Hominoids

Great and lesser apes; diversity; anatomy; behaviour; classification; intelligence; culture.

General Anatomy

The following features are used to identify hominoids (as well as the normal set of catarrhine features).

No tail

Shoulder blades at back rather than side

Y-5 pattern on molar teeth (not bilophodont)

Enlarged incisors

Large brains

Taxonomy

Hylobatidae

Lesser apes (gibbon and siamangs). Arboreal primate with specialised locomotor habit: brachiation (with bipedalism on the ground). They are usually grouped into a single genus, Hylobates, and they vary in size from about 5kg to 10kg. They are much smaller and more slender than the other hominoids, have proportionally longer arms, longer canine teeth and have buttock pads (ischial callosities). They live in rainforest in Southeast Asia in monogamous, territorial family groups of three or four individuals. The young leave the natal group after about 5 years. Territory defence is usually by calling - gibbons have a very loud spacing call. They are typically frugivorous. There is little sexual dimorphism (though there is sexual dichromism in some species).

Pongidae

I am using the traditional classification that separates the great apes from humans at the family level. This is much disputed, and probably incorrect: Gorilla and Pan should be moved into Hominidae. However, using this classification, there are 4 species of great ape in 3 genera.

Pongids are very large primates (up to 200kg) with large canines, arms longer than legs, and mainly quadrupedal locomotion with facultative bipedalism. Pongid skulls are long with large brains, though much smaller than extant hominids. They have large teeth with thin enamel and a U-shaped dental arcade due to the enlarging of the incisors. The mesial lower premolars are unicuspid ("sectorial") and are honed by the upper canine. There is a small gap (diastema) between the upper, distal incisors and the upper canines.

They tend to have robust skulls, with superior and inferior mandibular tori (the inferior one is also known as the "simian shelf"), and cresting on the skull at the attachment points of the chewing muscles - especially in males.

Pongo

One species, Pongo pygmaeus, lives on the islands of Borneo and Sumatra. The females weigh approximately 50kg and the males about 75kg, though in captivity, these figures can easily double. They have striking, range fur. The skull has no supra-orbital ridge nor frontal sinus. The teeth have wrinkled enamel, and the upper 2nd incisor is much smaller than the 1st. The eye sockets are tall and set close together. The nose is narrow with a smooth floor. The animals are solitary (mostly), but it has been suggested that this may be a recent feature due to predation pressure from humans.

Wild orangs show very limited tool use, but captive orangs are very inventive. They are mainly arboreal using a slow, quadrumanual climbing technique. Foraging tends to be by bending the fruit-laden small branches to the orang, rather than the orang moving towards the small branches. On the ground, they move rather slowly, mainly quadrupedally, but using the clenched fist rather than the true knuckle-walking found in the other great apes.

Gorilla

One species, Gorilla gorilla, lives in east and west coast lowland African equatorial rainforest and also in two mountain refugia in Rwanda and Bwindi. This is the largest extant primate with females weighing perhaps 100kg and the males 200kg. Again, these figures can double in captivity. They have black fur, and the adult males develop a silvery white saddle. Some sub-species have a brownish, or greyish coloration. The skull of the male is extremely robust, with huge nuchal and sagittal crests. The female skull is rather less impressively crested, but is still robust. Like chimps, the orbits are relatively widely space; they have a wide nose with a stepped floor; a frontal sinus; and the 2nd upper incisor is almost as big as the 1st.

They live in polygamous family groups, with a single male defending a harem of several sexually active females. They eat fruit and leaves and are mainly terrestrial, though they often make nests and sleep in trees. They, like chimpanzees, use a quadrupedal locomotion called "knuckle walking" where the hand is semi-clenched with the middle phalanx being held flat on the ground with hyper-extension of the metacarpo-phalangeal joint.

Again, tool use is not seen in wild animals, though a limited amount has been observed in captive animals.

Pan

There are two species of chimp: Pan troglodytes, the common chimpanzee; and Pan paniscus, the pygmy chimp or bonobo. Both species are smaller than the other two great apes with males weighing about 50kg and females about 30kg. The skull is less robust than the other great apes, with only a small sagittal crest being present. Like gorillas, the orbits are relatively widely space; they have a wide nose with a stepped floor; a frontal sinus; and the 2nd upper incisor is almost as big as the 1st.

Chimps live in extended, promiscuous, multi-male multi-female groups. There is a dominance hierarchy that controls preferred access to mates and food resources. They, like gorillas, use a quadrupedal locomotion called "knuckle walking" where the hand is semi-clenched with the middle phalanx being held flat on the ground with hyper-extension of the metacarpo-phalangeal joint. They are mostly terrestrial, but do sleep in nests in trees.

Tool use has been frequently observed in wild chimps: ant fishing; nut-cracking; leaf sponges. They are also thought to forage for specific medicinal plants. In addition to fruit, flowers and leaves, chimps also eat meat - often other primates, which they catch in a co-operative manner.

Hominidae

Only one living species, Homo sapiens.. Large primate (75kg males, 52kg females), with sparse pelage of one colour (black to yellow, or reddish), habitually upright and bipedal. Skull is comparatively gracile, with small teeth, thick enamel, and greatly reduced canines. We have a bicuspid first lower premolar, extremely large brains, no post-orbital constriction, vertical face, small jaws, anteriorly placed foramen magnum and no cresting. The dental arcade is parabolic. We share the frontal sinus, wide nose, large incisors with gorillas and chimps. Post-cranially, we are fully upright, with a short, broad, bucket shaped pelvis, long legs, and inwardly angled femur. Our big toes are aligned with the others for weight bearing and the foot has become modified as a walking platform with springy arches. We have a comparatively long thumb and a `precision grip'.

Currently, most humans live in monogamous (perhaps serially monogamous) pairs, with males contributing to infant care and living in family groups. However, there is a huge range of human social systems varying from polyandry through various degrees of promiscuous multi-male multi-female groupings through monogamy (lifelong and serial) to resource based polygamy. We do not know what the ancestral form was, and in any case, it was probably just as varied as today.

Primate behaviour

So what sort of behaviours do we see in primates?

Dominance

Primates, as (mostly) group living animals tend to form what are known as "dominance hierarchies". Animals higher in the hierarchy tend to displace lower ranked individuals from resources (mates, space, food). They tend to have higher reproductive success (either by mating more often, or by having more resources to invest in their offspring). The hierarchy is not fixed and depends on a number of changing factors (age, sex, aggression, intelligence perhaps), and may also depend on the support of others.

The rank is learned through play, agonistic interactions and affiliative interactions (and rather tautologically, that's exactly how it's measured too). This maintenance of social position, and social knowledge of ones rank is one of the postulated theories for why humans have been forced to evolve large brains.

Grooming

This is a common primate activity. Allogrooming (others) is an important affiliative mechanism. It can be used to strengthen links: subordinate animals tend to groom more dominate ones; males groom females for sexual access. Or for more practical purposes: mothers grooming infants to keep their fur clean? It is certainly the cement that keeps the primate social structure together.

Communication

This includes scents; body postures; gestures; vocalisations. Some of these appear to be autonomic responses indicating emotional states: fear, excitement, confidence, anger. Others seem to have a more specific purpose: loud ranging calls in Indri, howler monkeys and gibbons; quiet contact calls in lemurs to keep the group together; fear calls in lost infants, or on spotting predators. From our human perspective, we often find it easier to associate sounds with specific meaning, but among NHPs, gestures and actions are often used. Presentation and mounting behaviour is often used to diffuse potentially aggressive situations. Yawns exposing teeth are often threats, as is direct eye contact. Apparently this can cause problems when looking at baboons with binoculars: the front lenses look like bigger than normal eyes and this is seen as the observer being very aggressive.

Facial expression is important too. It's very obvious in chimps: their expression often appear all to human-like; but other primates also use stereotyped eyelid flashes or lip slaps.

In addition, there has recently been a great deal of success teaching chimps human language. This was initially American sign language, but has now been extended through the use of modified computer keyboards to really very high levels of sophistication (especially Kanzi, a pygmy chimp).

Reproduction

In all primates, except for humans (and perhaps chimps), the females are seasonally, or cyclically receptive. This is usually associated with visual changes such as perianal swelling, so that is clear when the females are in heat. Pair bonding of any sort is rare among primates, though gibbons seem to be lifelong monogamists, and some new world monkey groups, such as marmosets, have only one reproductively active pair in any group. Chimps, especially pygmy chimps, have been seen to have consortships of several weeks where copulation is frequent, but there is still no good evidence for paternal care of the infants.

Mothers and infants

This is the basic social group for many primates. It has been observed that this mother infant bonding is needed to allow the infant to be able to interact properly as an adult, and, if female, to be able to cope with offspring. This is one of the big problems with zoo animals where an individual has been hand reared by keepers. In some primates, this mother infant closeness continues after infancy. The females remaining in the group as a "matriline" and the males dispersing to other groups. The combined power of one of these female bonded matrilines is enough for the group of females to be more dominant than the alpha male, even though he is much bigger than an individual member.

Aggressive and affiliative behaviour

As mentioned before, many behaviours exist to keep the group structure running smoothly for the members of the group. There are occasions though when these behaviours (especially aggression) are directed outside the group. Baboons gang up to repel attacks by hyenas, and chimps have been known to systematically gang up on and destroy neighbouring groups of chimps.

One interesting argument here, is that the development of bipedalism has been seen by some to be driven by a root as an aggressive, dominance display behaviour. This is the gorilla standing bipedally and banging his chest, or a male chimp bipedally charging a subordinate. Most people would probably consider this to be an effect of a bipedal ability, rather than the cause.

Cultural behaviour

This is learned behaviour that is passed from generation to generation. You will hear a lot about this in humans, but it has been observed in primates too. One prime example is a group of Japanese macaques, where one individual accidentally learned that the sweet potatoes that they were being fed tasted better if the sand was washed off (this is not a normal food for these animals). This behaviour has now spread through the whole group, and is being passed on to infants. It is now part of their culture. Tool use abilities are often thought to be acquired and passed on in this way too - for example, termite fishing in chimps.

Primate cognitive abilities

I've mentioned a lot about behaviours without mentioning much about cognition. This is the amount of thought that went into a behaviour. There is a world of difference between an animal hitting a nut with a rock and cracking it by accident, and an animal thinking to itself: "I can't bite into this nut. I know, I need something to use as a hammer to crack it." However, it can be very difficult coming up with experiments to differentiate these two.

We can easily test mental skills such as recall and discrimination: e.g. Wisconsin general test apparatus and various training experiments. But it's much harder to work out the degree of thought required. This is still a big problem in evaluating the status of great apes. Just how nearly "sentient" are they?

Another feature that has come to light recently is "Machievellian Intelligence". Work especially with baboons seems to indicate that there is a lot of deliberate social deception going on: sneaky mating; passing the blame onto others; using infants for defence. This seems very complicated behaviourally, but again, it can (just about) be explained in a fairly minimally cognitive way.

Altruism of various sorts is also found in certain primates. The animals team up to gain various goals, whether it's hunting in chimps, or mate access in baboons. This would also seem to require a degree of cognition.

The signing chimp, Washoe, and the computer aided communication of Kanzi also indicate a high level of real intelligence. An interesting fact is that these language trained chimps do much better in the standardised intelligence tests too, indicating that we probably underestimate primate intelligence (primates are not all that interested in the colour of pencils, they want to know which of their friends are sleeping with each other - sound familiar?)

This is a thorny problem, with deep moral and political ramifications.

Bibliography

Library

 AUTHOR(S)       :Clark, Sir Wilfrid Edward Le Gros 1895-1971
 TITLE           :The antecedents of man : an introduction to the evolution of
                  the primates
 EDITION         :3rd ed.
 IMPRINT         :Edinburgh : University Press, 1971
 SERIES          :Edinburgh University publications. Science and mathematics
                  texts ; 2

 AUTHOR(S)       :Nowak, Ronald M.
 TITLE           :Walker's Mammals of the world
 EDITION         :5th ed. / Ronald M. Nowak
 IMPRINT         :Baltimore London : Johns Hopkins University Press, 1991

 AUTHOR(S)       :JONES, Steve  et al.,
 TITLE           :The Cambridge encyclopedia of human evolution / edited by
                  Steve Jones, Robert Martin and David Pilbeam / executive
                  editor Sarah Bunney
 IMPRINT         :Cambridge : Cambridge University Press, 1994.

 TITLE           :Machiavellian intelligence : social expertise and the
                  evolution of intellect in monkeys, apes, and humans / edited
                  by Richard W. Byrne and Andrew Whiten
 IMPRINT         :Oxford : Clarendon Press, 1988
 SERIES          :Oxford science publications

 TITLE           :"Language" and intelligence in monkeys and apes : comparative
                  developmental perspectives / edited by Sue Taylor Parker and
                  Kathleen Rita Gibson
 EDITION         :1st paperback ed.
 IMPRINT         :Cambridge : Cambridge University Press, 1994

 TITLE           :Primate societies / edited by Barbara B. Smuts ... [et al.]
 IMPRINT         :Chicago London : University of Chicago Press, 1987
      

Other

Conroy, G. C., 1990. Primate Evolution, Norton, London.

Napier, J. R., Napier P. H., 1985. The natural history of the primates, British Museum (Natural History), London.

Aiello, L., Dean, C., 1990. An introduction to human evolutionary anatomy. Academic Press, London.