The Primates: Overview

Primates are surprisingly variable in size. They range all the way from the pygmy mouse lemur that weighs less than 2 ounces (55 g.) and can fit in the palm of your hand to the gorilla that grows to as much as 440 pounds (200 kg.) in the wild and even heavier in captivity. Still larger primates existed in the past but are now extinct.

Primates are generally lively, clever, and very successful at adapting to different environmental opportunities. Physically, however, they are relatively unspecialized compared to animals such as birds, horses, and cats. Primates are not particularly fast runners, they do not have the sharpest hearing, they cannot fly, nor are they efficient hunters.

With the exception of spider monkeys, all of the primates have retained the ancient mammalian trait of pentadactylism

, which is having five fingers on each hand and five toes on each foot. An increasing refinement of the hands and feet for grasping objects has been a hallmark of primate evolution. Their fingers and toes are mobile and have very sensitive tactile pads at the tips, unlike most other mammals. At least some digits on all primates also have flat nails in place of rigid claws. This makes the manipulation of bits of food and other objects much easier.

The grasping, or prehensile

, ability of primate hands varies significantly also with the degree of opposability of the thumb. Partially rotating the thumb and pressing it forcefully towards the fingers provides a secure grip for hanging on branches and for manipulating small objects. All primates, with the exception of humans, have prehensile feet in addition to hands. However, few have the high degree of thumb opposability and strength typical of humans.

The small New World monkeys called marmosets and tamarins usually hang onto a tree branch by pressing against it with their palms and using all of their bent fingers together as counter pressure in a hook-like power grip. The more dexterous monkeys and apes can also use a secure power grip formed by the partial flexion of the fingers and the palm with counter force applied by the thumb. Humans and some apes also can employ precision grips formed by pinching with the tips of their flexed forefingers and the thumb. This allows their hands to be used effectively for manipulating small objects, such as the fork in the hand of the chimpanzee shown below.


Hook-like power grip (cotton-top tamarin)		Precision grip (chimpanzees)
	Secure power grip (orangutan)

All primates have a marked tendency towards erectness in their upper bodies. This can be seen in their sitting and standing postures as well as occasional bipedalism

. They also have shoulder joints that are unusually flexible for mammals. This is due to secure ball joints and strong clavicles

, or collarbones, which have allowed them to use their arms very effectively in climbing trees. Clavicles are another ancient mammalian trait that primates have retained.

In primate evolution, there was a progressive reduction in nose size and in the olfactory

areas of the brain. Lemurs are an exception. They are very much like the early primates 50 million years ago. Note the marked difference between lemur, monkey, and human noses in the photos below. Most monkeys and apes have evolved relatively small noses like us, while lemurs have retained long snouts similar to foxes and raccoons. It is not surprising that lemurs have a comparatively good sense of smell.

Most mammals have about the same number of genes that code for odor receptors in their nasal tracks. However, the majority of these genes in humans, and presumably other large primates, are no longer functional. Recent research has shown that of the approximately 1,000 human olfactory receptor genes, only 347 remain functional. The rest have mutations that deactivate them. Does this really mean that people have a poor sense of smell? The answer is both yes and no. We are poor at sensing short-chain chemical compounds but good at discriminating between complex long-chain ones such as those of many foods and flowers. Dogs are just the opposite in this. However, the smelling ability of dogs is far superior to people in one way. Fewer molecules of a substance are usually needed for dogs to detect it.

As the primate nose progressively shrank in size over millions of years, there was a corresponding increase in visual capabilities. As much as 50% of the cerebral cortex in some species of monkeys is involved in visual processing. Many primates have color vision comparable to our own. All have binocular vision with fields of view that significantly overlap, resulting in true three dimensional (3-D) depth perception or stereoscopic vision. At the same time, the field of view for peripheral vision was reduced.

drawing of a human head shown from above with the field of view of each eye superimposed to show the overlapping vision range
(view from above)

Human eyes have an overlapping field of view
of about 120�. It is only in this field that we
have stereoscopic vision. Beyond this 3-D
area out to 160-180�, we see things only in
two dimensions (2-D)--i.e., flat without depth.
Monkeys and apes have about the same
depth perception ability as humans, but the
eyes of lemurs are farther apart resulting in a
smaller overlapping field of view.

Additional information about primate color vision

Depth perception is an invaluable tool for animals that need to move quickly. It allows them to judge the distances to important objects in their environment. For monkeys jumping from branch to branch high up in the canopies of forests, this is a matter of life and death. Likewise, accurate depth perception is a critical capability for hunters, such as cats and hawks, who need to accurately judge distances in the pursuit of their prey. In contrast, it is often preferable to have greater peripheral vision if you are the prey. Gazelles, rabbits, and chickens have nearly a 360� field of view, but relatively little of that is stereoscopic vision.

Compared to most other animals, primate brains are large relative to their body size. Those areas of the brain that are involved with controlling manual dexterity, eye-hand coordination, and stereoscopic vision have particularly expanded. These traits were probably selected by nature mostly due to their usefulness in traveling in trees, manipulating objects, and getting food.

The more capable uterus and placenta of primates support long gestation periods, or pregnancies, for animals of their size. For example, the mouse lemur has a gestation period of 60 days, but it is only 20 days for a comparable size rodent mouse. Macaques, which are one of the most common groups of monkey species in the Old World, have 13.5-15.5 week gestations. Among comparable size domesticated dogs, it is only about 9 weeks. The result of long gestations is that offspring are born more mature and, therefore, have a greater chance of survival. Primate mothers also are generally very protective and nurturing with their young. Subsequently, primates do not need to have many offspring to maintain their population numbers. Multiple births at any one time are rare for them. The time gap between generations is surprisingly long for the larger primates. Among humans today, parents are on average 29 years older than their children. By comparison, chimpanzee mothers are on average 25 years older than their offspring. Among female gorillas, it is 19.3 years.

There frequently is a correlation between body size and longevity in the animal kingdom. Species with larger bodies generally live longer. However, primates have unusually long natural life spans for their size. For instance, mouse lemurs live about 8 years. Rodent mice rarely reach 2 years. Rats, which are slightly larger than mouse lemurs, can live 3 years. Big monkeys, such as baboons, live up to 30 years. Male baboons can weigh as much as 90 pounds. Comparable size domesticated dogs have a life expectancy of only 10-12 years. With good medical care, chimpanzees can live nearly as long as people (50-75+ years). This is not surprising given that they are anatomically very similar to us. Research done by Herman Pontzer of Hunter College suggests that the unusual longevity of primates is due to the fact that they expend only 50% as many calories a day compared to other animals their size. Slower metabolism rates are associated with slower rates of growth, reproduction, and aging in mammals.

Most primates have adapted to an arboreal, or tree living, way of life. Even the terrestrial

ones usually sleep in the trees. The most notable exceptions are humans and gorillas. Both of these species live on the ground with occasional tree climbing for fun, especially by children. Almost all primates are diurnal, which is to say, they are active during the day and sleep at night.

Generally, primates are highly social animals. Most of their waking hours are spent socializing with each other. This is made easier by complex vocalizations and visual displays. In addition, they regularly groom each other, thereby keeping clean and satisfying psychological needs at the same time. Grooming is a very pleasurable activity for primates, including humans. It is important to note that the few nocturnal

primate species are the least social.

Primates have been very successful animals due largely to the fact that they are intelligent and opportunistic in getting food. Most are unusually adaptable in diet. This has tremendous evolutionarily selective value. Many species are omnivorous

, though vegetable foods usually make up the bulk of calories consumed by most primate species because they are easier to obtain. By comparison, animals such as koalas and giant pandas are generally less successful because they are extremely limited in the kinds of foods that they can or will eat. Koalas subsist on the leaves of a few species of eucalyptus, and giant pandas primarily eat the shoots of a small number of bamboo species. If these food sources are not available, koalas and giant pandas die. Not surprisingly, their highly limited range of foods restricts where they can live. This is not the case with many primate species.

Often people who have not had the responsibility of taking care of a monkey or ape in captivity think that they would be great pets. After all, they are cute when they are youngsters and remarkably human-like. However, they generally make very poor pets. As one monkey owner said "People find out when it's too late. After six months, they're like a two year-old that can fly." Because they are intelligent, very active, sometimes unpredictable, and easily bored, they frequently get into trouble when left alone. In addition to being destructive, most are as messy as human babies. Unlike humans, however, they remain this way all of their unusually long lives. Owning large apes in particular is a lifelong commitment that takes considerable patience, energy, and money. Male chimpanzees can be surprisingly violent in their interactions with each other and humans. In recent years, several people have been extensively maimed and even killed by their pet chimpanzees in the U.S.


Spider monkey Count the number of fingers... (This is the only primate that has hands with 4 fingers and feet with 5 toes)



	Ring-tailed lemurs


	Lowland gorilla


	Upright posture (lowland gorilla)


Chimpanzees grooming		Humans grooming