Monkeys are divided into geographic groups, New World (The Americas) and Old World (Europe, Asia, Africa). Humans belong to the infraorder Catarrhini, which includes Old World monkeys (Super-family Cercopithecoidea) and apes (Superfamily Hominoidea).
Catarrhines have a 2:1:2:3 dental formula which means that per quadrant of the mouth (there are four: upper right jaw, upper left jaw, lower right jaw, lower left jaw) adults have two incisors, one canine, two pre-molars, and three molars. Baboons (Papio) and macaques (Macaca) dominate the Old World monkey group with their diversity and their impressive geographic coverage (baboons extend from north to South Africa and macaques cover much of south, east, and Southeast Asia and even reach to Western Europe).
New World monkeys (infraorder Platyrrhini) have one more premolar per quadrant than the catarrhines (2:1:3:3), which is the primitive condition. They also have grasping prehensile tails, some of which are furless on the underside with the equivalent of fingerprints for better grip. The New World monkeys also tend to be more acrobatic in the trees than Old World monkeys. Types of New World monkeys include capuchins, howlers, spider monkeys, squirrel monkeys, marmosets, and tamarins.
No living primate eats strictly leaves, fruits, or insects, but that does not prohibit them from evolving special dietary adaptations. Leaf-eating monkeys (colobines) have a long gut and a sacculated stomach for digesting cellulose, similar to the way cows digest grass. Their teeth are also adapted to shear leaves with sharp crests on the molars in a condition called bilophodonty. Colobines, are more sedentary than other monkeys and have smaller brain size relative to body size, which is expected given their low-quality diet.
Gibbons and siamangs (family Hylobatidae) from Indonesia and South Asia are the most suspensory of all primates and are known as the "lesser apes." They are also the least sexually dimorphic of the homi-noids. They move through the trees almost exclusively by two-armed brachiation and by four-legged climbing in between brachiating bouts. When on the ground, they walk bipedally, but it is rare. They have very long arms and hands and very short legs and feet (less weight on the bottom for better swinging through the trees). Gibbons and siamangs form monogamous mated pairs, so they have little sexual dimorphism as a result. Like Old World monkeys, they also have ischial callosities, which is an adaptation for sitting that includes roughened pads on the skin near the buttocks, and thickened bone underneath, but the other apes do not. The majority of the gibbon diet is fruit (figs) and they can weigh between 4 and 13 kg (between 9 and 29 lbs) depending on the species, and males and females average the same size.
Thanks to pioneering fieldwork by primatologistsJane Goodall (chimpanzees), Dian Fossey (gorillas), and Birute Galdikas (orangutans), incredible humanlike details about great ape behavior and biology are constantly forcing us to reevaluate what it means to be human. These women immersed themselves into ape life, getting up close and personal with the creatures and set the precedent for researchers and conservation efforts to follow.
The "great apes" are much larger than gibbons and siamangs and include the orangutans (the only Asian great ape), gorillas, chimpanzees, and bonobos. Unlike the lesser apes, they are not monogamous so their anatomy differs accordingly.
Orangutans (Pongo pygmaeus) are also known as the red ape and live quiet and mostly solitary lives in the diminishing forests on the islands of Borneo and Sumatra. Their reproductive strategy is known as a dispersed polygyny and they have the sexually dimorphic bodies to match. Males can weigh up to 200 lbs but females weigh less than half of that. Females and their offspring defend territories from other females, while males attempt to secure mating privileges by controlling several different female territories. Large dominant males have wide cheeks and also use their throat sacs to make loud roaring songs. Orangutans are mostly arboreal and eat fruits from the trees but sometimes males get too large to be completely arboreal and must spend more time on the ground.
The African great apes, existing mostly in regions of west and central Africa, are the most terrestrial of the hominoids (excluding humans which are the most terrestrial hominoid of all). African great apes include Gorilla gorilla (subspecies include the "western lowland," the "eastern lowland," and the highly endangered "mountain" gorilla) from Cameroon to the Virunga Mountains along the border of the Congo, Rwanda, and Uganda; Pan paniscus (the bonobo, formerly known as the "pygmy" chimpanzee) from the central Congo; and Pan troglodytes (the "common" chimpanzee) from west, central, and east Africa, mainly Tanzania, Ivory Coast, and Uganda. When walking quadrupedally, they all walk on the knuckles of the phalanges of their hands both on the ground and on horizontal tree branches.
With the largest body size, gorillas are the most terrestrial. Gorillas often feed on vegetation on the ground, as opposed to exploiting food sources like fruits and nuts in the trees like the smaller bodied chimpanzees and bonobos. As the largest living primates, gorilla males can weigh up to 400 pounds (200 kg) and females average half the size of males. They are gentle, social animals.
Chimpanzees and bonobos, however, are much livelier. Chimpanzees are much less sexually dimorphic with males weighing about 150 pounds (68 kg) and females about 120 pounds (56 kg). Bonobos are called "pygmy" chimpanzees because of their slightly smaller and slender bodies compared to common chimpanzees.
In large part due to body size, gorillas nest on the ground, as opposed to chimpanzees and bonobos who build sleeping nests in trees. When moving about arboreally, gorillas, bonobos, and chimpanzees use both quadrupedal and suspensory (hanging) locomotor behaviors. Although slow deliberation characterizes the majority of gorilla climbing bouts, even the largest ones are capable of very fast high-energy arm-swinging, like the displays or fleeing behaviors of chimpanzees and bonobos. The frequency of arboreal behaviors, especially for gorillas, depends upon the size of the trees that accommodate them. Habitats for mountain and lowland gorillas are distinct enough to warrant anatomical differences between the subspecies, because the montane habitats of mountain gorillas offer fewer climbable trees compared to the rainforests that lowland gorillas inhabit.
The reproductive strategies of gorillas and chimpanzees differ. Gorillas mate within a one or two male polygyny where one or two males have sole access to a group of females. Such a system is beneficial to both the males and the females so the term "harem" which comes with a stigma associated with female oppression is no longer applied to gorilla reproductive behaviors. Males benefit from knowing paternity and females benefit from the protection of the male from other males that may harm their offspring. Chimpanzees on the other hand have a much more fluid mating strategy. Their social units are called fission-fusion because groups join and then disperse from one another and then rejoin regularly. Under such conditions matings can take place between multiple different male-female pairings and social hierarchies within groups can determine who mates with whom. Like most primates and all the great apes, chimpanzee offspring remain with their mothers, not their fathers, throughout development.
All primates evolved from a common ancestor with fish and although it is tempting to assume humans were the only to become fishermen, chimpanzees are known to fish for termites (see Chapter 5). Still, humans have a fair share of differences from their closest chimpanzee relatives, apart from what prey items they choose to fish for, and these traits will be discussed further in the later chapters.
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