Faculty of Biological Sciences, University of Leeds

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Primate Behaviour

Dr. Bill Sellers

Introduction

Up till know I've talked mostly about physical features: how they apply to extant primates; how we use them for classification; how they apply to the fossil record. But human evolution isn't simply about how we have changed physically over the last 70 million years, but also about how our behaviour has changed.

If I asked you to define what is meant by the term "human", you could probably (hopefully) give me a list of shared, derived characters that anatomically define us. But at a philosophical level, I would hope that what you'd be really proud of is not that we normally walk bipedally, but that we can reason and imagine. Old Descartes put it succinctly: "I think, therefore I am" (but not quite in this context).

This lecture isn't about human behaviour per se, but about primate behaviour in general (and animal behaviour too), since just as we can use the morphology of living primates to give us clues and insights into the morphology of human ancestors, so we hope that the behaviours of non-human primates (NHPs) will be similarly enlightening for the behaviour of our ancestors.

The importance of primate studies

As I said, the study of primate behaviour is essential for giving us clues to the possible behaviours of our ancestors. However, it is an important study in its own right. It has now become our responsibility to manage much of the life on this planet, and in order to preserve what we currently have, we need to understand what the animals require in terms of space, diet, group organisation etc. to make sure that we leave sufficient wild habitats, or, if absolutely necessary, recreate suitable habitats in captivity.

Primate behavioural ecology

The most rewarding studies for conservation purposes are studies of the animals in their natural habitat. Studying an animal in relation to its environment in this way is called behavioural ecology. Obviously, defining an environment is a complex thing to do, including both biotic and abiotic factors.

Factors that might need to be taken into account include:

1. Quantity and quality of different kinds of food

2. Distribution of food resources

3. Distribution of water

4. Distribution and types of predators

5. Distribution of sleeping sites

6. Activity patterns (nocturnal/diurnal)

7. Relationship with other (non-predator) species

8. Impacts of human activity

These (and other) variables form a complex and dynamic web of interactions and it is the job of the behavioural ecologist to try and make some sense of them all.

For example, one variable that can be measures is average group size. It is often assumed that large groups can form in response to predator pressure (for example baboons living on the African savannah), where having more members in the group increases the likelihood of predator detection, and the animals may be able to gang together to drive off some of the predators.

However, there may be other strategies that can be pursued. The slender loris, for example, is largely solitary and nocturnal, and avoids predators by hiding, rather than attempting to run away.

Socio-biology

Socio-biology is to some extent an attempt to quantify behavioural ecology. Behavioural ecology attempts to identify a set of variables that have a qualitative effect on a behaviour: "an increase in predation pressure leads to an increase in group size." Socio-biology is an approach that says: "with a predator density of 2 lions per square kilometre, I predict that the group size will be 15 individuals." Obviously, this is a testable prediction and if the results are found to agree, it makes the cause and effect argument much harder to refute.

How does it work? Socio-biologist postulate that behaviour is an inherited trait, that is under the influence of natural selection much like any other trait (trait = phenotype). However, for it to be inherited, it must be influenced by genetics, and if this is the case, then its evolutionary impact can be directly measured by its impact on reproductive success.

Individuals whose genotype lead to higher reproductive success are fitter (by definition), and will pass these genes on at a faster rate, so these genes will spread (become fixed) in a population.

This is fine for animals where particular behaviours can be linked to genes (this has been done in fruit flies and some marine snails), but is open to question in a situation (as in primates) where much observed behaviour is a result of the environment (learning) rather than being innate (genetic), though even for learned behaviours, an animal is very likely to imitate its parents as opposed to other animals, so in this respect, behaviour is still inherited. And the behaviour itself, if widely copied, will spread through the population.

For the numerical predictions to work, the behaviour must be very largely inherited. It tends to work well with things where the animal is seen to have rather limited "conscious" control, such as clutch size in birds, interbirth interval in primates or sex of offspring. Behaviours are known to affect these things, but they seem to operate at a "low" cognitive level. They don't require thought.

However, the socio-ecological paradigm is very useful. It enables us to look at behaviours as a set of strategies in a game, with maximising individual reproductive success as the goal. We can then use game theory predictions in an attempt to explain the options available and their individual costs and benefits.

For example, look at the case of infanticide which is seen in a great many mammals, including primates. Hanuman langurs (India) live in social groups consisting of a single male and several females. Periodically, the resident male in these groups is challenged by an outsider, and if the outsider is successful in beating the resident, he takes over, and generally kills all the infants currently in the group.

Why? This seems to be bad for the species... But under the socio-biological tenet, we see that being good for the species (so called "group selection") is not important. To understand this behaviour, we must consider the male attempting to maximise his reproductive success. By killing the infants, he stops the mothers from continuing to lactate, and that way they become sexually receptive again quicker. He will then start to father his own infants sooner, and will therefore produce more offspring before he is in turn ousted from his position as the resident male in the group.

Reproductive Strategies

And that neatly brings us onto the question of reproductive strategies. In animals, the ultimate goal is to maximise reproductive success. In primates, this is generally achieved in females by having few young, and investing a great deal of time and effort into rearing them (lactating etc.). This large investment in a few offspring is often described as K-selected. Male primates, in general, take very little interest in helping to rear offspring. Their approach to maximising their reproductive success is to father as many offspring as possible and not to invest much in individual ones. This is often described as r-selection (the letters K and r are from a famous population dynamics equation that everyone forgets about).

There is certainly evidence that K-selection has increased in human evolution characterised by fewer offspring and prolonged periods of infancy. It is possible that the investment by males has also increased (but then again...).

There is an interesting strategy that has been reported on mangabeys. There appear to be 2 sorts of males: one very large and aggressive; the other smaller and more similar in size to a female or a juvenile. The large males are seen to mate very openly, whereas the smaller ones are seen to mate surreptitiously. It remains to be seen which strategy is the most successful in the long run since it's only with the advent of DNA fingerprinting that we can be certain about paternity.

There are some caveats with socio-biology. It relies very heavily on the assumption that animals are indeed well adapted to their environments, and whist this is probably often the case, it isn't necessarily so. However, if we are forced to conclude that an animal isn't well adapted then there doesn't seen any way forward, so we have to start assuming that is, and only conclude that an animal is the way it is because of history, and taxonomic inertia (slow change rates) as a last resort.

Primate social 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 tautological, 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 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.

Summary

This lecture has concentrated on social behaviour, because, to some extent, it's what's most interesting and also what makes NHPs so like us. However, behaviour also include locomotion (running, jumping, walking and climbing) and specifics of foraging behaviour. Socio-biological principles give us the tools to objectively investigate these things, but we shouldn't be blinded by the perfection of the method. In the end, most behavioural studies just give clues to the big picture, and the picture itself needs to be pieced together like a jigsaw. And like most of these things, it is open to interpretation too.

Spare Time

Talk about locomotor behaviour - specific adaptations and lifestyles - VCL, quadrupedalism, suspension, knuckle walking, bipedalism. No flying/gliding or burrowing (and most primates are not keen on water either).

Foraging - manipulative foraging, fruiting seasons, foraging strategies, resource defence, territory map as reasons for increasing intelligence.


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