because they propel with their legs
Strong legs for powerful leaping
In the trees: Arboreal quadrupeds
Moving quadrupedally means moving on four legs or feet, and it’s how many monkey species get around. It involves using both the hands and feet to grasp relatively horizontal tree limbs, which they walk on with great skill and a seemingly daredevil attitude. But evolution has shaped their instincts and abilities, and although accidents happen, they’re infrequent enough not to have extinguished this kind of locomotion. The arboreal quadrupeds have the following anatomical characteristics:
Strong arms and legs.
Relatively low body weight (most of them).
A divergent big toe, such that their feet look much like our hands, with the big toe sticking off to the side; this allows the feet to be used like hands, to grasp tree limbs.
A prominent tail (in most species) used as a balance; one kind of primate, the spider monkey, has a prehensile tail that can be carefully controlled to wrap around objects and hold them, just like a hand.
Soldiers beware: Terrestrial quadrupeds
The terrestrial quadrupeds get around on all fours, but on the ground rather than habitually in the trees. These animals include the baboons, which live in large, complex social groups (troops) and can be fearsome to humans. One troop in South Africa particularly disliked one turn-of-the-century British officer and regularly pelted him — and only him — whenever they saw him marching his own troops! The terrestrial quadrupeds have the following attributes:
Moderately strong arms and legs
Lack of massive upper- or lower-body build for either brachiating or clinging-and-leaping
Calloused feet, hands, and buttocks from spending so much time on the ground
Technically, the chimps and gorillas mix things up a little: They spend a lot of time on the ground, so they’re officially terrestrial quadrupeds, but they have the bodies of arboreal quadrupeds because they’ve only recently (in evolutionary time) come down from the trees in a substantial way. They have one important distinguishing characteristic: heavily built, locking knuckles that allow the heavy upper body to be supported with the knuckles of the hands by pressing down on the ground.
Other primates do some locomotor mixing as well. Bonobos, a kind of West African chimpanzee, are terrestrial quadrupeds, but they also spend some time brachiating and even walking on two legs. This walking is different than human walking, though, because the bonobos only do it on occasion, which is called opportunistic locomotion. Humans walk habitually, meaning their anatomy is adapted for this kind of locomotion.
THE GREAT WOMEN OF GREAT APE STUDIES
A great deal of what anthropology currently knows about the apes has come from long-term field studies carried out by some remarkable women. Jane Goodall began as a student of anthropologist Louis Leakey, who encouraged her to study the chimpanzees to better understand humanity. She did and for 45 years has observed these primates in great detail at a research station at Gombe, Tanzania. Recently Goodall has shifted from studying the chimpanzees to advocating for protection of chimpanzee habitat; like the other apes, the chimpanzee is endangered.
Another great ape, the orangutan of Borneo, has been studied for more than 30 years by Biruté Galdikas of Canada’s Simon Fraser University. Like Goodall, today Galdikas argues forcefully for protection of orangutan habitat, which is being deforested at an alarming rate; some estimate that the orangutan will be extinct by 2012. Dian Fossey (who, like Galdikas and Goodall, was also inspired by Louis Leakey) studied gorillas for nearly three decades, but she was murdered under mysterious circumstances in 1985, and today the gorilla is also becoming extinct, facing the deforestation of its habitat as well as a threat from the Ebola virus. For more on the extinction of primates, see the section “Primates Today (But For How Long?)” later in the chapter.
One of the most important things these women did was to study apes in the wild — not in zoos; you can imagine how different ape behavior would be in these situations. Remember, though, that even the observer’s presence would effect ape behavior, so rather than saying they were observing wild apes, anthropologists say they were studying habituated apes, apes that were accustomed to seeing human observers. Exactly what effects the observers have on ape behavior in non-zoo settings is debatable, but it’s very likely to be more “natural” than zoo behavior.
A group of one: Bipeds
Although many primates occasionally stand up to walk on two feet (and one gorilla in West Africa has even been observed to use a walking stick to cross a swampy patch of ground), they do so on occasion rather than habitually. Of the living primates, only Homo sapiens sapiens walks on two legs; I discuss why that’s a fascinating question in Chapter 6. For the moment, take a look at the main anatomical characteristics of bipedal primates:
Relatively long, strong legs
An S-shaped spinal column that acts as a spring to absorb stresses
A wide pelvis that keeps the thighs somewhat apart, helping balance
A parallel big toe lined up with the rest of the toes (rather than the divergent big toe used by other primates to grasp tree limbs)
Thighs that angle inward toward the knees and down from the pelvis, also assisting balance
Lateral and transverse arches built into the foot so that we aren’t flat-footed but supported by three main points of contact (the heel and under the big and small toes) in a stable, tripod-like structure
Monkey See, Monkey Do: Primate Social Groups and Behavior
Primates are very social creatures, and although other social mammals (like zebras) live in groups, primate social groups are extremely complex, with elaborate rank hierarchies and codes of conduct. Anthropologist Franz de Waal even called one book about chimpanzee behavior Chimpanzee Politics. Primate groups are also usually (but not always) quite large; baboon troops can have up to 300 members.
