These pages have been left in this location as a service to the numerous websites around the world which link to this content. The original authors are no longer at the University of Leeds, and the former Centre for Human Biology became the School of Biomedical Sciences which is now part of the Faculty of Biological Sciences.
Dr. Bill Sellers
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.
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.
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.
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.
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.
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).
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.
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.
Foraging - manipulative foraging, fruiting seasons, foraging strategies, resource defence, territory map as reasons for increasing intelligence.
This page is maintained by Steve Paxton