Primates


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Primates

The mammalian order to which humans belong. Primates are generally arboreal mammals with a geographic distribution largely restricted to the Tropics. Unlike most other mammalian orders, the primates cannot be defined by a diagnostic suite of specializations, but are characterized by a combination of primitive features and progressive trends. These include:

1. Increased dominance of vision over olfaction, with eyes more frontally directed, development of stereoscopic vision, and reduction in the length of the snout.

2. Eye sockets of the skull completely encircled by bone.

3. Loss of an incisor and premolar from each half of the upper and lower jaws with respect to primitive placental mammals.

4. Increased size and complexity of the brain, especially those centers involving vision, memory, and learning.

5. Development of grasping hands and feet, with a tendency to use the hands rather than the snout as the primary exploratory and manipulative organ.

6. Progressive elaboration of the placenta in conjunction with longer gestation period, small litter size (only one or two infants), and precocial young.

7. Increased period of infant dependency and more intensive parenting.

8. A tendency to live in complex, long-lasting social groups.

It has been recognized for a long time that many of these features are adaptations for living in trees. However, it has been proposed more recently that primates may have developed their specializations as a consequence of being visually directed predators, living among the smaller branches of the forest canopy or undergrowth, that captured insects with their hands.

Classification of the primates is as follows:

  • Order Primates
  • Suborder Strepsirhini
  • Infraorder Lorisiformes
  • Superfamily Lorisoidea
  • Family: Lorisidae (lorises)
  • Galagidae (bushbabies)
  • Infraorder Lemuriformes
  • Superfamily Lemuroidea
  • Family: Cheirogaleidae (dwarf lemurs)
  • Lepilemuridae (sportive lemur)
  • Lemuridae (true lemurs)
  • Indriidae (sifakas, indri, woolly lemur)
  • Daubentoniidae (aye-aye)
  • Suborder Haplorhini
  • Hyporder Tarsiiformes
  • Superfamily Tarsioidea
  • Family Tarsiidae (tarsiers)
  • Hyporder Anthropoidea
  • Infraorder Platyrrhini
  • Superfamily Ceboidea
  • Family: Callitrichidae (marmosets, tamarins)
  • Cebidae (capuchins, squirrel monkeys, douroucoulis, titis)
  • Atelidae (sakis, uakaris, howler monkeys, spider monkeys, woolly monkeys)
  • Infraorder Catarrhini
  • Superfamily Cercopithecoidea
  • Family Cercopithecidae (Old World monkeys)
  • Superfamily Hominoidea
  • Family: Hylobatidae (gibbons, siamang)
  • Hominidae (orangutan, gorilla, chimpanzees, humans)

There are two major groups of primates: the strepsirhines or “lower” primates, and the haplorhines or “higher” primates. Strepsirhines have elongated and forwardly projecting lower front teeth that form a toothcomb, used for grooming the fur and for obtaining resins and gums from trees as a source of food. The digits of the hands and feet bear flattened nails, rather than claws, except for the second toe, which retains a sharp toilet claw for grooming. They also have a moist, naked rhinarium and cleft upper lip (similar to the wet noses of dogs). Most strepsirhines are nocturnal, with large eyes and a special reflective layer (the tapetum lucidum) behind the retina that intensifies images in low light. Compared with haplorhines, the brain size is relatively small and the snout tends to be longer.

The strepsirhines are subdivided into two major groups: the lorisoids, which are found throughout tropical Africa and Asia, and the lemuroids, which are restricted to Madagascar.

The lorisoids include the galagids or bushbabies (Galago, Otolemur, Euoticus, and Galagoides) and the lorisids or lorises (Loris, Nycticebus, Perodicticus, Pseudopotto, and Arctocebus). They are small nocturnal primates, in which the largest species, the greater bushbaby, weighs only about 1 kg (2 lb). Their diet consists mainly of a combination of insects, fruits, and gums. Lorisoids are semisolitary, living in small, dispersed social groups.

The greatest diversity of strepsirhines is found on Madagascar, where more than 30 species are represented, belonging to five different families.

Tarsiers, tiny primates (weighing only about 120 g) from the islands of Southeast Asia, all belong to a single genus, Tarsius. They are nocturnal with the largest eyes of any primate, and other adaptations for a specialized lifestyle as vertical clingers and leapers. In the past, tarsiers have been grouped together with the strepsirhines as prosimians, because they retain many primitive features lost in higher primates. However, tarsiers share a number of distinctive specializations with anthropoids that suggest that they are more closely related to each other than either is to the strepsirhines. For this reason, tarsiers and anthropoids are classified together as haplorhines.

The anthropoids include the platyrrhines or New World monkeys and the catarrhines or Old World monkeys, apes, and humans. Anthropoids are distinguished from strepsirhines and tarsiers in having a larger brain, relatively small eyes (all anthropoids are diurnal, active by day, except for the nocturnal douroucouli from South America), eye sockets almost completely enclosed by a bony septum, the two halves of the lower jaw fused in the midline rather than separated by a cartilage, small and immobile ears, the hands and feet bearing nails with no toilet claws (except for the callitrichids that have secondarily evolved claws on all fingers and toes), a single-chambered uterus rather than two-horned, and a more advanced placenta.

The platyrrhines from South and Central America are a diverse group of primates comprising more than 50 species and 16 genera. Primatologists have had a difficult time establishing a classification of platyrrhines that reflects their evolutionary interrelationships, and no consensus has been reached. There is agreement, however, that three distinct clusters can be defined: the callitrichids, the pitheciines, and the atelines. The last two groups appear to be closely related and are commonly included together in the family Atelidae. The relationships of the remaining platyrrhines are uncertain, and they are often placed together for convenience in the Cebidae.

All platyrrhines are arboreal, and they are widely distributed throughout tropical forests extending from Mexico to northern Argentina. They are small to medium-sized primates ranging from 100 g to 15 kg (0.2 to 33 lb). Platyrrhines exhibit a variety of quadrupedal locomotor types ranging from squirrellike scrambling, to leaping and forelimb suspension. Atelines and capuchin monkeys are unique among primates in having a specialized prehensile tail that can grasp around branches for extra support.

The catarrhines include all anthropoid primates from Africa, Asia, and Europe. There are two main groups: the cercopithecids or Old World monkeys, and the hominoids or apes and humans. Catarrhines are distinguished from platyrrhines by a reduction in the number of premolars from three to two in each half of the upper and lower jaw, and the development of a tubelike (rather than ringlike) tympanic bone to supports the eardrum.

Old World monkeys are widely distributed throughout sub-Saharan Africa and tropical Asia, and also occur in the extreme southwestern tip of the Arabian Peninsula, northwest Africa, Gibraltar (their only European record), and East Asia. They are a highly successful group comprising more than 80 species. They are distinguished from other anthropoids in having bilophodont molar teeth that bear a pair of transverse crests. They also have naked, roughened sitting pads on their rumps, called ischial callosities—a feature that they share with hylobatids. In addition, most Old World monkeys are highly sexually dimorphic, with males considerably larger than females.

Hominoidea is the superfamily to which apes and humans belong. Hominoids are distinguished from cercopithecoids in having primitive nonbilophodont molars, larger brains, longer arms than legs (except in humans), a broader chest, a shorter and less flexible lower back, and no tail. Many of these specializations relate to a more upright posture in apes, associated with a greater emphasis on vertical climbing and forelimb suspension.

Hominoids can be classified into two families: the Hylobatidae, which includes the gibbons and siamang, and the Hominidae, which includes the great apes and humans. The gibbons and siamang (Hylobates) are the smallest of the hominoids (4–11 kg or 9–24 lb), and for this reason they are sometimes referred to as the lesser apes. The nine or so species are common throughout the tropical forests of Asia. They are remarkable in having the longest arms of any primates, which are 30–50% longer than their legs. This is related to their highly specialized mode of locomotion, called brachiation, in which they swing below branches using only their forelimbs. Gibbons are fruit eaters, while the larger siamang incorporates a higher proportion of leaves in its diet. Hylobatids live in monogamous family groups in which males and females are similar in size.

The great apes include the orangutan (Pongo) from Asia and the gorilla (Gorilla) and chimpanzees (Pan) from Africa. These were formerly included together in their own family, the Pongidae, to distinguish them from humans, who were placed in the Hominidae. However, recent anatomical, molecular, and behavioral evidence has confirmed that humans are closely related to the great apes, especially to the African apes, and for this reason most scientists now classify them together in a single family, the Hominidae. The orangutan is restricted to the tropical rainforests of Borneo and northern Sumatra. They are large, arboreal primates that climb cautiously through the trees using all four limbs for support. Orangutans subsist mainly on fruits.

The gorilla, the largest of the hominoids, has a disjunct distribution in tropical Africa. Because of their great size, gorillas are almost entirely terrestrial, although females and young individuals frequently climb trees. Nests are often built on the ground. Gorillas move quadrupedally, and like chimpanzees, the hands are specialized for knuckle walking in which the weight of the animal is borne on the upper surface of the middle joints of the fingers. Mountain gorillas eat a variety of leaves, stems, and roots, while lowland gorillas eat a greater proportion of fruits. Groups consist of a dominant male, called a silverback, as well as several adult females, subadults, and infants.

There are two species of chimpanzees, the common chimpanzee (Pan troglodytes) and the bonobo or pygmy chimpanzee (Pan panicus). The common chimpanzee is widely distributed in the forests and woodlands stretching across equatorial Africa, while the pygmy chimpanzee is restricted to the tropical rainforests of the Congo. Both species nest and feed in trees, but they mostly travel on the ground. Common chimpanzees have eclectic diets, including meat, which they obtain by hunting small to medium-sized mammals. Tool-using behaviors are common, and more than a dozen simple tool types have been identified. Chimpanzees are gregarious and sociable, and they live in large multimale communities that divide into smaller subgroups for foraging. See Mammalia

Primates

 

an order of mammals. Most zoologists divide the order into two suborders: Prosimii and Anthropoidea.

Body length ranges from 13–15 cm (mouse lemurs and pygmy marmosets) to 175 cm or more (gorilla). Weight varies from 60–100 g (mouse lemurs) to 180 kg. In the wild, the animals may attain an even greater weight (gorilla). The majority of primates have a tail. It may be longer or shorter than the body, or it may be approximately the same length. In some species the tail is very short (indri, slow loris, mandrill, and pig-tail macaque) or absent (slender loris, Barbary ape, and Simiidae). The pelage is well developed and often dense; prosimians have an undercoat. In many primates, the fur and naked parts of the skin are brightly colored. The eyes are brown or yellow. All apes have five well-developed digits on each limb. There is a clavicle, and the ulna and radius are not concresced. The thumbs are movable and in many species opposable to the other digits. The distal phalanges of the digits have nails (an exception are tree shrews). In primates having clawlike nails (Callithricidae) or claws on some fingers, the thumb always has a flat nail. The thumb is reduced in only a few primates. The limbs are usually longer than the spinal column; only for the tree shrew, Lorisidae, and Callithricidae are the limbs somewhat shorter.

Owing to their arboreal life, primates have well-developed senses of sight and hearing. Their sense of smell is secondary, and there are only three or four nasal conchae. The eyes are more or less directed forward; the orbits are separated from the temporal depression by a bony ring or a septum (in tarsiers and apes). Lower primates have four or five bundles of tactile hairs, or vibrissae, on their chin; higher primates have only two or three. Cutaneous creases are present in prosimians only on the pads of the palms and soles; in apes and man they are present over the entire surface.

The brain in prosimians has a small number of furrows and convolutions. The olfactory centers are well developed, and the cerebellum is not completely covered by the hemispheres. In apes and man, the large hemispheres of the brain have numerous furrows and convolutions (only Callithricidae have few) and cover the cerebellum. The olfactory centers are not pronounced, and the visual regions are well developed. Vision is for the most part binocular; apes and man can distinguish colors. In most primates the facial part of the skull is elongated. The dental system is heterodont (incisors, canines, premolars, and molars) and diphyodont (milk teeth and permanent ones). The large molars have three to five cusps. Man and Catarrhini have 32 teeth, and Cebidae and most prosimians 36. The dental formula in most primates is as follows:

There are about 200 extant species of primates, distributed mainly in tropical and subtropical forests of Africa, Madagascar, Asia, many islands of the Malay Archipelago, Central America, and South America.

Primates live mainly in trees, but there are semiterrestrial and terrestrial forms. The animals live in troops or, less frequently, in couples. Some lead a solitary existence. Their diet is mixed, but with a predominance of plant substances. A few representatives feed only on insects. The stomach is simple or, in a few herbivorous species (langurs), compound. Primates reproduce year-round, with the females commonly bearing a single offspring. Lower primates produce two or three young per birth. Most females have one pair of mammae, but many prosimians have two or three pairs (tree shrew, tarsiers, some Lemuridae and Lorisidae).

The ancestors of primates were probably primitive insectivorous mammals similar to extant tree shrews. The most ancient representative of that original group is Zalambdalestes, the incomplete cranium, hands, and feet of which were found in Upper Cretaceous deposits of Mongolia. During the Cretaceous period, the oldest primates left Asia and resettled in America, Europe, and Africa. From them were descended various lemuroids and tarsioids, whose numerous remains are known from Paleocene and Eocene deposits of Europe and North America. Platyrrhine and catarrhine monkeys apparently developed from primitive tarsiers. The ancestors of American platyrrhine monkeys, which arose independently of Old World catarrhine monkeys, left North America and entered Central and South America, where they developed over a long period of time and adapted to arboreal life. Fossil anthropoid apes only appeared in the Oligocene. Of the greatest interest are the remains of various species of Dryopithecus from Miocene deposits in Europe, which are considered to be the common ancestors of anthropoid apes and man.

The practical significance of primates is small, although many of them have been hunted for a long time. The flesh is used as food by local populations, and the hides are used for making clothing. Primates are captured for zoos and as laboratory animals for scientific institutions. Owing to the sharp decrease in their numbers, many primates are in need of protection. A number of them have been entered in the Red Data Book. These include Lemuridae, Indridae, Daubentonidae, many platyrrhines (lion-headed marmoset and ouakari), and catarrhines (red and black colobus monkey, snub-nosed langur, Pygathryx, some gibbons, orangutans, and chimpanzees).

REFERENCES

Weber, M. Primaty. Moscow-Leningrad, 1936. (Translated from German.)
Nesturkh, M. F. Primatologiia i antropogenez. (Obez’iany, poluobez’iany i proiskhozhdenie cheloveka). Moscow, 1960.
Zhizn’ zhivotnykh, vol. 6. Moscow, 1971.
Sanderson, I. T., and G. Steinbacher. Knaurs Affenbuch: Alles über halbaffen, Affen und andere Herrentiere. Munich-Zurich, 1957.
Napier, J. B., and P. Napier. A Handbook of Living Primates. London-New York, 1967.

T. D. GLADKOVA

Primates

[prī′mād·ēz]
(vertebrate zoology)
The order of mammals to which man belongs; characterized in terms of evolutionary trends by retention of a generalized limb structure and dentition, increasing digital mobility, replacement of claws by flat nails, development of stereoscopic vision, and progressive development of the cerebral cortex.
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