Primate - biology. I INTRODUCTION Primate, order of mammals that includes humans, apes, which are the closest living relatives to humans, monkeys, and some less familiar mammals, such as tarsiers, lorises, and lemurs. Humans and other primates share a common evolutionary descent. For this reason, primates have always fascinated scientists because their physical features, social organization, behavioral patterns, and fossil remains provide clues about our earliest human ancestors. Primates evolved from tree-dwelling ancestors. Although some species, such as humans, have since taken to the ground, all primates share features that are related to their tree-climbing ancestry. These include arms and legs that can move more freely than those of most other mammals, flexible fingers and toes, forward-facing eyes that can judge distances accurately--a vital aid when moving about high above the ground--and large brains. Primates live in a wide range of habitats but are restricted by their need for warmth. Most primates live in tropical jungles or dry forests, but some live in dry grasslands, and others have settled in cold, mountainous regions of China and Japan. The world's most northerly primate, the Japanese macaque, has learned to bathe in hot springs to survive through the winter snows. In parts of the tropics, monkeys can be seen within a few miles of busy city centers, but despite this adaptability, the majority of the world's primates still retain a close dependence on trees. Apart from humans, baboons are the only primates that have fully made the transition to life out in the open, and even they instinctively climb to safety if danger threatens. Some primates, especially the smaller species, are active only at night, or nocturnal, while others are diurnal, active during the day. Most primate species--particularly monkeys--are highly sociable animals, sometimes living in troops of more than 100 members. Smaller primates, especially nocturnal ones, tend to be solitary and secretive. Primates range in size from quite small to quite large. The world's largest species, the lowland gorilla at 200 kg (400 lb) is more than 6,000 times the weight of the smallest primate, the pygmy mouse lemur from Madagascar. Measuring just 20 cm (8 in) from nose to tail, and weighing about 30 g (1 oz), this tiny animal was first identified about two centuries ago, but was later assumed to be extinct until its rediscovery in 1993. II TYPES OF PRIMATES There are about 235 species of primates. Scientists use more than one way to classify primates, and one system divides the order into two overall groups, or suborders: the prosimians and the anthropoids. The prosimians, or 'primitive primates,' make up the smaller of these two groups, with about 60 species, and include lemurs, pottos, galagos, lorises, and, in some classification systems, tarsiers. Lemurs are only found on the islands of Madagascar and Comoros, where they have flourished in isolation for millions of years. Pottos and galagos are found in Africa, while lorises and tarsiers are found in southeast Asia. Typical prosimians are small- to medium-sized mammals with long whiskers, pointed muzzles, and well-developed senses of smell and hearing. Most prosimians are nocturnal, although in Madagascar some of the larger lemurs are active by day. In the past, tree shrews were often classified as primates, but their place in mammal classification has been the subject of much debate. Today, based on reproductive patterns and on new fossil evidence, most zoologists classify them in an order of their own, the Scandentia. The remainder of the world's primates make up the anthropoid, or "humanlike" suborder, which contains about 175 species. This group consists of humans, apes, and monkeys. Most anthropoids, apart from baboons, have flat faces and a relatively poor sense of smell. With a few exceptions, anthropoids are almost always active during the day, and they find their food mainly by sight. Apes are found only in Africa and Asia. They have no tails, and their arms are longer than their legs. Monkeys from Central and South America, known as New World monkeys, have broad noses and nostrils that open sideways. They are called platyrrhines, which means broad-nosed. Monkeys from Africa and Asia, known as Old World monkeys, have narrow noses and nostrils that face downward--a characteristic also seen in apes and humans. Old World Monkeys are called catarrhines, which means downward-nosed. III PRIMATE CHARACTERISTICS During the course of evolution, primates have kept several physical features that most other mammals have lost. One of these is the clavicle, or collarbone. In primates, the clavicle forms an important part of the shoulder joint. It helps to stabilize the shoulder, permitting a primate to support its weight by hanging from its arms alone--something that few other mammals can do. Some primates, particularly gibbons and the siamang, use this ability to move through the trees from one branch to another by swinging from arm to arm. This type of locomotion is called brachiation. Over the course of evolution, many mammals have gradually lost limb bones as they have adapted to different ways of life: horses, for example, have lost all but a single toe on each foot. Nearly all primates, by contrast, have retained a full set of five fingers and toes, and in most cases these digits have become increasingly flexible as time has gone by. In the aye-aye, a prosimian from Madagascar, the third finger on each hand is long and thin with a special claw at the end. Aye-ayes use these bony fingers to extract insect grubs from bark. Evolution has had a marked effect on the thumbs and big toes of primates. In most mammals, these digits bend in the same plane as the other fingers and toes. But in many primates, the thumbs or big toes are opposable, meaning that they are set apart in a way that permits them to meet the other digits at the tips to form a circle. This enables primates to grip branches, and equally importantly, pick up and handle small objects. Instead of having claws, most primates have flat nails that cover soft, sensitive fingertips--another adaptation that helps primates to manipulate objects with great dexterity. Tails are absent in humans and apes, but in most monkeys and prosimians, the tail plays a special role in maintaining balance during movement through the treetops. Many New World monkeys have prehensile tails, which can be wrapped around branches, gripping them like an extra hand or foot. Primate skulls show several distinctive features. One of these is the position of the eyes, which in most species is on the front of the skull looking forward, rather than on the side of the skull looking to the side as in many other mammals. The two forward-facing eyes have overlapping fields of view, which give primates stereoscopic vision. Stereoscopic vision permits accurate perception of distance, which is helpful for handling food or swinging from branch to branch high above the ground. Another distinctive feature of primate skulls, in anthropoids particularly, is the large domed cranium that protects the brain. The inside surface of this dome clearly shows the outline of an unusually large brain--one of the most remarkable characteristics of this group. The shapes of anthropoid brains are different from other mammals; the portion of the brain devoted to vision is especially large, while the portion devoted to smell is comparatively small. IV FEEDING HABITS The primate order includes a handful of species that live entirely on meat (carnivores) and also a few that are strict vegetarians (herbivores), but it is composed chiefly of animals that have varied diets (omnivores). The carnivorous primates are the four species of tarsiers, which live in Southeast Asia. Using their long back legs, these pocket-sized nocturnal hunters leap on their prey, pinning it down with their hands and then killing it with their needle-sharp teeth. Tarsiers primarily eat insects but will also eat lizards, bats, and snakes. Other prosimians, such as galagos and mouse lemurs, also hunt for insects, but they supplement their diet with different kinds of food, including lizards, bird eggs, fruit, and plant sap. This opportunistic approach to feeding is seen in the majority of monkeys and also in chimpanzees. Several species of monkeys, and chimpanzees, but not the other apes, have been known to attack and eat other monkeys. Baboons, the most adept hunters on the ground, often eat meat and sometimes manage to kill small antelope. Most apes and monkeys eat a range of plant-based foods, but a few specialize in eating leaves. South American howler monkeys and African colobus monkeys eat the leaves of many different trees, but the proboscis monkey on the island of Borneo is more selective, surviving largely on the leaves of mangroves. These leaf-eating monkeys have modified digestive systems, similar to cows, which enable them to break down food that few other monkeys can digest. Other apes and monkeys eat mostly fruit, while some marmosets and lemurs depend on tree gum and sap. V REPRODUCTION Compared to many other mammals, primates have relatively few young, and their offspring take a long time to develop. The gestational period, the time between conception and birth, is remarkably long compared to other mammals of similar size. A tarsier, for example, gives birth to a single young after a gestational period of nearly six months. By contrast, a similarly sized rodent will often give birth to six or more young after a gestational period lasting just three weeks. Most primates usually give birth to a single baby, although some species, such as dwarf lemurs, usually have twins or triplets. Once the young are born, the period of parental feeding and protection can be even more drawn out. In small prosimians the young are often weaned after about five weeks, but in apes they are often fed on their mother's milk for three or four years, and they may continue to rely on her protection for six or more years. This long childhood--which reaches its extreme in humans--is a crucial feature of a primate's life because it enables complex patterns of behavior to be passed on by learning. Some primates have fixed breeding seasons, but many can breed at any time of the year. In many species, females signal that they are in estrus--receptive and ready to mate--by releasing special scents. In other species, females develop conspicuous swelling around their genitals to signal their readiness for mating. Such swelling is especially noticeable in chimpanzees. While most copulation occurs when the females are receptive, in some species, such as humans and pygmy chimpanzees, copulation frequently occurs even if the female is not in estrus. Primates display a wide range of mating behaviors. Solitary primates, such as aye-ayes and orangutans, have relatively simple reproductive behavior. Within the territory that each male controls, several females live, each with their own territory. The male mates with any females within his territory that are receptive. Other species, such as gibbons, form small family groups consisting of a monogamous pair and their young. Gorillas form harems, in which one adult male lives with several adult females and their young. Among social primates, breeding can be complicated by the presence of many adults. Males may cooperate in defending their troop's territory, but they often fight each other for the chance to mate. In some species, only the dominant male mates with the females in the group. Chimpanzee females mate promiscuously with several adult males, although they usually pair up with one of the high-ranking males during the final few days of estrus, spending all of their time together and mating together exclusively. VI PRIMATE INTELLIGENCE Primates have the most highly developed brains in the animal kingdom, rivaled only by those of dolphins, whales, and possibly elephants. Anthropoid primates in particular are intelligent and inquisitive animals that are quick to learn new patterns of behavior. This resourcefulness enables them to exploit a wide range of foods and may help them to escape attacks by predators. Many zoologists believe that primates' large brains initially evolved in response to their tree-dwelling habits and their way of feeding. Anthropoid primates, which have the largest brains, live in a visual world, relying on sight to move about and to locate and manipulate food. Unlike smell or hearing, vision generates a large amount of complex sensory information that has to be processed and stored. In primate brains, these operations are carried out by a portion of the brain called the cerebral cortex, which evolved into such a large structure that the rest of the brain is hidden beneath it. Some unrelated mammals, such as squirrels, also live in trees, but they have less-developed eyesight and much smaller brains. Increased brainpower has had far-reaching effects on the way primates live. It has helped them to move about and find food as well as enabled them to develop special skills. One of the most remarkable of these is toolmaking, seen in chimpanzees and, to a far greater extent, in humans. Toolmaking, as opposed to simple tool use, involves a preconceived image of what the finished tool should look like--something that is only possible with an advanced brain. The intelligence of primates is also evident in their social behavior. For species that live in groups, daily life involves countless interactions with relatives, allies, and rivals. Mutual cleaning and grooming of the fur, which removes parasites, helps to reinforce relationships, while threats--sometimes followed by combat--maintain the hierarchy of dominance that permeates typical primate troops. VII PRIMATE COMMUNICATION Primates use a variety of methods to communicate. In solitary prosimians, when animals are not within sight of each other, communication is often accomplished by using scents. Such animals use urine, feces, or special scent glands to mark territory or to communicate a readiness to mate. In social anthropoids, visual and vocal signals are much more important. Most monkeys and apes communicate with a complex array of facial expressions, some of which are similar to the facial expressions used by humans. Primates also communicate with a repertoire of sounds. These range from the soft clicks and grunts of the colobus to the songs of the gibbon and the roaring of the howler monkey, which can sometimes be heard more than 3 km (2 mi) away. Far-carrying calls are used in courtship, both to keep group members from getting separated and to mark and maintain feeding territories. Some primate calls convey more precise messages, often denoting specific kinds of danger. In the wild, researchers have observed that chimpanzees use as many as 34 different calls, and evidence suggests that they can pass on information-such as the location of foodusing this form of communication. VIII THE ORIGINS OF PRIMATES Relatively little is known about the origins of primates compared to many other groups of mammals, because primates have left relatively few fossil remains. The chief reason for the scarcity of fossils is that forests, the primary home for most early primates, do not create good conditions for fossilization. Instead of being buried by sediment, the bodies of early primates were more likely to have been eaten by scavengers and their bones dispersed. The earliest fossils of primates that have been discovered date from the end of the Cretaceous Period, about 65 million years ago. These early fossils include specimens of a species called Notharctus, which resembles today's lemurs and had a long pointed snout. The ancestors of another prosimian group, the tarsiers, are known from fossils that date back to the early Eocene Epoch, about 50 million years ago. In 1996 researchers in China recovered fossil bones of a primitive primate no bigger than a human thumb. The animal, named Eosimias, lived 45 million years ago. Many scientists believe that Eosimias is an example of a transitional animal in the evolution of prosimians to anthropoids. The origin of anthropoids has proven to be difficult to pin down. A single anthropoid fossil has been found that may come from the Eocene Epoch, but conclusive fossil evidence of anthropoids does not appear until the Oligocene Epoch, which began 38 million years ago. These early anthropoids belonged to a lineage that led to the catarrhine primates--the Old World monkeys, apes, and humans. The platyrrhine primates, which include all New World monkeys, are presumed to have diverged from the Old World monkeys during the Eocene Epoch. They evolved in isolation on what was then the island continent of South America. Genetic analysis shows that New World monkeys clearly share a common ancestry with the catarrhines, which means that they must have reached the island continent from the Old World. Exactly how they did this is unclear. One possibility is that they floated across from Africa on logs or rafts of vegetation, journeying across an Atlantic Ocean that was much narrower than it is today. Of all primate groups, the apes and the direct ancestors of humans have been the most intensively studied. One key question concerns when the two groups diverged. Based on the comparisons of genes and the structure of body parts, scientists think that the line leading to the orangutan diverged from the one leading to humans about 12 million years ago. The ancestral line leading to chimpanzees did not diverge until more recently, probably between 5 and 7 million years ago. This evidence strongly suggests that chimpanzees are our closest living relatives. IX HUMANS AND OTHER PRIMATES The word primate means 'the first." When it was originally coined over two centuries ago, it conveyed the widely held idea that primates were superior to all other mammals. This notion has since been discarded, but nonhuman primates still generate great interest because of their humanlike characteristics. In scientific research, much of this interest has focused on primate behavior and its correspondence with human behavior. Attempts have been made to train chimpanzees and orangutans to mimic human speech, but differences in anatomy make it very difficult for apes to produce recognizable words. A more revealing series of experiments has involved training chimpanzees, and later gorillas, to understand words and to respond using American Sign Language. In the late 1960s, a chimp named Washoe learned more than 130 signs. In the 1970s and 1980s, a gorilla named Koko learned to use more than 500 signs and to recognize an additional 500 signs. One of the outcomes of these long-running experiments was that the chimps or gorillas occasionally produced new combinations of signs, indicating that the animals were not simply repeating tricks that they had learned. More recently, chimps have been trained to communicate with humans by using colored shapes or computer keyboards. They too have shown an ability to associate abstract symbols with objects and ideas--the underlying basis of language. Apes and monkeys also play an important role in the field of medical research. Because their body systems work very much like our own, new vaccines and new forms of surgery are sometimes tried on apes and monkeys before they are approved for use on humans. Species that are most often used in this way include chimpanzees, baboons, and rhesus monkeys. This kind of animal experimentation has undoubtedly contributed to human welfare, but the medical use of primates is an increasingly controversial area, particularly when it involves animals captured in the wild. X ENDANGERED PRIMATES According to figures published by the World Conservation Union (IUCN), more than 110 species of primates--nearly half of the world's total--are currently under threat of extinction. This makes the primates among the most vulnerable animals on earth. The species most under threat are those that have been affected by deforestation. This has been particularly severe in Madagascar, the only home of the lemurs, and it is also taking place at a rapid rate in Southeast Asia, threatening gibbons and orangutans. The almost total destruction of Brazil's Atlantic rainforest has proved catastrophic for several species, including the lion tamarins, which are found only in this habitat. Primates are also threatened by collection for the pet trade and by hunting. Illegal hunting is the chief threat facing the mountain gorilla, a rare African subspecies that lives in the politically volatile border region straddling Uganda, Rwanda, and the Democratic Republic of the Congo. In the face of these threats, urgent action is currently underway to protect many of these endangered species. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) currently forbids the export of many primates, although not all countries have chosen to follow this law. More direct methods of species preservation include habitat protection and captive breeding programs. In some cases--for example, the lion tamarin--these programs have met with considerable success. However, without the preservation of extensive and suitable natural habitats, many primate species are destined for extinction. Scientific classification: Primates make up the order Primates in the class Mammalia. Prosimians make up the suborder Prosimii. Anthropoids make up the suborder Anthropoidea. Contributed By: David Burnie Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
