand only 24 cm at the shoulder and weigh a mere 1.5 kg.

Not surprisingly for animals with long slender yet powerful legs, many antelopes have long strides and can run fast. Some (e.g. Klipspringer) are also adapted to climbing in rock kopjes. Both Dibatags and Gerenuks habitually stand on their two hind legs to reach acacia and other tree foliage. Different antelope have different body types which can affect movement. Duikers are short, bush dwelling antelope that can pick through dense foliage and dive into the shadows rapidly. Gazelles and Springbok are known for their speed and leaping abilities. Even larger antelope, such as Nilgai, Elands, and Kudus, are capable of jumping 8 feet or greater, although their running speed is restricted by their greater mass.

Antelopes have a wide variety of coverings, through most have a dense coat of short fur. In most species, the coat (pelage) is some variation of a brown colour (or several shades of brown); often with white or pale under-bodies. Exceptions include the zebra-marked Zebra Duiker, the grey, black and white Jentink's Duiker and the Black Lechwe. Most of the "spiral-horned" antelopes have pale vertical stripes on their backs. Many desert and sub-desert species are particularly pale, some almost silvery or whitish (e.g. Arabian Oryx); the Beisa and Southern Oryxes have gray and black pelage with vivid black-and-white faces. Common features of various gazelles are a white rump, which flashes a warning to others when they run from danger, and a dark stripe mid-body (the latter feature is also shared by the Springbok and Beira). The Springbok also has a pouch of white brushlike hairs running along its back, which opens up when the animal senses danger, causing the dorsal hairs to stand on end.

Antelopes are ruminants, and thus have well-developed molar teeth, which grind cud (food balls stored in the stomach) into a pulp for further digestion. They have no upper incisors, but rather a hard upper gum pad, against which their lower incisors bite to tear grass stems and leaves.

Like many other herbivores, antelopes rely on keen senses to avoid predators. Their eyes are placed on the sides of their heads, giving them a broad radius of vision with minimal binocular vision. The fact that most species have their pupils elongated horizontally also helps in this respect. Acute senses of smell and hearing, give antelope the ability to perceive danger at night out in the open (when predators are often on the prowl). These same senses play an important role in contact between individuals of the same species: markings on head, ears, legs and rumps are used in such communication—many species "flash" such markings, as well as their tails; vocal communications include loud barks, whistles, "moos" and trumpeting; many species also use scent marking to define their territories or simply to maintain contact with their relatives and neighbors.

Many antelope are sexually dimorphic. In most species, both sexes have horns, but those of males tend to be larger. There is a tendency for males to be larger than the females; however, exceptions in which the females tend to be heavier than the males include the Bush Duiker, Dwarf Antelope, Cape Grysbok and Oribi, all rather small species. A number of species have hornless females (e.g. Sitatunga, Red Lechwe, and Suni). In some species, the males and females have different coloured pelage (e.g. Blackbuck and Nyala).

Size and shape of horns varies immensely. Those of the duikers and dwarf antelopes tend to be simple "spikes", but differ in the angle to the head from backward curved and backward pointing (e.g. Yellow-backed Duiker) to straight and upright (e.g. Steenbok). Other groups have twisted (e.g. Common Eland), spiral (e.g. Greater Kudu), "recurved" (e.g. the reedbucks), lyrate (e.g. Impala), or long, curved (e.g. the oryxes) horns. Horns are not shed and are not made of bone, which distinguishes them from antlers.

Horns are efficient weapons and tend to be better developed in those species where males fight over females (large herd antelope) than in solitary or lekking species. With male-male competition for mates, horns are clashed in combat. It is much more common for males to use their horns against each other than against another species.

The boss of the horns is typically arranged in such a way that two antelope striking at each other's horns cannot crack each other's skulls, making a fight via horn more ritualized than dangerous. Many species have ridges in their horns for at least 2/3 the length of their horns, but these ridges are not a direct indicator of age.

Antelope are often classified by their reproductive behavior.

Forest-dwelling bushbuck.Small antelope, such as dik-diks, tend to be monogamous. They live in a forest environment with patchy resources, and a male is unable to monopolize more than one female due to this sparse distribution. Larger forest species often form very small herds of 2-4 females and 1 male.

Some species such as lechwe pursue a lek breeding system. In this, the males gather on a lekking ground and compete for a small territory, while the females appraise males and choose one with whom to mate.

Large grazing antelope, such as impala or wildebeest form large herds made up of many females and a single breeding male, which excludes all other males, often by combat.

Antelope pursue a number of defense strategies, often dictated by their morphology.

Large antelope that gather in large herds, such as wildebeest or cape buffalo rely on numbers and running speed for protection. In some species, adults will circle around the offspring, protecting them from predators when threatened. Many forest antelope rely on cryptic coloring and good hearing to avoid predators. Forest antelope often have very large ears and a dark or striped coloration. Small antelope, especially duikers, evade predation by jumping into dense bush where the predator cannot pursue. Springboks use a behavior known as stotting to confuse predators.

Fast running gazelles prefer open grassland habitatOpen grassland species have nowhere to hide from predatorsm so they tend to be fast runners. They are agile and have good endurance. These are advantages when pursued by sprint-dependent predators like cheetah, which are the fastest of land animals but tire quickly. Reaction distances vary with predator species and predator behaviour. For example, gazelles may not flee from a lion until it is closer than 200 m (650 ft) lions hunt as a pride or by surprise, usually by stalking, one that can be seen clearly is unlikely to attack. However, sprint-dependent cheetahs will cause gazelles to flee at a range of over 800 m (0.5 mile).

It is difficult to determine how long antelope live in the wild. With the preference of predators towards old and infirm individuals who can no longer sustain peak speeds, few wild prey-animals live as long as their biological potential. In captivity, wildebeest have lived beyond 20 years old, and Impalas have reached their late teens. In the wild, few individuals of prey species live to old age, as the old and weak fall easier prey to their predators; antelopes are no exception to this rule.

Domestication of animals requires certain traits in the animal which antelope do not typically display. Most species are difficult to contain in any density, due to aggression or territoriality of males. Because many have extremely good jumping abilities, providing adequate fencing is a challenge. Also, antelope will consistently display a fear response to perceived predators, such as humans, making them very difficult to herd or handle.

Although antelopes have a diet and rapid growth rate highly suitable for domestication, this tendency to panic and their non-hierarchical social structure explains why not a single one of ninety or so species of antelope has been domesticated.

A wide variety of antelope hybrids have been recorded in zoos, game parks, and wildlife ranches. This is due to either a lack of more appropriate mates in enclosures shared with other species or a misidentification of species. The ease of hybridization shows how closely related some antelope species are. With few exceptions, most hybrid antelope occur only in captivity.

Most hybrids occur between species within the same genus. All reported examples occur within the same sub-family. As with most mammal hybrids, the less closely related the parents, the more likely that the offspring will be sterile.

Bird

Birds (class Aves) are winged, bipedal, endothermic (warm-blooded), vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the 5 cm (2 in) Bee Hummingbird to the 3 m (10 ft) Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 150–200 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 155–150 Ma. Most paleontologists regard birds as the only clade of dinosaurs that survived the Cretaceous–Tertiary extinction event approximately 65.5 Ma.

Modern birds are 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. All birds have forelimbs modified as wings and most can fly, with some exceptions including ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.

Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviours including 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 breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.

Many species are of economic importance, mostly as sources of food acquired through hunting or farming. Some species, particularly songbirds and parrots, are popular as pets. Other uses include the harvesting of guano (droppings) for use as a fertiliser. Birds figure prominently in all aspects of human culture from religion to poetry to popular music. About 120–130 species have become extinct as a result of human activity since the 17th century, and hundreds more before then. Currently about 1,200 species of birds are threatened with extinction by human activities, though efforts are underway to protect them.

Birds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the Snow Petrel's breeding colonies up to 440 kilometres (270 mi) inland in Antarctica. The highest bird diversity occurs in tropical regions. It was earlier thought that this high diversity was the result of higher speciation rates in the tropics, however recent studies found higher speciation rates in the high latitudes that were offset by greater extinction rates than in the tropics. Several families of birds have adapted to life both on the world's oceans and in them, with some seabird species coming ashore only to breed and some penguins have been recorded diving up to 300 metres (980 ft).

Many bird species have established breeding populations in areas to which they have been introduced by humans. Some of these introductions have been deliberate; the Ring-necked Pheasant, for example, has been introduced around the world as a game bird. Others have been accidental, such as the establishment of wild Monk Parakeets in several North American cities after their escape from captivity. Some species, including Cattle Egret, Yellow-headed Caracara and Galah, have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.

Birds have two sexes: male and female. The sex of birds is determined by the Z and W sex chromosomes, rather than by the X and Y chromosomes present in mammals. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ).

In nearly all species of birds, an individual's sex is determined at fertilization. However, one recent study demonstrated temperature-dependent sex determination among Australian Brush-turkeys, for which higher temperatures during incubation resulted in a higher female-to-male sex ratio.

Brood parasitism, in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism. After a parasitic bird lays her eggs in another bird's nest, they are often accepted and raised by the host at the expense of the host's own brood. Brood parasites may be either obligate brood parasites, which must lay their eggs in the nests of other species because they are incapable of raising their own young, or non-obligate brood parasites, which sometimes lay eggs in the nests of conspecifics to increase their reproductive output even though they could have raised their own young. One hundred bird species, including honeyguides, icterids, estrildid finches and ducks, are obligate parasites, though the most famous are the cuckoos. Some brood parasites are adapted to hatch before their host's young, which allows them to destroy the host's eggs by pushing them out of the nest or to kill the host's chicks; this ensures that all food brought to the nest will be fed to the parasitic chicks.

Governments and conservation groups work to protect birds, either by passing laws that preserve and restore bird habitat or by establishing captive populations for reintroductions. Such projects have produced some successes; one study estimated that conservation efforts saved 16 species of bird that would otherwise have gone extinct between 1994 and 2004, including the California Condor and Norfolk Island Green Parrot.

Reptiles

Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded amniotes that have skin covered in scales or scutes as opposed to hair or feathers. They are tetrapods (having or having descended from vertebrates with four limbs) and lay amniote eggs, whose embryos are surrounded by the amnion membrane. Modern reptiles inhabit every continent with the exception of Antarctica, and four living orders are currently recognized:

Crocodilia (crocodiles, gavials, caimans, and alligators): 23 species

Sphenodontia (tuatara from New Zealand): 2 species

Squamata (lizards, snakes, and amphisbaenids, worms) approximately 7,900 species

Testudines (turtles and tortoises): approximately 300 species

The majority of reptile species are oviparous (egg-laying) although certain species of squamates are capable of giving live birth. This is achieved, either through ovoviviparity (egg retention), or viviparity (offspring born without use of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, that grows to only 1.6 cm (0.6 in), to the saltwater crocodile that may reach 6 m in length and weigh over 1,000 kg. The science dealing with reptiles is called
herpetology.

The reptiles were from the outset of classification grouped with the amphibians. Linnaeus working from species poor Sweden where the common adder and grass Snake are often found hunting in water, included all reptiles and amphibians in class "III - Amphibia" in his Systema Naturae. The terms "reptile" and "amphibian" were largely interchangeable, "reptiles" being preferred by the French. Josephus Nicolaus Laurenti were the first to formally use the term "Reptilia" for an expanded, though basically similar selection of reptiles and amphibians to that of Linnaeus. Not until the turn of the century did it become clear that reptiles and amphibians are in fact quite different animals, and Pierre André Latreille erected the class Batracia for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians, birds and mammals.

The origin of the reptiles lays about 320-310 million years back, in the steaming swamps of the late Carboniferous, when the first reptiles evolved from advanced reptilomorph labyrinthodonts. The oldest traces of reptiles is a series of footprints from the fossil strata of Nova Scotia, dated to 315 million years old. The tracks are attributed to Hylonomus, the oldest known reptile in the biological sense of the word. It was a small, lizard-like animal, about 20 to 30 cm (8-12 inche) long, with numerous sharp teeth indicating an insectivorous diet. Other examples include Westlothiana (for the moment considered to be more related to amphibians than amniotes) and Paleothyris, both of similar build and presumably habit. One of the best known early reptiles is Mesosaurus, a genus of early reptiles from the early Permian that had returned to water, living off fish. The earliest reptiles were largely overshadowed by bigger labyrinthodont amphibians such as Cochleosaurus, and remained a small, inconspicuous part of the fauna until after the small ice age at the end of the Carboniferous.

The first reptiles are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving Anapsids, as they also share this skull structure, but this point has become contentious lately, with some arguing that turtles reverted to this primitive state in order to improve their armor. Both sides have strong evidence, and the conflict has yet to be resolved.

Very early after the first reptiles appeared, two branches split off. One lead to the Synapsida (the "mammal-like reptiles" or "stem mammals"), having two openings in the skull roof behind the eyes high , the other group, Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull.

The function of the holes in bout groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite. The diapsids and later anapsids are classed as the "true reptiles", the Sauropsida.

With the close of the Carboniferous, reptiles became the dominant tetrapod fauna. While the terrestrial reptilomorph labyrinthodonts still existed, the mammal-like reptiles evolved the first terrestrial megafauna in the form of pelycosaurs like Edaphosaurus and the carnivorous Dimetrodon. In the mid-Permian the climate turned dryer, resulting in a faunal turnover. The primitive pelycosaurs where replaced by the more advanced therapsids.

The anapsid reptiles, with their massive skulls without postorbital holes, continued and flourished throughout the Permian. The pareiasaurs reached giant proportions in the late Permian, eventually disappearing at the close of the period (the turtles being possible survivors).

Early in the period, the diapside reptiles split into two lineages, the lepidosaurs (forefathers of modern snakes, lizards, and tuataras). The group remained lizard-like and relatively small and inconspicuous during the whole periode.

The close of the Permian saw the greatest mass extinction known (see the Permian–Triassic extinction event). Most of the earlier anapsid/synapsid megafauna disappeared, making room for the archosauromorph diapsids. The archosaurs was characterized by elongated hind-legs and an erect pose, the early forms looking somewhat like long legged crocodiles. The archosaurs became the dominant group during the Triassic, developing into the well known dinosaurs and pterosaurs, as well as crocodiles and phytosaurs. Some of the dinosaurs developed into the largest land animals ever to have lived, making the Mesozoic popularly known as the "Age of Reptiles". The dinosaurs also deveoped smaller forms, including the feather-bearing smaller theropds. In the mid Jurassic, these gave rise to the first birds.

The lepidosauromorph diapsids may have been ancestral to the sea reptiles. Developing into the ichthyosaurs and sauropterygians, they came to dominate the Mesozoic seas.

The Therpasids came under increasing pressure from the archosaurs the early Mesozoic and developed into increasingly smaller and more nocturnal forms, the first mammals being the only survivors of the line by late Jurassic.

The close of the Cretacious saw the demise of the Mesozoic reptilian megafauna (see the Cretaceous–Tertiary extinction event). Of the large marine reptiles, only the sea turtles are left, and of the dinosaurs, only the small feathered theropods survived in the form of birds. The major surviving reptilian line is the lepidosaurs, of which the snakes are currently the most numerous and widespread representatives. The end of the “Age of Reptiles”, opened up for the “Age of Mammals”. Despite this, reptiles are still a major fauna component, particularly in tropical climates. There are about 8200 extant species of reptiles (whereof almost half are snakes), compared to 5400 species of mammals (of which two/thirds are rodents and bats). The most numerous modern group with reptilian roots are the birds, with over 9000 species.

Most reptiles have a three-chamber heart consisting of two atria, one variably-partitioned ventricle, and two aorta that go the systemic circulation. The degree of mixing of oxygenated and deoxygenated blood in the three-chamber heart is variable depending on the species and physiological state. Under different conditions, deoxygenated blood can be shunted back to the body or oxygenated blood can be shunted back to the lungs. This variation in blood flow has been hypothesized to allow more effective thermoregulation and longer diving times for aquatic species, but has not been shown to be a fitness advantage.

There are some interesting exceptions to the general physiology. For instance, crocodilians have an anatomically four-chambered heart, but also have two systemic aorta and are therefore capable only of bypassing their pulmonary circulation. Also, some snake and lizard species (e.g., monitor lizards and pythons) have three-chamber hearts that become functional four-chamber hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during ventricular diastole and completely divides it during ventricular systole.

Because of this ridge, some of these squamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.

All reptiles breathe using lungs. Aquatic turtles have developed more permeable skin, and some species have modified their cloaca to increase the area for gas exchange (Orenstein, 2001). Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. In squamates, the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of this constraint, most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it.

Varanids, and a few other lizard species, employ buccal pumping as a complement to their normal "axial breathing." This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time. Tegu lizards are known to possess a proto-diaphragm, which separates the pulmonary cavity from the visceral cavity.

While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs (Klein et al., 2003). Crocodilians actually have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis (part of the pelvis, which is movable in crocodilians) back, which brings the liver down, thus freeing space for the lungs to expand.

This type of diaphragmatic setup has been referred to as the "hepatic piston."

How turtles and tortoises breathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how turtles do it. The results indicate that turtles & tortoises have found a variety of solutions to this problem. The problem is that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles such as the Indian flapshell (Lissemys punctata) have a sheet of muscle that envelops the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a diaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction). Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements (Landberg et al., 2003). They are probably using their abdominal muscles to breathe during locomotion. The last species to have been studied is red-eared sliders, which also breathe during locomotion, but they had smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells.

Most reptiles lack a secondary palate, meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains from getting kicked in by struggling prey). Skinks (family Scincidae) also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead.

Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.Reptilian skin is covered in a horny epidermis, making it watertight and enable reptiles to live on dry land, in contrast to the amphibians. Compared to mammals, reptilian skin is rather thin, and lack the thick dermal layer that produces leather in mammals. Exposed parts of reptiles are protected by scales or scutes, sometimes with a bony base, forming armour. In turtles, the body is hidden inside a hard shell composed on fused scutes. In the lepidosaurians like lizards and snakes, the whole skin is covered in epidermal scales. Such scales where once thought to be typical of the class Reptilia as a whole, but are actually found only in lepidosaurians. The scales found in turtles and crocodiles are of dermal origin rather than epidermal, and are properly termed scutes.

Excretion is performed mainly by two small kidneys. In diapsids, uric acid is the main nitrogenous waste product; turtles, like mammals, mainly excrete urea.

Unlike the kidneys of mammals and birds, reptile kidneys are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialized structure present in the nephrons of birds and mammals, called a Loop of Henle. Because of this, many reptiles use the colon to aid in the reabsorption of water. Some are also able to take up water stored in the bladder. Excess salts are also excreted by nasal and lingual salt glands in some reptiles.

Most reptiles are carnivorous and have rather simple and not overly long guts, meat being fairly simple to break down and digest. Digestion is slower than in mammals, reflecting about the fact that they can not divide and masticate their food like mammals do, and their lower metabolism. Being cold blooded their energy requirement is about a 5th to a 10th of that of a mammal of the same size. Large reptiles like crocodiles and the large constrictors can basically live from a single large meal for months, digesting it slowly.

While modern reptiles are predominately carnivorous, this has not always been so. During the early history of reptiles, several groups produced big-bodied herbivorous megafauna, in the Paleozoic the Pareiasaurs and the synapsid Dicynodonts, and in the Mesozoic several lines of Dinosaurs. Today the turtles are the only predominantly herbivorous reptile group, but several lines of agams and iguanas have developed to live wholly or partly from plants.

Herbivorous reptiles face the same problems of mastication as herbivorous mammals, but lacking the complex mammal teeth, quite a few species swallow rocks and pebbles to aid in digestion, so called gastrolithes. The rocks are washed around in the stomach helping to grind up plant matter. Fossil gastrolithes has also been found associated with sauropods. Sea turtles, crocodiles and marine iguanas also use the gastrolithes as ballast, helping them to dive.

The reptilian nervous system contains the same basic part of the amphibian brain, but the reptile cerebrum and cerebellum are slightly larger. Most typical sense organs are well developed with certainexceptions most notably the snake's lack of external ears (middle and inner ears are present). There are twelve pairs of cranial nerves.

Reptiles are not generally considered particularly intelligent when compared to mammals and birds. Their brains fall well below those of mammals in size relative to the body, the encephalisation quotient being about one tenth of that of mammals. The crocodiles have brains in the higher size range and show a fairly complex social structure.

Larger lizards like the monitors are known to exhibit complex behaviour, including cooporation. The Komodo dragon is known to engage in play.

Most reptiles are diurnal animals. The vision is typically adapted to daylight condition, with colour vision and advanced visual depth perception compared to amphibians and most mammals. In some species vision is reduced, such as blindsnakes. Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form of pits sensitive to infrared radiation (heat). Such heat sensitive pits are particularly well developed in the pit vipers, but also found in boas and pythons. These allows the snakes to sense the body heat from birds and mammals, making pitvipers able to hunt rodents in the dark.

Most reptiles reproduce sexually, though some are capable of asexual reproduction. All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Tuataras lack copulatory organs, so the male and female simply press their cloacas together as the male excretes sperm.

Most reptiles, however, have copulatory organs, which are usually retracted or inverted and stored inside the body.

In turtles and crocodilians, the male has a single median penis, while squamates including snakes and lizards possess a pair of hemipenes.

Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion, chorion, and allantois are present during embryonic life. There are no larval stages of development. Viviparity and ovoviviparity have only evolved in Squamates, and a substantial fraction of the species utilize this mode of reprduction, including all boas and most vipers. The degree of viviparity varies: some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a placenta.

Asexual reproduction has been identified in squamates in six families of lizards and one snake.

In some species of squamates, a population of females are able to produce a unisexual diploid clone of the mother. This asexual reproduction called parthenogenesis occurs in several species of gecko, and is particularly widespread in the teiids (especially Aspidocelis) and lacertids (Lacerta). In captivity, Komodo dragons (varanidae) have reproduced by parthenogenesis.

Parthenogenetic species are also suspected to occur among chameleons, agamids, xantusiids, and typhlopids.

Primates

A primate is a member of the biological order Primates, the group that contains lemurs, the Aye-aye, lorisids, galagos, tarsiers, monkeys, and apes, with the last category including great apes. With the exception of humans, who inhabit every continent on Earth, most primates live in tropical or subtropical regions of the Americas, Africa and Asia. Primates range in size from the Pygmy Mouse Lemur weighing only 30 grams (1.1 oz) to the Mountain Gorilla weighing 200 kilograms (440 lb). According to fossil evidence, the primitive ancestors of primates may have existed in the late Cretaceous period around 65 million years ago, and the oldest known primate is the Late Paleocene Plesiadapis, c. 55–58 million years ago. Molecular clock studies suggest that the primate branch may be even older, originating in the mid-Cretaceous period around 85 mya.

The Primates order has traditionally been divided into two main groupings: prosimians and simians. Prosimians have characteristics most like those of the earliest primates, and included the lemurs of Madagascar, lorisiforms, Aye-aye and tarsiers. Simians included the monkeys and apes. More recently, taxonomists have created the suborder Strepsirrhini, or "curly-nosed" primates, to include non-tarsier prosimians and the suborder Haplorrhini, or "dry-nosed" primates, to include tarsiers and the simians. Simians are divided into two groups: the platyrrhines ("flat nosed") or New World monkeys of South and Central America and the catarrhine ("narrow nosed") monkeys of Africa and southeastern Asia. The New World monkeys include the capuchin, howler and squirrel monkeys, and the catarrhines include the Old World monkeys (such as baboons and macaques) and the apes. Humans are the only catarrhines that have spread outside of Africa, South Asia, and East Asia, although fossil evidence shows many species once existed in Europe as well.

Considered generalist mammals, primates exhibit a wide range of characteristics. Some primates (including some great apes and baboons) do not live primarily in trees, but all species possess adaptations for climbing trees.

Locomotion techniques used include leaping from tree to tree, walking on two or four limbs, knuckle-walking, and swinging between branches of trees (known as brachiation). Primates are characterized by their large brains, relative to other mammals, as well as an increased reliance on stereoscopic vision at the expense of smell, the dominant sensory system in most mammals. These features are most significant in monkeys and apes, and noticeably less so in lorises and lemurs.

Three-color vision has developed in some primates. Most also have opposable thumbs and some have prehensile tails. Many species are sexually dimorphic, which means males and females have different physical traits, including body mass, canine tooth size, and coloration. Primates have slower rates of development than other similarly sized mammals, and reach maturity later but have longer lifespans. Some species live in solitude, others live in male–female pairs, and others live in groups of up to hundreds of members.

The primate lineage is thought go back to at least 65 mya, even though the oldest known primate from the fossil record is Plesiadapis (c. 55–58 mya) from the Late Paleocene. Other studies, including molecular clock studies, have estimated the origin of the primate branch to have been in the mid-Cretaceous period, around 85 mya.

In modern cladistic reckonings, the Primates order is monophyletic. The suborder Strepsirrhini, the curly-nosed or "wet-nosed" primates, is generally thought to have split off from the primitive primate line about 63 mya (million years ago), although earlier dates are also supported. The seven strepsirhine families are the four related lemur families and the three remaining families that include the Aye-aye, the lorisids, and the galagos. Older classification schemes wrap the Lepilemuridae into the Lemuridae and the Galagidae into the Lorisidae, yielding a three-two family split instead of the four-three split as presented here. During the Eocene, most of the northern continents were dominated by two groups, the adapiforms and the omomyids. The former is considered a member of Strepsirrhini, but it does not have a toothcomb like modern lemurs; recent analysis has suggested Darwinius masillae fits into this grouping. The latter was related closely to tarsiers, monkeys, and apes. It is unclear exactly how these two groups relate to extant primates. Omomyids perished about 30 mya, while Adapids survived until about 10 mya.

According to genetic studies, the lemurs of Madagascar diverged from the lorisiforms approximately 75 mya. These studies, as well as chromosomal and molecular evidence, also show that lemurs are more closely related to each other than to other strepsirrhine primates. However, Madagascar split from Africa at 160 mya and from India at 90 mya.

For lemurs to be more closely related to each other than other strepsirrhine primates, it is thought that a very small ancestral population came to Madagascar via a single rafting event between 50 and 80 million years ago.

Other colonization options have been examined, such as multiple colonizations from Africa and India, but none are supported by the genetic and molecular evidence.

Until recently the Aye-aye has been difficult to place within Strepsirrhini. Theories had been proposed that its family, Daubentoniidae, was either a lemuriform primate (meaning its ancestors split from lemur line more recently than the lemurs and lorises split) or a sister group to all the other strepsirrhines. In 2008, the Aye-aye family (Daubentoniidae) was confirmed to be a lemuriform and descended from the same ancestral lemur population that rafted to the island, it is contained within the Chiromyiformes infraorder, forming a sister clade to the lemurs.

The suborder Haplorrhini, the simple-nosed or "dry-nosed" primates, is composed of two sister clades. The prosimian tarsiers in family Tarsiidae (monotypic in its own infraorder Tarsiiformes), represent the most primitive division at about 58 mya. The Simiiformes infraorder emerged about 40 mya, and contains the two clades: the parvorder Platyrrhini that developed in South America and contains New World monkeys, and the parvorder Catarrhini that developed in Africa and contains the Old World monkeys, humans and the other apes. A third clade, which included the eosimiids, developed in Asia but went extinct millions of years ago.

Like the lemurs, the New World monkeys have unclear origins. Molecular sequence studies based on concatenated sequences have yielded wide variations in the estimated date of the divergence between platyrrhines and catarrhines, ranging from 33 to 70 mya, while studies based on mitochondrial sequences suggest a more consistent date of 35 mya.

It has been postulated that there is a single origin for the anthropoids in Africa some migrated and subsequently speciation occurred. It is possible that the anthropoid primates traversed the Atlantic ocean during the Eocene, facilitated by Atlantic Ocean ridges and a lowered sea level, then island-hopped to South America. Once again, a rafting event may explain this transoceanic colonization. Due to continental drift, the young Atlantic Ocean was not nearly as wide as it is today, and research suggests that a small 1 kg (2.2 lb) primate could have survived 13 days on a raft of vegetation. Given estimated current and wind speeds, this would have provided enough time to make the voyage between the continents.

Apes and monkeys spread from Africa into Europe and Asia starting in the Miocene. Soon after, the lorises and tarsiers made the same journey. The first hominid fossils were discovered in Northern Africa and date back 5–8 mya. Old World monkeys disappeared from Europe about 1.8 mya. Some molecular and fossil studies generally show that modern humans originated in Africa 100–200 tya (thousand years ago).

Although primates are well studied in comparison to other animal groups, several new species have been recently discovered; genetic tests on some populations have revealed previously unrecognised species. Primate Taxonomy listed about 350 species of primates in 2001, the author, Colin Groves, increased that number to 376 for his contribution to the third edition of Mammal Species of the World (MSW3). However, MSW3 falls short of current understanding as its collection was completed in 2003; a number of publications have since pushed the number to 424 species and 658 species and subspecies. Notable new species not listed in MSW3 include the Bemaraha Woolly Lemur (Avahi cleesei, named after British actor and lemur enthusiast John Cleese) and the GoldenPalace.com Monkey (whose name was put up for auction).

Primate hybrids usually arise in captivity, but there have also been examples in the wild. Hybridization occurs where two species' range overlap to form hybrid zones; hybrids may be created by humans when animals are placed in zoos or due to environmental pressures such as predation. Intergeneric hybridizations, hybrids of different genera, have also been found in the wild. Although they belong to genera that have been distinct for several million years, interbreeding still occurs between the Gelada and the Hamadryas Baboon.

Primates have forward-facing eyes on the front of the skull; binocular vision allows accurate distance perception, useful for the brachiating ancestors of all great apes. There is a bony ridge above the eye sockets; this ridge reinforces weaker bones in the face which are put under strain during chewing. Strepsirrhines have a postorbital bar, a bone which runs around the eye socket, to protect their eyes; in contrast, the higher primates, haplorrhines, have evolved fully enclosed sockets.

The primate skull has a large domed cranium which is particularly prominent in anthropoids. The cranium protects the large brain, a distinguishing characteristic of this group. The endocranial volume (the volume within the skull) is three times greater in humans than in the greatest non-human primate, reflecting a larger brain size.

The mean endocranial volume is 1201 cubic centimeters in humans, 469 cm3 in gorillas, 400 cm3 in chimpanzees and 397 cm3 in orangutans. The primary evolutionary trend of primates has been the elaboration of the brain, in particular the neocortex (a part of the cerebral cortex), which is involved with sensory perception, generation of motor commands, spatial reasoning, conscious thought and, in humans, language. While other mammals rely heavily on their sense of smell, the arboreal life of primates has led to a tactile, visually dominant sensory system, a reduction in the olfactory region of the brain and increasingly complex social behavior.

Primates generally have five digits on each limb (pentadactyly), with keratin nails on the end of each finger. The bottom sides of the hands and feet have sensitive pads on the fingertips. Most have opposable thumbs, a characteristic primate feature; however, opposing thumbs are not limited to this order (opossums, for example, also have them). Thumbs allow some species to use tools. In primates, the combination of opposing thumbs, short fingernails (rather than claws) and long, inward-closing fingers is a relic of the ancestral practice of gripping branches, and has, in part, allowed some species to develop brachiation (swinging by the arms from tree limb to tree limb) as a significant means of transportation. Prosimians have clawlike nails on the second toe of each foot, called toilet-claws, which they use for grooming.

The primate collar bone is retained as prominent element of the pectoral girdle; this allows the shoulder joint broad mobility. Apes have more mobile shoulder joints and arms due to the dorsal position of the scapula, broad ribcages that are flatter front-to-back, and a shorter, less mobile spine compared to Old World monkeys (with lower vertebrae greatly reduced, resulting in tail loss in some species). Old World monkeys are unlike apes in that most have tails. The only primate family with prehensile tails are the New World Atelids, including the howler, spider and woolly monkeys.

Primates show an evolutionary trend towards a reduced snout. Technically, Old World monkeys are distinguished from New World monkeys by the structure of the nose, and from apes by the arrangement of their teeth. In New World monkeys the nostrils face sideways; in Old World monkeys, they face downwards. There is a considerably varied dental pattern in primates and although some have lost most of their incisors, all retain at least one lower incisor.

In most strepsirhines, the lower incisors and canines form a toothcomb, which is used in grooming and sometimes foraging, and the first lower premolar is shaped like a canine. Old World monkeys have eight premolars, compared with twelve in New World monkeys. The Old World species are divided into apes and monkeys depending on the number of cusps on their molars; apes have five, Old World monkeys have four, although humans may have 4 or 5. The main hominid molar cusp (hypocone) evolved in early primate history, while the cusp of the corresponding primitive lower molar (paraconid) was lost. Prosimians are distinguished by their immobilized upper lips, the moist tip of their nose and forward-facing lower front teeth.

The evolution of color vision in primates is unique among most eutherian mammals. While the remote vertebrate ancestors of the primates possessed three color vision (trichromaticism), the nocturnal, warm-blooded, mammalian ancestors lost one of three cones in the retina during the Mesozoic period. Fish, reptiles and birds are therefore trichromatic or tetrachromatic while all mammals, with the exception of some primates and marsupials, are dichromats or monochromats (totally color blind). Nocturnal primates, such as the night monkeys and bush babies, are often monochromatic. Catarrhines are routinely trichromatic due to a gene duplication of the red-green opsin gene at the base of their lineage, 30 to 40 million years ago. Platyrrhines, on the other hand, are trichromatic in a few cases only. Specifically, individual females must be heterozygous for two alleles of the opsin gene (red and green) located on the same locus of the X chromosome. Males, therefore, can only be dichromatic, while females can be either dichromatic or trichromatic. Color vision in strepsirrhines is not as well understood; however, research indicates a range of color vision similar to that found in platyrrhines.

Like catarrhines, Howler monkeys (a family of platyrrhines) show routine trichromatism that has been traced to an evolutionarily recent gene duplication. Howler monkeys are one of the most specialized leaf-eaters of the New World monkeys; fruits are not a major part of their diet, and the type of leaves they prefer to consume (young, nutritive, and digestible) are detectable only by a red-green signal. Field work exploring the dietary preferences of howler monkeys suggests that routine trichromaticism was environmentally selected for.

Sexual dimorphism, the variation between individuals of different sex in the same species, is often exhibited in simians, though to a greater degree in Old World species (apes and some monkeys) than New World species. Recent studies involve comparing DNA to examine both the variation in the expression of the dimorphism among primates and the fundamental causes of sexual dimorphism. Primates usually have dimorphism in body mass and canine tooth size along with pelage and skin color. The dimorphism can be attributed to and affected by different factors, including mating system, size, habitat and diet.

Comparative analyses have generated a more complete understanding of the relationship between sexual selection, natural selection, and mating systems in primates. Studies have shown that dimorphism is the product of changes in both male and female traits. Ontogenetic scaling, where relative extension of a common growth trajectory occurs, may give some insight into the relationship between sexual dimorphism and growth patterns. Some evidence from the fossil record suggests that there was convergent evolution of dimorphism, and some extinct hominids probably had greater dimorphism than any living primate.

Primate species move by brachiation, bipedalism, leaping, arboreal and terrestrial quadrupedalism, climbing, knuckle-walking or by a combination of these methods. Several prosimians are primarily vertical clinger and leapers.

These include many bushbabies, all indriids (i.e., sifakas, avahis and indris), sportive lemurs, and all tarsiers. Other prosimians are arboreal quadrupeds and climbers. Some are also terrestrial quadrupeds, while some are leapers. Most monkeys are both arboreal and terrestrial quadrupeds and climbers. Gibbons, muriquis and spider monkeys all use brachiation extensively. Woolly monkeys also sometimes brachiate. Orangutans use a similar form of locomotion called quadramanous climbing, in which they use their arms and legs to carry their heavy bodies through the trees. Chimpanzees and gorillas knuckle walk, and can move bipedally for short distances. Although numerous species, such as the Australopithecines and early hominids, have exhibited fully bipedal locomotion, humans are the only extant species with this trait.

Several species of primates are known to associate in the wild. Some of these associations have been extensively studied. In the Tai Forest of Africa several species coordinate anti-predator behavior. These include the Diana Monkey, Campbell's Mona Monkey, Lesser Spot-nosed Monkey, Western Red Colobus, King Colobus and Sooty Mangabey, which coordinate anti-predator alarm calls. Among the predators of these monkeys is the Common Chimpanzee.

The Red-tailed Monkey associates with several species, including the Western Red Colobus, Blue Monkey, Wolf's Mona Monkey, Mantled Guereza, Black Crested Mangabey and Allen's Swamp Monkey. Several of these species are predated on by the Common Chimpanzee.

In South America, squirrel monkeys associate with capuchin monkeys. This may have more to do with foraging benefits to the squirrel monkeys rather than anti-predation benefits.

Primates have advanced cognitive abilities: some make tools and use them to acquire food and for social displays; some have sophisticated hunting strategies requiring cooperation, influence and rank; they are status conscious, manipulative and capable of deception; they can recognise kin and conspecifics; and they can learn to use symbols and understand aspects of human language including some relational syntax and concepts of number and numerical sequence.

Research in primate cognition explores problem solving, memory, social interaction, a theory of mind, and numerical, spatial, and abstract concepts.

Lemurs, lorises, tarsiers, and New World monkeys rely on olfactory signals for many aspects of social and reproductive behavior. Specialized glands are used to mark territories with pheromones, which are detected by the vomeronasal organ; this process forms a large part of the communication behavior of these primates. In Old World monkeys and apes this ability is mostly vestigial, having regressed as trichromatic eyes evolved to become the main sensory organ. Primates also use vocalizations, gestures, and facial expressions to convey psychological state.

Primates have slower rates of development than other mammals. All non-human primate infants are breastfed by their mothers and rely on them for grooming and transportation. In some species, infants are protected and transported by males in the group, particularly males who may be their fathers. Other relatives of the infant, such as siblings and aunts, may participate in its care as well. Most primate mothers cease ovulation while breastfeeding an infant; once the infant is weaned the mother can reproduce again. This often leads to weaning conflict with infants who attempt to continue breastfeeding.

Primates have a longer juvenile period between weaning and sexual maturity than other mammals of similar size. During the juvenile period, primates are more susceptible than adults to predation and starvation; they gain experience in feeding and avoiding predators during this time they learn social and fighting skills, often through playing.

Primates, especially females, have longer lifespans than other similarly sized mammals.

Whale

Whales are marine mammals of order Cetacea which are neither dolphins—members, in other words, of the families Delphinidae or Platanistoidae—nor porpoises. They include the blue whale, the largest living animal. Orcas, colloquially referred to as "killer whales", and pilot whales have whale in their name but for the purpose of biological classification they are actually dolphins. For centuries whales have been hunted for meat and as a source of valuable raw materials. By the middle of the 20th century, large-scale industrial whaling had left many
species seriously endangered.

All cetaceans, including whales, dolphins and porpoises, are descendants of land-living mammals of the Artiodactyl order (even-toed ungulate animals). Both cetaceans and artiodactyl are now classified under the super-order Cetartiodactyla which includes both whales and hippopotamuses. In fact, whales are the closest living relatives of hippos; they evolved from a common ancestor at around 54 million years ago. Whales entered the water roughly 50 million years ago. Cetaceans are divided into two suborders:

The baleen whales are characterized by baleen, a sieve-like structure in the upper jaw made of keratin, which they use to filter plankton from the water. They are the largest suborder of whale.

The toothed whales have teeth and prey on fish, squid, or both. An outstanding ability of this group is to sense their surrounding environment through echolocation.

Like all mammals, whales breathe air into lungs, are warm-blooded, feed their young milk from mammary glands, and have some hair, although very little.

The body is fusiform. The forelimbs, also called flippers, are paddle-shaped. The end of the tail holds the fluke, or tail fins, which provide propulsion by vertical movement. Although whales generally do not possess hind limbs, some whales (such as sperm whales and baleen whales) sometimes have rudimentary hind limbs; some even with feet and digits. Most species of whale bear a fin on their backs known as a dorsal fin.

Beneath the skin lies a layer of fat, called blubber. It serves as an energy reservoir and also as insulation. Whales have a four-chambered heart. The neck vertebrae are fused in most whales, which provides stability during swimming at the expense of flexibility. They have a pelvis bone, which is a vestigial structure. Whales breathe through their blowholes, located on the top of the head so the animal can remain submerged. Baleen whales have two; toothed whales have one. The shapes of whales' spouts when exhaling after a dive, when seen from the right angle, differ between species. Whales have a unique respiratory system that lets them stay underwater for long periods of time without taking in oxygen. Some whales, such as the Sperm Whale, can stay underwater for up to two hours holding a single breath. The Blue Whale is the largest known mammal that has ever lived, and the largest living animal, at up to 35 m (105ft) long and 150 tons. Whales generally live for 40–90 years, depending on their species, and on rare occasions can be found to live over a century. Recently a fragment of a lance used by commercial whalers in the 19th century has been found in a bowhead whale caught off Alaska, which showed the whale to be between 115 and 130 years old. Furthermore, a technique for dating age from aspartic acid racemization in the whale eye, combined with a harpoon fragment, indicates an age of 211 years for one male, making bowhead whales the longest lived extant mammal species. Whale flukes often can be used as identifying markings, as is the case for humpback whales. This is the method by which the publicized errant Humphrey the whale was identified in three separate sightings.

A toothed whale, like the sperm whale, possess teeth with cementum cells overlying dentine cells. Unlike human teeth which are comprised mostly of enamel on the tooth portion outside of the gum, whale teeth have cementum outside the gum. Only in larger whales does some enamel show where the cementum has been worn away on the tip of the tooth revealing the underlying enamel.

Whales' ears have specific adaptations to their underwater environment. In humans, the middle ear works as an impedance matcher between the outside air’s low impedance and the cochlear fluid’s high impedance. In aquatic mammals such as whales, however, there is no great difference between the outer and inner environments. Instead of sound passing through outer ear to middle ear, whales receive sound through their lower jaw, where it passes through a low-impedance, fat-filled cavity.

Whales are widely classed as predators, but their food ranges from microscopic plankton to very large fish. Males are called bulls; females, cows. The young are called calves.

As mammals, whales breathe air and must surface to get oxygen. This is done through a blowhole. Many whales also exhibit other surfacing behaviours such as breaching and tail slapping.

Because of their environment (and unlike many animals), whales are conscious breathers: they decide when to breathe. All mammals sleep, including whales, but they cannot afford to fall into an unconscious state for too long, since they need to be conscious to breathe. It is thought that only one hemisphere of their brains sleeps at a time, so that whales are never completely asleep, but still get the rest they need. This is thought because whales often sleep with only one eye closed.

Whales communicate with each other using lyrical sounds, called whale song. Being so large and powerful, these sounds are also extremely loud (depending on the species); sperm whales have only been heard making clicks, as all toothed whales (Odontoceti) use echolocation and can be heard for many miles. They have been known to generate about 20,000 acoustic watts of sound at 163 decibels.

Females give birth to a single calf. Nursing time is long (more than one year in many species), which is associated with a strong bond between mother and young. In most whales reproductive maturity occurs late, typically at seven to ten years. This mode of reproduction spawns few offspring, but provides each with a high probability of survival in the wild.

The male genitals are retracted into cavities of the body during swimming, so as to be streamlined and reduce drag and to prevent injury. Most whales do not maintain fixed partnerships during mating; in many species the females have several mates each season. At birth newborn are delivered tail-first, minimising the risk of drowning.

Whale cows nurse by actively squirting milk the consistency of toothpaste into the mouths of their young preventing loss to the surrounding aquatic environment.


Some species of large whales are listed by various watchdog groups and governments as endangered due to reduced population resulting from commercial whaling. Large whales have been hunted commercially for whale oil, meat, baleen and ambergris (a perfume ingredient from the intestine of sperm whales) since the 1600s. By the middle of the 20th century, whaling left many populations severely depleted.

The International Whaling Commission introduced a six year moratorium on all commercial whaling in 1986, which has been extended to the present day. The moratorium is not absolute, however, and some whaling continues to be practiced under the auspice of research or aboriginal rights; current whaling nations are Norway, Iceland and Japan and the aboriginal communities of Siberia, Alaska and northern Canada.

Several species of small whales are caught as bycatch in fisheries for other species. In the tuna fishery in the Eastern Tropical Pacific thousands of dolphins were drowned in purse-seine nets, until measures to prevent this were introduced. Fishing gear and deployment modifications, and eco-labelling (dolphin-safe or dolphin-friendly brands of canned tuna), have contributed to a reduction in the mortality of dolphins by tuna fishing vessels in recent years. In many countries, small whales are still hunted for food, oil, meat or bait.