In this section, we discuss the major primate subgroups. , we present a brief description of each major grouping, taking a somewhat closer look at the apes.
Lemurs and Lorises
The suborder Strepsirhini includes the lemurs and lorises, the most nonderived, or primitive, living primates. Remember that by “primitive” we mean that lemurs and lorises are more similar anatomically to their earlier mammalian ancestors than are the other primates (tarsiers, monkeys, apes, and humans). For example, they retain certain ancestral characteristics, such as a greater reliance on olfaction. Their greater olfactory capabilities (compared with other primates) are reflected in the presence of a relatively long snout and a moist, fleshy pad, or rhinarium, at the end of the nose. Many other characteristics distinguish lemurs and lorises from the other primates, including eyes placed more to the side of the face, differences in reproductive physiology, and shorter gestation and maturation periods. Lemurs and lorises also have a unique derived trait called a “dental comb” , formed by forwardprojecting lower incisors and canines. These modified teeth are used in grooming and feeding.
Lemurs
Lemurs are found only on the island of Madagascar and adjacent islands off the east coast of Africa . As the only nonhuman primates on Madagascar, lemurs diversified into numerous ecological niches without competition from monkeys and apes. Thus, the approximately 60 surviving species of lemurs on Madagascar today represent an evolutionary pattern that vanished elsewhere.
Lemurs range in size from the small mouse lemur, with a body length (head and trunk) of only 5 inches, to the indri, with a body length of 2 to 3 feet (Nowak, 1999). Typically, the larger lemurs are diurnal and eat a wide variety of foods,such as leaves, fruits, buds, bark, and shoots, but the tiny mouse and dwarf lemurs are nocturnal and insectivorous. There’s a great deal of behavioral variation among lemurs. Some are mostly arboreal, but others, such as ringtailed lemurs are more terrestrial. Some arboreal species are quadrupeds, and others (sifakas and indris) are vertical clingers and leapers . Several species (for example, ring-tailed lemurs and sifakas) live in groups of 10 to 25 animals composed of males and females of all ages. However, indris live in “family” units composed of a mated pair and dependent offspring, and several nocturnal species are mostly solitary.
Lorises
Lorises , which somewhat resemble lemurs, were able to survive in mainland areas by becoming nocturnal. In this way, they were (and are) able to avoid competition with more recently evolved primates (the diurnal monkeys). There are at least eight loris species, all of which are found in tropical forest and woodland habitats of India, Sri Lanka, Southeast Asia, and Africa. Also included in the same general category are six to nine galago species, also called bush babies (Bearder, 1987; Nowak, 1999), which are widely distributed throughout most of the forested and woodland savanna areas of sub-Saharan Africa).
Locomotion in some lorises is a slow, cautious, climbing form of quadrupedalism. All galagos, however, are highly agile vertical clingers and leapers. Some lorises and galagos are almost entirely insectivorous, while others also eat fruits, leaves, and other plant products. Lorises and galagos frequently forage alone, but feeding ranges can overlap, and two or more females may feed and even nest together. Females also leave young infants behind in nests while they search for food, a behavior not seen in most primate species.
Tarsiers
There are five recognized tarsier species (Nowak, 1999), all of which are restricted to islands of Southeast Asia (Malaysia, Borneo, Sumatra, the Philippines), where they inhabit a wide range of habitats, from tropical forest to backyard gardens . Tarsiers are nocturnal insectivores that leap from lower branches and shrubs onto small prey. They appear to form stable pair bonds, and the basic tarsier social unit is a mated pair and their young offspring (MacKinnon and MacKinnon, 1980). Tarsiers are highly specialized (derived) animals that have several unique characteristics. In the past, primatologists thought that tarsiers were more closely related to lemurs and lorises than to other primates because they share several traits with them. However, they actually present a complex blend of characteristics not seen in any other primate. One of the most obvious is their enormous eyes, which dominate much of the face and are immobile within their sockets. To compensate for the inability to move their eyes, tarsiers, like owls, can rotate their heads 180°.
Anthropoids: Monkeys, Apes, and Humans
Although there is much variation among anthropoids, they share certain features that, when taken together, distinguish them as a group from lemurs and lorises.
Here’s a partial list of these traits:
- 1.Larger average body size
- 2. Larger brain in absolute terms and relative to body weight
- 3. Reduced reliance on the sense of smell, as indicated by the absence of a rhinarium and reduction of olfactory-related brain structures
- 4. Increased reliance on vision, with forward-facing eyes placed more to the front of the face
- 5. Greater degree of color vision
- 6. Back of eye socket protected by a bony plate
- 7. Blood supply to the brain different from that of lemurs and lorises
- 8. Fusion of the two sides of the mandible at the midline to form one bone (in lemurs and lorises, they’re two distinct bones joined by cartilage at the middle of the chin)
- 9. More generalized dentition, as seen in the absence of a dental comb
- 10. Differences in female internal reproductive anatomy
- 11. Longer gestation and maturation periods
- 12. Increased parental care
- 13. More mutual grooming
Approximately 85 percent of all primates are monkeys. Primatologists estimate that there are about 195 species, but it’s impossible to give precise numbers because the taxonomic status of some monkeys remains in doubt, and previously unknown species are still being discovered. Monkeys are divided into two groups separated by geographical area (New World and Old World) as well as at least 35 million years of separate evolutionary history.
New World Monkeys
The approximately 70 New World monkey species can be found in a wide range of environments throughout most forested areas in southern Mexico and Central and South America . They exhibit a wide range of variation in size, diet, and ecological adaptations . In size, they vary from the tiny marmosets and tamarins that weigh only about 12 ounces to the 20- ound howler monkeys . New World monkeys are almost exclusively arboreal, and some never come to the ground. Like the Old World monkeys, all except one species (the owl monkey) are diurnal. In addition to being the smallest of all monkeys, marmosets and tamarins have several other distinguishing features. They have claws instead of nails, and unlike other primates, they sually give birth to twins instead of one infant. They live in social groups usually composed of a mated pair, or a female and two adult males, and their offspring. This type of mating pattern is rare among mammals, and marmosets and tamarins are among the few primate species in which males are extensively involved in infant care.
Other New World species range in size from squirrel monkeys (weighing only 1.5 to 2.5 pounds and having a body length of 12 inches) to the larger howlers (as much as 22 pounds in males and around 24 inches long). Diet varies, with most relying on a combination of fruits and leaves supplemented to varying degrees with insects. Most are quadrupedal; but some, such as spider monkeys (Fig. 6-24), are semibrachiators. Howlers, muriquis, and spider monkeys also have prehensile tails that are used not only in locomotion but also for hanging from branches. Socially, most New World monkeys live in groups composed of both sexes and all age categories. Some (such as titis) form monogamous pairs and live with their subadult offspring.
Old World Monkeys
Except for humans, Old World monkeys are the most widely distributed of all living primates. They’re found throughout sub-Saharan Africa and southern Asia, ranging from tropical jungle habitats to semiarid desert and even to seasonally snow-covered areas in northern Japan.
Conveniently, all Old World monkeys are placed in one taxonomic family, Cercopithecidae. In turn, this family is divided into two subfamilies: the cercopithecines and the colobines. Most Old World monkeys are arboreal, but some (such as baboons) spend a lot of time on the ground and return to the trees for the night. They have areas of hardened skin on the buttocks called ischial callosities that serve as sitting pads, making it possible to sit and sleep on tree branches for hours at a time.
The cercopithecines are more generalized than the colobines. They’re more omnivorous, and as a group, they eat almost anything, including fruits, seeds, leaves, grasses, tubers, roots, nuts, insects, birds’ eggs, amphibians, small reptiles, and small mammals (the last seen in baboons).
The majority of cercopithecine species, such as the mostly arboreal guenons and the more terrestrial savanna and hamadryas baboons , are found in Africa. The many macaque species (including the well-known rhesus monkeys), however, are widely distributed across southern Asia and India.
Colobine species have a narrower range of food preferences and mainly eat mature leaves, which is why they’re also called “leaf-eating monkeys.” The colobines are found mainly in Asia, but both red colobus and black-and-white colobus are exclusively African.
Locomotion in Old World monkeys includes arboreal quadrupedalism in guenons, macaques, and langurs; terrestrial quadrupedalism in baboons and macaques; and semibrachiation and acrobatic leaping in colobus monkeys. Marked differences in body size or shape between the sexes, referred to as sexual dimorphism, are typical of some terrestrial species and are especially pronounced in baboons. For example, male baboons, which can weigh 80 pounds, can be up to twice the size of females. Females of several species (especially baboons and some macaques) have pronounced cyclical changes of the external genitalia. These changes, which include swelling and redness, are associated with estrus, a hormonally initiated period of sexual receptivity in female nonhuman mammals that is correlated with ovulation. They serve as visual cues to males that females are sexually receptive.
Old World monkeys live in a few different kinds of social groups. Colobines tend to live in small groups, with only one or two adult males. Savanna baboons and most macaque species are found in large social units comprising several adults of both sexes and offspring of all ages. Monogamous pairing isn’t common in Old World monkeys, but it’s seen in a few langurs and possibly one or two guenon species.
Hominoids: Apes and Humans
Apes and humans are classified together in the same superfamily, the hominoids. Apes are found in Asia and Africa. The small-bodied gibbons and siamangs live in Southeast Asia, and the two orangutan subspecies live on the islands of Borneo and Sumatra . In Africa, until the mid- to late twentieth century, gorillas, chimpanzees, and bonobos occupied the forested areas of western, central, and eastern Africa, but their habitat is now extremely fragmented, and all are threatened or highly endangered. Apes and humans differ from monkeys in numerous ways:
- 1.Generally larger body size (except for gibbons and siamangs)
- 2. No tail
- 3. Lower back shorter and more stable
- 4. Arms longer than legs (only in apes)
- 5. Anatomical differences in the shoulder joint that facilitate suspensory feeding and locomotion
- 6. Generally more complex behavior
- 7. More complex brain and enhanced cognitive abilities
- 8. Increased period of infant development and dependency
Gibbons and Siamangs
The eight gibbon species and closely related siamangs are the smallest of the apes, with a long, slender body weighing 13 pounds in gibbons and around 25 pounds in siamangs. Their most distinctive anatomical features are adaptations to feeding while hanging from tree branches, or brachiation. In fact, gibbons and siamangs are more dedicated to brachiation than any other primate, a fact reflected in their extremely long arms, long, permanently curved fingers, short thumbs, and powerful shoulder muscles. (Their arms are so long that when they’re on the ground, they have to walk bipedally with their arms raised to the side.)
Gibbons and siamangs mostly eat fruits, although they also consume a variety of leaves, flowers, and insects. The basic social unit of gibbons and siamangs is an adult male and female with dependent offspring. Although they’ve been described as monogamous, in reality, members of a pair do sometimes mate with other individuals. As in marmosets and tamarins, male gibbons and siamangs are very much involved in rearing their young. Both males and females are highly territorial and protect their territories with elaborate whoops and siren-like “songs,” lending them the name “the singing apes of Asia.”
Orangutans: Orangutans (Pongo pygmaeus; ) are represented by two subspecies found today only in heavily forested areas on the Indonesian islands of Borneo and Sumatra. The name orangutan (which has no final “g” and should never be pronounced “o-rang-utang”) means “wise man of the forest” in the language of the local people. But despite this somewhat affectionate-sounding label, orangutans are severely threatened with extinction in the wild due to poaching by humans and continuing habitat loss on both islands.
Orangutans are slow, cautious climbers whose locomotion can best be described as four handed—referring to their use of all four limbs for grasping and support. Although they’re almost completely arboreal, orangutans sometimes travel quadrupedally on the ground. Orangutans exhibit pronounced sexual dimorphism; males are very large and may weigh more than 200 pounds, while females weigh less than 100 pounds. In the wild, orangutans lead largely solitary lives, although adult females are usually accompanied by one or two dependent offspring. They’re primarily frugivorous but may also eat bark, leaves, insects, and (rarely) meat.
Gorillas The four generally recognized gorilla (Gorilla gorilla) subspecies are the largest of all the living primates. Today, they are restricted to forested areas of western and eastern equatorial Africa . Gorillas exhibit marked sexual dimorphism. Males may weigh as much as 400 pounds, while females weigh around 150 to 200 pounds. Adult gorillas, especially males, are primarily terrestrial, and like chimpanzees, they practice a type of quadrupedalism called knuckle walking.
Western lowland gorillas are found in several countries of westcentral Africa. In 1998, Doran and McNeilage estimated their population size at perhaps 110,000, but Walsh and colleagues (2003) suggested that their numbers were far lower. Staggeringly, in August 2008, the Wildlife Conservation Society reported the discovery of an estimated 125,000 western lowland gorillas in the northern region of the Democratic Republic of the Congo (DRC—formerly Zaire)! This is extremely encouraging news, but it doesn’t mean that gorillas are out of danger. To put this figure into perspective, consider that a large football stadium can hold around 70,000 spectators. So, next time you see a stadium packed with fans, think about the fact that you’re looking at a crowd that numbers around half of all the western lowland gorillas on earth.
Small populations of Cross River gorillas, another West African subspecies, are found in areas along the border between Nigeria and Cameroon (Sarmiento and Oates, 2000). Primatologists believe that there may be only 250 to 300 of these animals; thus, Cross River gorillas are among the most endangered of all primates. Currently, the International Union for the Conservation of Nature and Natural Resources (IUCN) is developing plans to protect this vulnerable and little-known subspecies (Oates et al., 2007).
Eastern lowland gorillas, which haven’t really been studied, live near the eastern border of the DRC. At present, their numbers are unknown but suspected to be around 12,000. Considering that warfare is common in the region, researchers fear that many of these gorillas have been killed, but it’s impossible to know how many.
Mountain gorillas , the most extensively studied of the four subspecies, are restricted to the mountainous areas of central Africa in Rwanda, the DRC, and Uganda. There have probably never been many mountain gorillas, and today they number only about 700 animals.
Mountain gorillas live in groups consisting of one, or sometimes two, large silverback males, a variable number of adult females, and their subadult offspring. (The term silverback refers to the saddle of white hair across the backs of fully adult males that appears around the age of 12 or 13.) A silverback male may tolerate the presence of one or more young adult “blackback” males (probably his sons) in his group. Typically, but not always, both females and males leave their natal group as young adults . Females join other groups; and males, who appear to be less likely to emigrate, may live alone for a while or may join up with other males before eventually forming their own group. Systematic studies of free-ranging western lowland gorillas weren’t begun until the mid-1980s, so even though they’re the only gorillas you’ll see in zoos, we don’t know as much about them as we do about mountain gorillas. The social structure of western lowland gorillas is similar to that of mountain gorillas, but groups are smaller and somewhat less cohesive.
All gorillas are almost exclusively vegetarian. Mountain and western low land gorillas concentrate primarily on leaves, pith, and stalks, but western lowland gorillas eat more fruit. Western lowland gorillas, unlike mountain gorillas (which avoid water), also frequently wade hrough swamps while foraging on aquatic plants (Doran and McNeilage, 1998).
Perhaps because of their large body size and enormous strength, gorillas have long been considered ferocious; in reality, they’re usually shy and gentle. But this doesn’t mean they’re never aggressive. In fact, among males, competition for females can be extremely violent. As might be expected, males will attack and defend their group from any perceived danger, whether it’s another male gorilla or a human hunter. Still, the reputation of gorillas as murderous beasts is the result of uninformed myth making and little else.
Chimpanzees
Chimpanzees are probably the best known of all nonhuman primates, even though many people think they’re monkeys . Today, chimpanzees are found in equatorial Africa, in a broad belt from the Atlantic Ocean in the west to Lake Tanganyika in the east . But within this large geographical area, their range is very patchy, and it’s becoming even more so with continuous habitat destruction.
In many ways, chimpanzees are anatomically similar to gorillas. However, the ecological adaptations and behaviors of chimpanzees and gorillas differ, with chimpanzees spending more time in the trees. Chimpanzees are also frequently excitable, active, and noisy, while gorillas tend to be placid and quiet.
Chimpanzees are smaller and less sexually dimorphic than orangutans and gorillas. A male chimpanzee may weigh 150 pounds, but females can weigh at least 100 pounds. In addition to quadrupedal knuckle walking, chimpanzees may brachiate. When on the ground, they frequently walk bipedally for short distances when carrying food or other objects.
Chimpanzees eat a huge variety of foods, including fruits, leaves, insects, birds’ eggs, and nuts. Moreover, both males and females occasionally take part in group hunting efforts to kill small mammals such as young bushpigs and antelope. Their prey also includes monkeys, especially red colobus. When hunts are successful, the members of the hunting party share the meat. Chimpanzees live in large communities ranging in size from 10 to as many as 100 individuals. A group of closely bonded males forms the core of chimpanzee communities in many locations, especially in East Africa (Wrangham and Smuts, 1980; Goodall, 1986; Wrangham et al., 1992). But for some West African groups, females appear to be more central to the community (Boesch, 1996; Boesch and Boesch-Acherman, 2000; Vigilant et al., 2001). Relationships among closely bonded males aren’t always peaceful or stable, yet these males cooperatively defend their territory and are highly intolerant of unfamiliar chimpanzees, especially males.
Even though chimpanzees live in communities, it’s rare for all members to be together at the same time. Rather, they tend to come and go, so that the individuals they encounter vary from day to day. Adult females usually forage either alone or with their offspring, a grouping that might include a number of animals, since females with infants sometimes accompany their own mothers and siblings. These associations have especially been reported for the chimpanzees at Gombe National Park, Tanzania, where about 40 percent of females remain in the group they were born in (Williams, 1999). But in most other areas, females leave their natal group to join another community. This behavioral pattern reduces the risk of mating with close male relatives, because males apparently never leave the group in which they were born.
Chimpanzee social behavior is complex, and individuals form lifelong attachments with friends and relatives. If they remain in their natal group, the bond between mothers and infants can remain strong until one of them dies. This may be a long time, because many wild chimpanzees live well into their 40s and occasionally even longer.
Bonobos
Bonobos (Pan paniscus) are found only in an area south of the Zaire River in the DRC. Not officially recognized by European scientists until the 1920s, they remain among the least studied of the great apes. Although ongoing field studies have produced much information (Susman, 1984; Kano, 1992), research has been hampered by civil war. There are currently no accurate counts of bonobos, but their numbers are believed to be between 29,000 and 50,000 (IUCN, 2011).
The 50,000 estimate is probably optimistic at this point, and bonobos are highly threatened by human hunting, warfare, and habitat loss.
Because bonobos bear a strong resemblance to chimpanzees, but are slightly smaller, they’ve been inappropriately called “pygmy chimpanzees.” Actually, the differences in body size aren’t great, although bonobos are less stocky. They also have longer legs relative to arms, a relatively smaller head, and a dark face from birth.
Bonobos are more arboreal than chimpanzees, and they’re less excitable and aggressive. While aggression isn’t unknown, it appears that physical violence both within and between groups is uncommon. Like chimpanzees, bonobos live in geographically based fluid communities, and they eat many of the same foods, including occasional meat derived from small mammals (Badrian and Malenky, 1984). But bonobo communities aren’t centered around a group of males. Instead, male-female bonding is more important than in chimpanzees and most other nonhuman primates (Badrian and Badrian, 1984).
This may be related to bonobo sexuality, which differs from that of other nonhuman primates in that copulation is frequent and occurs throughout a female’s estrous cycle, so sex isn’t linked solely to reproduction. In fact, bonobos are famous for their sexual behavior, copulating requently and using sex to defuse potentially tense situations. Sexual activity between members of the same sex is also common (Kano, 1992; de Waal and Lanting, 1997).
Humans
Humans (Homo sapiens) are the only living representatives of the habitually bipedal primates (hominin tribe). Our primate heritage is evident in our overall anatomy, genetic makeup, and many behavioral aspects. Except for reduced canine size, human teeth are typical primate (especially ape) teeth. The human dependence on vision and decreased reliance on olfaction, as well as flexible limbs and grasping hands, are rooted in our primate, arboreal past.
Humans in general are omnivorous, although all societies observe certain culturally based dietary restrictions. Even so, as a species with a rather generalized digestive system, we’re physiologically adapted to digest an extremely wide assortment of foods. Perhaps to our detriment, we also share with our relatives a fondness for sweets that originates from the importance of highenergy fruits eaten by many nonhuman primates.
But humans are obviously unique among primates and indeed among all animals. No member of any other species has the ability to write or think about issues such as how it differs from other life-forms. This ability is rooted in the fact that during the last 800,000 years of human evolution, brain size has increased dramatically, and there have also been many other neurological changes.
Humans are also completely dependent on culture. Without cultural innovation, it would never have been possible for us to leave the tropics. As it is, humans inhabit every corner of the planet except for Antarctica, and we’ve established outposts there. And, lest we forget, a fortunate few have even walked on the moon! None of the technologies (indeed, none of the other aspects of culture) that humans have developed over the last several thousand years would have been possible without the highly developed cognitive abilities we alone possess. Nevertheless, the neurological basis for intelligence is rooted in our evolutionary past, and it’s something that connects us to other primates. Indeed, research has demonstrated that several nonhuman primate species—most notably chimpanzees, bonobos, and gorillas—display a level of problem solving and insight that most people would have considered impossible just 30 years ago.
Humans are uniquely predisposed to use spoken language, and for the last 5,000 years or so, we’ve also used written language. This ability exists because during the course of human evolution, certain neurological and anatomical structures have been modified in ways not observed in any other species. But while nonhuman primates aren’t anatomically capable of producing speech, research has shown that to varying degrees, the great apes are able to communicate by using symbols, which is a foundation for language that humans and the great apes (to a limited degree) have in common.
Aside from cognitive abilities, the one other trait that sets humans apart from other primates is our unique (among mammals) form of striding, habitual bipedal locomotion. This particular trait appeared early in the evolution of our lineage, and over time, we’ve become more efficient at it because of changes in the musculoskeletal anatomy of our pelvis, leg, and foot. Still, while it’s certainly true that human beings are unique intellectually, and in some ways anatomically, we’re still primates. As a matter of fact, humans are basically exaggerated African apes