The Primates: Apes

Adult female orangutan

Adult male orangutan

Leaning form
of brachiation
(young orangutan)

Orangutan mother with her
child "fishing out" food from
a rock crevice with a stick

The smallest and the most arboreal apes are the 12-13 species of gibbons. Because of their diminutive size, these members of the family Hylobatidae are also referred to as the "lesser apes." Most adult gibbons are only about 3 feet (90 cm.) tall standing upright and 12-20 pounds (5.5-9 kg.) in weight. Males in the biggest gibbon species, known as siamangs

, are up to 30 pounds (13.5 kg.). Siamangs are different enough from other gibbons to be in their own genus. All gibbons are very slender. Long bushy hair on their bodies makes them look stockier than they actually are. Gibbons have little sexual dimorphism in body size, with the exception of siamangs. However, the sexual dimorphism of siamangs is slight compared to that of the great apes.

Gibbons are excellent brachiators

. That is, they move around in trees by swinging under branches with a hand over hand motion. This is also referred to as suspensory climbing. At times, gibbons walk bipedally, or two footed, on top of branches. However, they are more efficient at brachiation, and 90% of their locomotion is by this means. Gibbons have long arms with sturdy rotary cup shoulder joints, comparatively short legs, permanently curved fingers, and light weight bodies. The rotary cup joints allow them to easily raise their arms straight up above their heads. They also allow gibbons to swing long distances from branch to branch with little effort. Each swing can transport a gibbon 20 feet (6 m.) at speeds approaching 35 miles (56 km.) an hour. Their upper torsos are flattened from front to back and their shoulders are far apart, giving them a mechanical advantage in thrusting each arm forward in turn.

Brachiation is a rare ability for primates. None of the prosimians do it. Among the monkeys, only a few New World species have the capability, and they don't use it as frequently as the gibbons. All of the great apes have the anatomical characteristics that allow brachiation, but most of them rarely use this mode of locomotion because they are too heavy to be supported by small branches. Humans also have retained most of the upper body traits that allow us to brachiate, though our legs grew significantly longer and more muscular as we became habitually bipedal. As a consequence of weakened arm muscles and added weight from our legs, we now are inefficient brachiators at best. However, our small children are still light enough to enjoy swinging on playground "monkey bars".

One other anatomical feature sets the gibbons apart from the other apes. They have ischial callosities like many of the Old World monkeys. No great ape has them.

Gibbons are monogamous

in their mating patterns and form nuclear family groups. That is to say, their communities consist of a single mated pair of adults with their juvenile offspring. They live in well defined territories in the tree tops and rarely go down to the forest floor. Adults regularly defend their territory against others of their species with piercingly loud whooping and hooting vocalizations, much like the indris of Madagascar and the howler monkeys of the New World. However, the calls of the latter two primates sound very different. The calls of different gibbon species are easily distinguished from each other as well. When they are vocalizing, the front of the necks of gibbons and siamangs expand with air, much like the flexible bag on a bagpipe.

Siamang neck
swollen with air
for vocalizing

Adult male and female siamang
      mated pair vocalizing in a duet
      recording by Dennis O'Neil
      (length = 57 secs.)

Orangutans are the largest and the rarest of the Asian apes. Males often grow to 175-200 pounds (80-90 kg.) and 4½ feet (1.4 m.) tall. At this size, they are usually too large to cross from one tree to another by the branches and must go down to the ground and walk quadrupedally between them. There is marked sexual dimorphism among the orangutans. Males have huge fleshy pads framing the upper part of their faces. In addition, females weigh only about half as much as the males (73-99 lbs or 33-45 kg.). Being lighter, females and juveniles often stay in the trees and use a leaning form of brachiation--they carefully shift their body weight to bend a supporting branch and then grab the next one before the first one breaks.

Orangutans have largely hairless faces, but the rest of their bodies are covered with long reddish brown hair. The body hair of adult males becomes so long and intertwined that it appears almost to be in unkempt "dreadlocks".

Orangutans are intelligent and generally peaceful vegetarians. Most of the time, they live solitary lives quietly browsing fruits and leaves high up in trees. They are nearly as intelligent as the African apes, but have much more introverted personalities.

Unfortunately, the orangutans are in danger of extinction in the wild because they are hunted for the illegal international pet trade. Their forest territories are also being rapidly cut down for the lumber and cleared by burning for farming, especially in Indonesia where most of them live. Conservation International estimates that the 15,000 remaining Indonesian orangutans are currently disappearing at a rate of 1,000 per year. When population size shrinks, there is a corresponding decrease in genetic diversity. Species that have little genetic diversity are more easily driven to extinction by a changing environment. The loss of habitat has decimated gibbons in Indonesia as well. However, they are not at as much a risk of becoming extinct because their range extends widely over Southeast Asia.

The largest apes are the gorillas of Africa. Adult males are 5½- 6 feet (1.7-1.8 m.) tall and have 9-10 foot (2.7-3.0 m.) arm spans. They have massive heads with heavy, thick muscles on top that are used to close crushing jaws. Their bodies are stout and very muscular.

Like humans, gorillas are terrestrial animals. They are quadrupedal

knuckle walkers. That is, they walk on the soles of their feet but not on the palms of their hands. They bend their fingers and support the upper end of their bodies with their knuckles instead of their open palms.

They are shy, peaceful vegetarians who usually flee from humans and rarely fight even among themselves. They live in family groups consisting of a dominant adult male with several adult female mates and their children. These groups grow over time to 20 or more individuals. Subadult males are tolerated in the family as long as they are not actively competing with the dominant male for mates. Gorillas are highly sexually dimorphic. Adult males average about 350 pounds (160 kg.) and reach 400 pounds (181 kg.) in the wild, while most adult females are only about 155 pounds (70 kg.) and much less muscular.

There are three varieties, or subspecies, of gorillas. The rare mountain gorilla variety lives at high altitude in sparse woodlands in the mountains of Central Africa. As of 2011, there were only 786 of them still alive. The more numerous lowland gorilla varieties live in the dense forests of West Africa. Most zoo gorillas are of the lowland varieties.

The common chimpanzees more closely resemble humans than do the gorillas. Male chimps grow to 5½ feet (1.7 m.) tall and average about 100 pounds (45 kg.) with 6 foot (1.8 m.) arm spans, while females are usually only around 82 lbs. (37 kg.) and are less muscular. Chimpanzees are comfortable walking quadrupedally on the ground in addition to climbing in trees. They are knuckle walkers like gorillas. The natural habitat of chimpanzees includes both tropical forests and bordering savannas

in Africa.

Chimps are intelligent animals with generally pleasant personalities. However, the males are less peaceful than the smaller females. This behavior difference is typical of most primate species, including humans. The chimpanzee diet is usually at least 90% vegetarian. Males are more likely to eat meat than are females, although chimp hunting skills are relatively poor.

Chimpanzees live in fluid societies of 10-50 individuals without monogamous mating bonds. Membership changes through time as females move from one community to another apparently seeking new mates. In contrast, males usually stay together in their natal community throughout their lives and act as a group in defending the food resources of their territory against incursions by other chimpanzee communities. These interactions can be quite noisy, violent, and sometimes fatal.

Bonobos are close relatives of common chimpanzees in the same genus, Pan

. Their similarity is due to the fact that they had a common ancestor only about two million years ago. Bonobos are sometimes referred to as pygmy chimpanzees. Despite this name, they are only slightly smaller than the common chimps. Bonobos usually have blacker hair with tufts at the side of their faces, longer arms and legs, as well as slimmer bodies. Their high pitched vocalizations are also quite different from those of the common chimpanzees. Like many of the Old World monkey species, adult female bonobos normally have prominent "sexual skins." However, unlike monkeys, bonobo females are sexually receptive throughout the year.

Bonobos have fluid social groupings similar to the common chimpanzees, although bonobos are less excitable and aggressive. Male-female alliances also are more important for bonobos. Older females at times even become group leaders. Bonobos are unique among nonhuman primates in primarily engaging in sexual intercourse face to face. Gorillas do it occasionally. Both heterosexual and homosexual intercourse are common among bonobos. Copulation occurs frequently as a means of reducing tension in the community and has become recreational for them. In this and other traits, bonobos are quite similar to humans.

Today, the bonobo range is limited to the forests south of the Zaire River in West Central Africa, and there are considerably fewer of them than the common chimpanzees.

Chimps and Bonobos--an explanation of how they became different species
This link takes you to a video at an external website. To return here, you must click
the "back" button on your browser program. (length = 3 mins, 47 secs)

Common chimpanzees are the most successful of all apes in that there are more of them and they have the widest geographic range. However, their numbers have been significantly reduced. A century ago, there were millions of them in the wild. Today, there are less than 200,000. This sharp decline apparently has been mostly due to the rapid increase in human populations and the accompanying natural habitat decimation. An additional factor has been the desire of many people in West and Central Africa to eat chimpanzees and other non-human primate species. At least 4,000 chimpanzees are killed for their meat every year. Most apes and monkeys are relatively large, noisy animals that are easy targets for hunters. Chimpanzees and gorillas have suffered devastating Ebola epidemics as well. During 2002 and 2003, approximately 5,000 gorillas succumbed to this highly contagious, almost always fatal disease. The lowland gorilla population has dropped by at least 80% over the last 3 generations. Only 100,000-125,000 survive today. The bonobos are at an even greater risk of disappearing since there may be only about 6,000 of them remaining in the wild. The African apes have the misfortune of living mostly in nations in which wildlife protection has been severely disrupted by civil wars and the breakdown of effective national authority over the last two decades. It is unlikely that the populations of these apes would be able to spring back rapidly even if they were more carefully protected because they have low reproductive rates. Under the best conditions, adult female chimps usually only have one baby every 5 years. The other great apes are similar.

The great apes are not alone in being threatened by humans. The International Primatological Congress has estimated that 48% of primate species are in danger of extinction within the next decade. Many will disappear with little known about them. They are at their greatest risk in Asia. The percentage of threatened primate species is 79% in China, 83% in Laos, 84% in Indonesia, 86% in Vietnam, and 90% in Cambodia. In most parts of the world, the main threat is loss of habitat due to forest clearance. However, conservation efforts have had some successes in Brazil. The numbers of Golden lion tamarins and black lion tamarins there have been increasing to the point that these species are no longer considered critically endangered. Also on the hopeful side is the recent discovery of approximately 125,000 western lowland gorillas in a largely unexplored forest region of northern Democratic Republic of Congo. Impacts on this area by humans have been minimal so far.


Gibbon


	Siamang brachiating


	Knuckle walking (subadult male lowland gorilla)




	Mutual grooming (common chimpanzees)