Primates

DIETARY HABITS

SOCIAL SYSTEMS AND BEHAVIOR

BIBLIOGRAPHY

No simple set of diagnostic characteristics defines “primates.” Rather, members of the order Primates share, to varying degrees, several suites of features that reflect a generally arboreal lifestyle. These attributes include:

1. Pentadactyl, prehensile hands and/or feet with nails (rather than claws) on the digits, of which the thumb and/or big toe are opposable.
2. Pronounced sensory emphasis on vision, notably through development of binocular and stereoscopic vision, as well as expansion of the visual cortex.
3. A generalized postcranial anatomy tending toward orthrograde (upright) posture.
4. A trend toward enlargement of the cerebral cortex, especially in monkeys and apes.

Primate taxonomy is debated, but 200 to 350 species are recognized. The majority are distributed in tropical and subtropical regions (roughly between the Tropics of Cancer and Capricorn) and throughout sub-Saharan Africa, south and southeast Asia, and South and Central America, although a few forms, such as the Japanese macaque, inhabit decidedly temperate habitats. Primates are chiefly rain forest and tropical forest dwellers, although many other biomes are ecologically relevant for certain groups, including savanna, woodland scrub, evergreen temperate forest, desert, and high-altitude eflinwood meadow.

Six major groups of primates are usefully distinguished:

1. Lemuriformes: the small nocturnal and larger diurnal lemurs of Madagascar.
2. Lorisiformes: the nocturnal, small-bodied lorises and bushbabies of Africa, south Asia, and island southeast Asia.
3. Tarsiiformes: the small, nocturnal tarsiers of Sumatra, Borneo, Sulawesi, and the Philippines.
4. Playtrrhini: the small- to large-bodied monkeys of Central and South America.
5. Cercopithecoidea: the medium- to large-bodied monkeys of Africa and Asia.
6. Hominoidea: the large apes of Africa and Asia, as well as humans.

The Lemuriformes and Lorisiformes form a coherent suborder, the Strepsirhini, unified by various traits, notably retention of the “rhinarium” (the naked, “wet” mammalian nose), increased reliance on olfaction, and greater seasonality of sexual behavior (“estrus”). Many strepsirhines are nocturnal. The second suborder, the Haplorhini, comprises the primates of the remaining three taxa, which exhibit predominantly diurnal habits (with the exception of tarsiers and one monkey species), enhanced vision and visual communication, and comparatively more continuous (less seasonal) sexual activity and reproduction.

DIETARY HABITS

Diversity and omnivory generally typify primate feeding strategies, rather than marked dietary specialization such as the bovid emphasis on grass. The three principal primate foods are fruits, leaves, and insects, but their relative dietary proportions vary considerably across species. Frugivory is most widely distributed: Most primates eat some fruit, and species strongly committed to frugivory are found in virtually every family. Figs (Moraceae, Ficus species) are an often a crucial keystone fruit source for forest primates. Because of metabolic implications of body size, folivory is more common among larger primates (greater than or equal to 13.5 pounds), such as the Old World “leaf monkeys” (Colobinae), neotropical woolly spider monkeys, and mountain gorillas. Conversely, insects are generally consumed in higher proportions by small primates (less than or equal to 3.5 pounds), such as the nocturnal strepsirhines, tarsiers, and neotropical marmosets, tamarins, and squirrel monkeys. Thus, with increasing body size, primate diets typically grade from insectivory, to insectivory-frugivory, frugivory, frugivoryfolivory, and finally to folivory.

Some less common foods are nevertheless important for certain primates or in specific seasons. For example, marmosets, tamarins, and some strepsirhines rely heavily on exudates (gum), particularly seasonally. Underground storage organs (e.g., rhizomes, tubers, corms) are paramount in baboon foraging. These parts are modified stems or leaves in which the plant stores starch, minerals, and water (the potato being a well-known example utilized by humans). Nectar (for some lemurs and neotropical monkeys), grass (for gelada baboons), and seeds (for saki monkeys) are other examples. Carnivory (predation) occurs at significant rates only among baboons (Papio species), neotropical capuchin monkeys (Cebus species), and most notably chimpanzees (Pan troglodytes ), the latter of which prey heavily upon red colobus monkeys. In contrast to the more individualistic and “opportunistic” prey capture of Papio and Cebus, chimpanzee “hunting” is performed simultaneously by multiple adult males. Researchers debate, however, whether male chimpanzees act independently of one another or coordinate their actions cooperatively through an organized division of labor (different hunting “roles”) and the use of foresight and mental attribution.

The omnivorous, “generalist” feeding inclination of primates has not precluded some adaptive specializations in dentition, physiology, and behavior. One noteworthy example is the gastrointestinal tract of Colobine monkeys: In a manner directly analogous to ruminants, it contains a large, multichambered stomach with variable chemical environments facilitating fermentation by symbiotic, cellulolytic bacteria. Thus, these monkeys are able to metabolize and subsist on otherwise difficult-to-digest, mature foliage.

SOCIAL SYSTEMS AND BEHAVIOR

Perhaps the most striking features of primate biology are the diversity and complexity of social systems. Many nocturnal strepsirhines—as well as the orangutan—live in “dispersed” societies in which adult males and females inhabit individually separate home ranges wherein daily activities are independently pursued. These home ranges may partly overlap and, in some species, differ in size between the sexes. Thus, a single polygynous male may occupy and defend from other males (territoriality) a large home range encompassing several females’ individual ranges. The original characterization of this arrangement as a “solitary” or “asocial” existence was understandable but inaccurate. Not only do “solitary” individuals interact at meaningful rates through indirect olfactory and vocal modalities, they may even aggregate regularly, for example, during feeding in orangutans, or at sleeping sites in some nocturnal strepsirhines. Recent mitochondrial DNA evidence from nocturnal lemurs reveals that such sleeping aggregations comprise related females, thereby suggesting a “hidden” matrilineal dimension of as yet unclear significance in these dispersed social systems.

In contrast, the diurnal lemurs and all monkeys are highly gregarious, living in permanent social groups. With few exceptions, primate societies are generally closed (interaction—particularly affiliatively—between individuals of separate groups is rare) and age-graded (immatures of multiple ages/generations are present). The size and composition of groups vary greatly, as do the nature and patterning of constituent social relationships. This variation is expressed not only between species but also often across populations of the same species.

As with mammals generally, female primates often remain in their natal area and social unit for most if not all of their lives, whereas males disperse to other groups around the age of sexual maturity. The resulting female-bonded societies are thus based on “core” affiliative networks of related females—matrilines—to which males associate in variable numbers and time periods.

For example, in some species (leaf monkeys and guenons) only one adult male lives in a group of several (typically three to ten) adult females and their offspring. Group membership is likely to confer polygynous reproductive advantages to males, although in at least some species, females copulate with other males in the population, which either live alone or in all-male “bachelor” units.

A “multimale” variant of this social system generally emerges with relatively larger female group sizes (e.g., ten to fifty). In such cases, three to fifteen adult (nonnatal) males typically reside with the females and their offspring. This social structure predominates in many of the Old World monkeys—notably the baboons and macaques—in addition to neotropical forms such as capuchin and squirrel monkeys. Relationships among females are highly differentiated, most often manifested as relatively rigid, linear dominance hierarchies organized around kinship (matrilineal relatedness). Several field and laboratory studies suggest a substantive cognitive dimension to nonhuman primate understanding of both “status” and “matrilineal kinship.” Females invest heavily in activities that maintain their status, such as grooming and coalitionary support. Males similarly maintain dominance relationships that are, however, much more dynamic and unrelated to kinship.

In 10 to 15 percent of species—most notably the gibbons (Hylobatidae) and species of small neotropical monkeys—the group comprises one adult of each sex accompanied by several immature individuals of various ages. This social system was originally designated “monogamy” or the “nuclear family,” but the term “social monogamy” is now preferred, particularly in light of avian research. Individuals in over 90 percent of bird species live and breed in heterosexual pairs, but genetic data have revealed that in many species extra-pair paternity may be significantly high (e.g., 40 to 50%). Although not all extra-pair sexual behavior results in fertilization, it is clear that one can no longer assume that sexual or reproductive monogamy results invariably from the monogamous social arrangement of birds. Correspondingly, “extra-pair” copulations, pair-bond termination (“divorce”), and “non-nuclear” families do occur among socially monogamous primates, but the generality and significance of these phenomena remains less clear than is the case for birds.

A polyandrous mating system is found in some marmosets and tamarins that live in groups comprising one adult female, two adult males, and youngsters of various ages. Males, as well as young adult offspring of either sex, are primarily responsible for care of the breeding female’s (fraternal) twin offspring. The reproduction of adult daughter “caretakers” may, in fact, be behaviorally or physiologically suppressed by the dominant female.

Great ape societies are characterized by comparatively attenuated female relationships. As noted, female orangutans pursue largely separate lives. Although gorillas live in unimale, multifemale groups, dispersal of females (as well as males) at sexual maturity constrains the formation of matrilineal relationships; female gorillas instead direct more social attention to the resident male. Individual female chimpanzees occupy separate home ranges, interact infrequently, and have weak dominance relationships. Females disperse at sexual maturity, whereas males remain in their natal community. Thus, the kinship element of chimpanzee society is patrilineal in nature, based on relationships among related males that collectively maintain a territory encompassing the individual home ranges of numerous “solitary” females. These males and females may join and leave temporary foraging parties that vary greatly in size and composition, dynamically undergoing “fusion” and “fission” with other foraging parties and individuals of the community over days, if not hours. Cooperation in competition is a hallmark of male social life. Hunting is one such context, as is vying for dominance and the sometimes violent territorial aggression directed against neighboring communities of related males. Although the patrilineal structure and “fusion-fission” system of social foraging also characterizes bonobos (as well as neotropical spider monkeys), bonobos are noteworthy for the much more amicable nature of their social relationships and apparent female dominance over males.

Negotiating complex relationships involving status, kinship, and affiliation over their relatively long lives is believed to have been an important impetus in the evolution of social cognition in monkeys and, especially, apes.

SEE ALSO Alpha-male; Anthropology; Anthropology, Biological; Kinship; Kinship Selection; Social Status

BIBLIOGRAPHY

Bergman, T. J., J. C. Beehner, D. L. Cheney, and R. M. Seyfarth. 2003. Hierarchical Classification by Rank and Kinship in Baboons. Science 302: 1234–1236.

Cheney, D. L., and R. M. Seyfarth. 1990. How Monkeys See the World: Inside the Mind of Another Species. Chicago: University of Chicago Press.

Dixson, Alan F. 1998. Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes, and Human Beings. Oxford: Oxford University Press.

Fleagle, John G. 1999. Primate Adaptation and Evolution. 2nd ed. San Diego, CA: Academic Press.

Kappeler, Peter M., and Michael E. Pereira, eds. 2003. Primate Life Histories and Socioecology. Chicago: University of Chicago Press.

Kappeler, Peter M., and Carel P. van Schaik. 2002. Evolution of Primate Social Systems. International Journal of Primatology 23 (4): 707–740.

Martin, Robert D. 1990. Primate Origins and Evolution: A Phylogenetic Reconstruction. Princeton, NJ: Princeton University Press.

Ryne A. Palombit

PRIMATES

Nonhuman primates provide valuable experimental models for many aspects of aging research. These include diseases of aging (such as diabetes, cardiovascular dysfunction, and osteoporosis), reproductive senescence, neurobiological aging and related cognitive decline, and interventions to alleviate age-related deterioration. The choice of species is dependent on the purpose of the investigation and degree of similarity between the primate model and humans. Many types of nonhuman primates have been employed in gerontological studies, and these include various macaques (cynomologous, pigtails, and rhesus), lemurs, squirrel monkeys, baboons, chimpanzees, and others. By far, the most frequently employed nonhuman primate in gerontological and biomedical studies in general is the rhesus monkey, known by some as the "E. coli (the best characterized and most popular research bacteria) of primates." Mark Lane, at the National Institute on Aging, observed that the number of research articles on the aging of nonhuman primates has increased dramatically, from a total of less than 150 in the period between 1940 and 1978, to over 400 between 1995 and 1999.

Diseases of aging and "normal" aging

Since the major causes of death in humans are cardiovascular, it is not surprising that primate models have been used extensively to study cardiovascular disease and related problems such as obesity and diabetes. Some primates develop atherosclerosis spontaneously, while others must be fed special diets to induce this disease. The latter are usually high in cholesterol and fat. William Cefalu and his colleagues at Bowman Gray School of Medicine have examined various aspects of diet and atherosclerosis in monkeys and compared the manifestations of this disease with those in humans. Dietary lipid is important for both species, and similarities also exist with respect to plasma lipoprotein contributions (i.e., "good" cholesterol or high density lipoprotein, termed HDL, is protective, while "bad" cholesterol, or low density lipoprotein/LDL, contributes to cardiovascular disease), genetic and gender specific susceptibility, and pathology.

Obesity is a risk factor for both cardiovascular problems and diabetes in human and nonhuman primates. Although both obesity and diabetes occur without feeding special diets to nonhuman primates, neither are as prevalent as in humans, at least in the populations of most developed countries. As in humans, diabetes in monkeys is characterized by fasting hypoglycemia (low blood sugar), glucose intolerance, and reduced sensitivity to insulin (the hormone responsible for regulating sugar entry into cells).

Also as in humans, monkeys lose bone mass during aging. In its most extreme form, this constitutes the disease osteoporosis. Females of both species suffer more from this malady than their male counterparts. Part of the problem may stem from the senescence of the reproductive system, and a critical factor in this process is loss of estrogen in females. Rhesus monkeys exhibit regular menstrual cycles of approximately monthly duration until very late in life. Menopause, which is similar to that in humans, occurs with reduced concentrations of blood estrogen and increased concentrations of follicle stimulating hormone (necessary for maturation of ova, but increased in most primates when estrogen levels decline). Loss of estrogen has been linked to both bone loss and cardiovascular disease in humans and monkeys.

Estrogens may also protect, to some extent, against memory (cognitive) loss in many primate species. Menopausal monkeys have more difficulty remembering than pre-menopausal counterparts (Roberts et al.). For this and other reasons, nonhuman primates are used to examine both cognitive changes and overall aging of the brain and nervous system. Various memory tests have been devised to measure different aspects of monkey learning and memory, with the general conclusion being that, like older humans, aged nonhuman primates can still learn new tasks but have difficulty in proportion to the complexity of the challenge. Also like older humans, senescent rhesus monkeys exhibit either neuronal (nerve cell) loss or structural deterioration in certain brain regions. One aspect of the latter, the appearance of so-called senile plaques, parallels a similar phenomenon in humans with Alzheimer's disease.

Finally, it should be mentioned that nonhuman primates also provide important models for evaluating the effects of various interventions on aging and the above age-associated diseases. The most robust such manipulation, reduced dietary caloric intake, has been employed in both rhesus and squirrel monkeys since 1987 (Ingram et al.). It should also be noted that, in addition to information about the diseases of aging, monkeys provide much valuable insight into "healthy" aging as well (Ingram et. al.; Short, Williams, and Bowden). This includes normal age changes in hormones, blood chemistry, and blood cells as well as physiological and behavioral processes.

Advantages and disadvantages of nonhuman primates as aging models

As with essentially all experimental models, monkeys offer a variety of advantages and disadvantages. Clearly, the most pronounced of the former is their biological similarity to humans. It is estimated that the DNA of rhesus monkeys possesses approximately 90 percent similarity with that of man. Chimpanzees are almost 99 percent identical to humans. Consequently, it is not very surprising that so many aspects of aging are similar across the phylogenetic range of primates. Thanks to their relatively shorter life spans, monkey aging can be observed faster than that of humans. Other advantages include an ability to control experimental conditions (e.g., long-term diet, environment, etc.) more rigorously than would be possible in studies employing humans. In addition, interventions (such as evaluation of new drugs) that are not ethically acceptable in man can be applied to nonhuman primates.

Unfortunately, the length of the monkey life span can also be a disadvantage. Although considerably shorter than for humans, the twenty-five to forty-year longevity of a rhesus monkey is still too long to conduct many experiments. Moreover, the cost of such monkey studies, both in terms of labor and initial animal procurement, may also be prohibitive in many cases. Even the availability of many nonhuman primate species may preclude their experimental use, as increasing numbers of investigators recognize the unique value of well-controlled data in species so similar to humans. Furthermore, despite the widespread use of monkeys such as the rhesus, their genetic heterogeneity is far greater than in many shorter-lived, cheaper animal models (e.g., rats and mice). Lastly, the ever-increasing level of restrictions and regulations concerning the use of nonhuman primates in research constitutes a major burden for scientists desiring such models.

George S. Roth

See also Accelerated Aging: Animal Models; Diabetes Mellitus; Genetics; Memory; Nutrition, Caloric Restriction; Physiological Changes, Organ Systems: Bone; Physiological Changes, Organ Systems: Cardiovascular.

BIBLIOGRAPHY

Cefalu, W. T., and Wagner, J. D. "Aging and Atherosclerosis in Human and Nonhuman Primates." Age 20 (1997): 15–28.

Ingram, D. K.; Cutler, R. G.; Weindruch, R.; Renquist, D. M.; Knapka, J. J.; April, M.; Belcher, C. T.; Clark, M. A.; Hatcherson, C. H.; Marriott, B. M.; and Roth, G. S. "Dietary Restriction and Aging: The Initiation of a Primate Study." Journal of Gerontology Biological Sciences 48 (1990): B148–B163.

Lane, M. A. "Nonhuman Primate Models in Biogerontology." Experimental Gerontology. Forthcoming.

Roberts, J. A.; Gilardi, K. V.; Lasley, B.; and Rapp, P. R. "Reproductive Senescence Predicts Cognitive Decline in Aged Female Monkeys." Neuroreport 27 (1997): 2047–2051.

Short, R. A.; Williams, D. D.; and Bowden, D. M. "Cross-sectional Evaluation of Potential Biological Markers of Aging in Pigtailed Macaques: Effects of Age, Sex and Diet." Journal of Gerontology 42 (1987): 644–654.

Primates

Primates are one group (order) of mammals that evolved about 65 million years ago (mya). The early primates were probably small tree-dwelling (arboreal ) animals that hunted for insects at night. Today we recognize at least 167 living species in the order primates. We can classify these species (and the primate fossils we find) into two major groups (suborders): prosimians (at least 38 species) and anthropoids (at least 129 species). Prosimians include lemurs (Lemuriformes), galagos and lorises (Lorisiformes) and tarsiers (Tarsiiformes). Anthropoids include New World monkeys (Platyrrhini; from South America), Old World monkeys (from Africa and Asia), apes and humans (Catarrhini). However, some scientists feel that tarsiers should be classified as anthropoids rather than as prosimians.

Prosimians

The first primates to evolve were prosimians. These early primates split into several subgroups about 55 mya. One lineage (see graph) gave rise to Lorisiformes and Lemuriformes, and the other lineage gave rise to Tarsiiformes and the ancestors of all other primate groups (anthropoids). The first New World monkeys probably evolved about 45 mya, followed by Old world monkeys about 40 mya. The first apes appeared about 25 mya, human ancestors (e.g. Australopithecus ) about 4 mya, and humans (Homo ) about 2 mya.

Many modern prosimians still resemble the early primates in appearance and life style. For example galagos, lorises, tarsiers and some lemurs (e.g. from the dwarf lemur family) are all small arboreal animals that are active at night (nocturnal), hunting for insects or feeding on gum, tree sap, fruit, flowers or leaves. Nocturnal prosimians range in size from 60 g (mouse lemur) to about 2800 g (aye-aye). Only the larger lemurs (about 2-10 kg) have made the transition to being active during the day (diurnal). We assume that their increased size allowed them to make this transition because larger animals are less susceptible to being caught by diurnal predators. Diurnal lemurs mostly feed on fruits, flowers or leaves, and some of them may even come down from the trees to travel on the ground (e.g. ring-tailed lemurs and sifakas). The largest prosimians that ever lived were gorilla-sized lemurs on the island of Madagascar. However, when humans colonized the island about 1000 years ago these large lemurs were hunted to extinction. Lemuriformes occur exclusively on the island of Madagascar. Lorisiformes can be found all over Africa and Asia, and Tarsiiformes are restricted to Asia.

Anthropoids

All anthropoids (monkeys, apes and humans) are diurnal with one exception: the owl monkey from South America. Monkeys occur in the Old and New World, but apes are restricted to Africa and Asia. Humans probably originated in Africa, but are now distributed worldwide. Monkeys and apes range in size from about 150g (pigmy marmosets) to about 180 kg (gorillas). They have variable diets including insects, fruits, leaves, meat and grains. Among the Old-World monkeys and apes we find many terrestrial species that have left the trees for the ground. One advantage of terrestrial life is that these primates can now use their hands more freely to manipulate objects rather than holding on to branches, and that they can gather in larger social groups.

The social life of primates is much more complex than that of other mammals. Among primate species we find a wide range of gregariousness from the small nocturnal primates who spend most of their time looking for food alone, only occasionally meeting others (e.g. mouse lemurs), to primates living in small family groups (e.g. tamarins), and to those that live in large social groups (e.g. baboons) with many individuals of different ages, sex, dominance ranks and relatedness. Individuals within such large groups may compete with each other for food and mating partners, they may collaborate together against others, and may even form friendships.

Primate Evolution

Many of the primate characteristics that we see in primates today (e.g. opposable thumbs, nails instead of claws, forward facing eyes) probably evolved as an adaptation to life in trees. For example primate hands have opposable thumbs that allows them to grasp tree branches, but also allows the handling of tools in apes and humans. The replacement of claws by nails went along with an increased sensitivity of the hands and fingertips and an improved ability to manipulate objects. During primate evolution we also see a trend from having eyes on the side of the face, as most lemurs do, to having both eyes up front as in monkeys, apes and humans. Moving the eyes up front reduces the visual field on the side of the head, but it increases the overlap of the area seen by both eyes simultaneously, allowing for improved depth perception. Now distances (e.g. between branches) can be better estimated, and vision overall is improved. As the eyes are moved up front we also see a shortening of the snout and a reduction of the sense of smell. As a consequence visual signals are now replacing chemical signals as the main form of social communication.

Dispersal

There are still many open questions about primate evolution and how primates became dispersed to the localities where we find them today. For example we do not know how lemurs reached Madagascar. When prosimians evolved in Africa about 65 mya, Madagascar had already separated from the mainland. One possibility that is being discussed is that a few prosimians may have reached Madagascar by rafting on large trees floating in the ocean, and upon arrival, gave rise to all the lemur species. As an alternative our estimated timeline of primate evolution may be incorrect and prosimians may have evolved much earlier. However, this is highly unlikely and not supported by fossil evidence. A similar problem is posed by the exclusive presence of New World monkeys on the South American continent, which was isolated from Africa and North America when anthropoids evolved. Rafting from North America or from Africa is being discussed, but morphological evidence and the direction of past ocean currents seems to favor Africa as the origin of New World monkeys. And finally, the discovery of new anthropoid fossils from Asia in the 1990s suggests that the first anthropoids may have evolved in Asia rather than in Africa.

In addition to the dispersal questions there are still many puzzling questions about the evolution of the diverse primate social systems. For example researchers are studying which factors may determine whether primates live alone or in groups, or which circumstances may shape the nature of the interactions between individual primates.

Anthropologists are interested in studying non-human primates in the hope to gain insight into the evolution of the complex human social behavior. At present, 50 percent of all non-human primate species are threatened by extinction, and unless their habitats are protected from the ever-expanding human population in the near future, many primates will be lost forever.

Kathrin F. Stanger-Hall

Bibliography

Fleagle, John G. Primate Adaptation and Evolution. Academic Press, 1988.

Smuts, Barbara B., Dorothy L. Cheney, Robert M. Seyfarth, Richard W. Wrangham, and Thomas, T. Struhsaker, eds. Primate Societies. Chicago: The University of Chicago Press, 1987.

Internet Resources

Beard, Chris. "Vertebrate Paleontology." Carnegie Museum of Natural History. <http://www.primate.wisc.edu/pin/factsheets/links.html>.

Primate Ancestors in China. <http://www.cruzio.com/~cscp/beard.html>.

Primates

The mammals (warm-blooded animals) called primates include the lower primates (lemurs, lorises, and tarsiers) and the higher primates (monkeys, apes, and humans). Mostly occurring in tropical areas, primates first evolved more than 50 million years ago from shrewlike, insect-eating mammals. Many present-day primates are arboreal (tree-dwellers), with long, agile limbs for climbing and four fingers and an opposable thumb covered by nails for grasping branches. (An opposable thumb is one that is able to be placed against the other fingers.) The eyes of primates are located in the front of their heads, allowing depth perception. Their diet consists of fruit, leaves, stems, buds, and insects, although some primates are carnivores (meat-eaters). Primates have large brains, with the higher primates showing a marked intelligence.

Lower primates: Lemurs, lorises, tarsiers

The lower primates, including the lemurs, lorises, and tarsiers, were the first primates, occurring in North America, Europe, and Asia. Lemurs now occur only on Madagascar, an island off the coast of Africa. They are mostly tree-dwelling, nocturnal (active at night) animals with a moist snout (nose) and a long, furry tail. Lorises are slow-moving, tailless, and nocturnal and live in trees. They are found in southeast Asia and Africa. Tarsiers are small primates with large bulging eyes and a long, thin, naked tail. They are mainly tree-dwelling, nocturnal creatures of the islands of southeast Asia.

Higher primates: Monkeys, apes, humans

Monkeys are mostly tree-dwelling, social mammals that are chiefly diurnal (active during daytime). Old World monkeys (those originating in Africa and Asia) have narrow nostrils that face downward, a fully opposable thumb, and average-sized or absent tails. Included in this group are the baboons and macaques, which are ground-dwellers. New World monkeys (those originating in Central and South America) have rounded nostrils set fairly far apart. Their thumbs are smaller than Old World monkeys, and they typically have long arms and legs and long tails for wrapping around tree limbs. Spider monkeys, marmosets, and capuchins are examples of New World monkeys.

Apes—the group of primates most closely related to humans—include gorillas, orangutans, chimpanzees, and gibbons. The hands, feet, and face of an ape are hairless, while the rest of its body is covered with coarse black, brown, or red hair.

Words to Know

Arboreal: Living in trees.

Diurnal: Occurring or active during the daytime.

Nocturnal: Occurring or active at night.

Opposable thumb: A thumb that occurs opposite the fingers so that it can be placed against them to grasp objects.

Gorillas are ground-dwelling mammals that inhabit the forests of central Africa. They are the largest and most powerful of all primates, reaching a length of 6 feet (1.8 meters) and weighing up to 500 pounds (227 kilograms).

Orangutans are chestnut-colored, long-haired apes live only in the rain forests of the Indonesian islands of Sumatra and Borneo. They are the largest living arboreal mammals, spending the daylight hours moving through the forest canopy in search of fruit, leaves, tree bark, and insects. Many orangutans live to 50 or 60 years of age.

Chimpanzees are partly arboreal, partly ground-dwelling primates that live in the forests of west, central, and east Africa. They are agile creatures that can move rapidly through treetops. On the ground, chimpanzees usually walk on all fours (called knuckle-walking), since their arms are longer than their legs. They make and use a variety of tools, throw sticks and stones as weapons, and hunt and kill young monkeys. Chimpanzees are the most intelligent of the apes and are capable of learning complex sign language. Their closest relatives are humans, who share much of the same genetic material.

Gibbons are the smallest of the apes and are found in southeast Asia, China, and India. They spend most of their lives at the tops of trees in the jungle, using their long arms to swing their agile bodies from tree to tree in search of leaves and fruit. Gibbons are known for their loud calls and songs, which they use to defend their territory. They are devoted parents who show extraordinary affection in caring for offspring.

Humans and their related ancestors belong to a group called hominids, primates with an upright posture and bipedal (two-footed) locomotion. Humans differ from apes in that their brains are larger in relation to their bodies and their faces are flatter and do not have a bony ridge over the eyes. The human skeleton is similar to that of a chimpanzee or gorilla but is modified for walking on two legs. Also, human teeth are smaller than those of apes, with the canine teeth much less pronounced.

Bridging the gap between lower and higher primates

The human line split off from the one leading to chimpanzees and apes 5 million to 7 million years ago. Scatterings of fossils have led scientists to speculate that the earliest primates of any kind appeared about 55 million years ago, mainly in Asia. But just when the two lines of primates (lower and higher) separated had seemed to be lost in the wide gaps in the fossil record.

In March 2000, however, scientists announced they had discovered the fossil bones of an animal they believed to be the earliest known relative in the primate lineage that led to monkeys, apes, and humans. The animal, which the scientists named Eosimias for "dawn monkey," lived some 45 million years ago in a humid rain forest in what is now China. The smallest primate ever found, alive or extinct, it was no bigger than the length of a human thumb and weighed less than 1 ounce (28 grams). It was also probably nocturnal and solitary and fed on insects and fruits. Scientists believe Eosimias to be a transitional figure when lower primates, known as prosimians, went their separate way, eventually developing into present-day lemurs, lorises, and tarsiers. Eosimias was part of the diverging higher primates, known as anthropoids, that eventually evolved into today's monkeys, apes, and humans.

Primate

A primate is a type of mammal with flexible fingers and toes, forward-pointing eyes, and a well-developed brain. Humans are primates, as are lemurs, monkeys, and apes. Except for humans, primates, are found in mostly tropical habitats. Primates are the most highly developed group in the animal kingdom.

Most primates either live in trees or have evolved from tree-dwelling ancestors. All are placental mammals meaning that before birth, their young are nourished by a structure in the mother's body called a placenta. There are about 180 species of primates, all which make up the order called Primata. This name comes from the Latin word primus meaning first, and the case can be made that primates are in may ways first among animals.

CHARACTERISTICS OF PRIMATES

Primates have many physical attributes that account for and contribute to their being considered in this manner. First, they have binocular vision, meaning that they use both eyes together. Because both eyes are located in the front of their faces and point forward rather than from the sides, they have sharp three-dimensional vision and good depth perception. Primates also have specially adapted hands for grasping things. Most have five flexible fingers on each hand and five flexible toes on each foot (with flattened nails on their ends instead of claws), as well as an opposable thumb. With a thumb that can be opposed to or that is able to touch the other fingers, primates can fully encircle or grasp something like a tree branch or a tool with their hand. They also can reach out and bring food to their mouth. Some primates are equally agile with their feet as well. All primates have large, well-developed and highly complex brains. Convolutions are folds in the brain that increase surface area and allow for a greater number of nerve cells. This brain enables chimpanzees to make and use tools and humans to reason and solve problems. Primates also have all four different kinds of teeth, meaning that they can eat all types of food. Primate teeth are less specialized for tearing and ripping and more useful for grinding and chewing a plant-based diet.

All primates also have a clavicle or collarbone that gives them a flexible shoulder whose joint allows free movement of the arm in all directions. Wrist and elbow joints are also highly mobile. Primates usually have only two mammae (or breasts), and give birth to one or sometimes two offspring per pregnancy. Compared to other mammals, their young are dependent for quite some time and take a long time to mature.

Primates are very social and exhibit the most complex behavior of all the mammals. They bond together in pairs of two, in larger family groups, or in even larger bands. In the larger groups, there is usually a leader or dominant male and a hierarchy after him. Communication is very important among primate species, and they use many visual signals as well as sounds to interact with one another. Primates regularly send each other messages, whether they are warnings of danger or calls for mating.

PROSIMIANS AND ANTHROPOIDS

Most biologists divide primates into two groups: prosimians and anthropoids. Prosimians are tree-dwelling, squirrel-like insect eaters including lemurs, lorises, and tarsiers. They were the first primates to evolve and are considered primitive compared with anthropoids. Most primates belong to the second group, the anthropoids. Monkeys, apes, and humans are all anthropoids. There are two types of monkeys who were probably split apart when the continents of South America and Africa drifted away from one another. New World monkeys are found in the tropical forests of Central and South America and include howler monkeys, capuchins, marmosets, and tamarins. Many of these monkeys are small and highly vocal. Old World monkeys are found in the tropics of Asia and Africa and include baboons, colobuses, and macaques. They are usually larger than their New World counterparts but have much shorter tails. Another way to tell them apart is by their noses. New World monkeys have flattened noses with widely set nostrils, while Old World monkeys have more definite noses and closely set nostrils.

HOMINOIDS

A separate and useful "superfamily" or extra grouping of primates, called hominoids, is often used by life scientists when they are discussing both apes and hominids. Hominids are humans and their direct ancestors of humans (meaning that humans are the only living species of hominid). Unlike other primates, hominoids have large heads and no tails. They also can move about on their back legs (although many apes cannot do this for a long distance). The ape family is made up of gibbons, who swing from tree to tree with their extra long arms, as well as orangutans, gorillas, and chimpanzees. These are humans' closest relatives in the animal kingdom, and they are able to see in color, move their lips, and even make a great number of facial expressions, all human-like actions. As for hominids, there is only one species living today, and that is Homo sapiens or human beings. While humans are the most intelligent species of primate (Homo sapiens means "wise human") gorillas are the largest and most powerful primate. Deoxyribonucleic acid (DNA) comparisons have shown that chimpanzees are the closest living relatives of human beings.

[See alsoHomo sapiens; Human Evolution; Mammals ]