To distract me from dangerous goings-on outside my window, where men are cutting down tall trees, Chris & Uta have asked me about the social lives of marmosets. This is not really altruism on their part. They will use the information for their book.
CDF: We want to know about the origins of human social cognition. Where should we look? Bees and ants? Other primates?
RR: I think you need to look at all and any species and work out the packages of behaviour that different species use.
UF: What do you mean by packages of behaviour?
RR: Behaviour, and social structure, depend on the constraints of environment and evolution and both boil down to reproductive success. Does the species have to get through a cold winter? Does the species live up trees? It is factors like these that will determine whether the species produces a large number of offspring with low parental investment or a low number of offspring with high parental investment. These strategies come as packages and much social structure follows therefrom.
UF: So, is reproductive behaviour, mating and nurturing the young, at the biological root of social behaviour?
RR: This is where we find differences between species, because of the constraints I just mentioned. Old World monkeys tend to have a hierarchical mating system. Here males compete with other males for access to large numbers of females – though it all gets complicated with alliances and pay-offs. New World monkeys tend to live in more or less monogamous families – though it all gets complicated with adultery and incest. New World monkeys tend to live up trees, like birds, which are also usually monogamous so perhaps trees matter.
UF: Are humans more similar in their mating strategy to New World monkeys?
RR: Humans exhibit different strategies in different societies depending on food supply and other external threats. We can infer a lot about Human ancestral social behaviour by the fact that males are slightly bigger than females. This size difference occurs in any species where males compete with other males for resources and for females. The ultimate aim here is to win wars, get rid of other males, and acquire as many available females as possible. Of course, the size difference in humans is not large compared to the difference in some species. This implies a long history of more family based behavioural packages as well.
UF: Mike Tomasello’s lab has a vast number of studies comparing the joint interactions of great apes and those that happen between human children. There are similarities, but one of the main differences seems to be that, while human social interactions are basically cooperative, those of chimpanzees, gorillas and orang-utan’s are mostly driven by competition. Tomasello’s conclusion is that great apes are extremely limited in their cooperative capacities, and this is their essential difference to humans.
CDF: But unlike chimpanzees, Marmosets seem to be extremely cooperative, showing, for example, cooperative breeding. Is this true?
RR: Kin selection explains many aspects of superficially altruistic behaviour. Co-operative breeding in marmosets usually means assisting in the rearing of the offspring of relatives because of the family structure of groups of marmosets. But an owner of a captive breeding colony of marmosets will often attempt to cross foster a neonate when the breeding female of one family has had just one baby and the breeding female of another family has just had triplets. This is often successful because the family can’t be sure that the newcomer is not related to them (and marmosets aren’t very good at counting either). In the wild, quite a large troop will contain mainly related animals. So a certain amount of general co-operation could be sustained by kin selection.
Several additional explanations have been put forward to explain co-operative behaviour in co-operative breeding species such as marmosets. These are all plausible, but none are truly altruistic since they increase the reproductive success or survival of the protagonist.
* Helpers that have cared for other infants have greater reproductive success when they become parents than individuals that do not have previous infant care experience. This is undoubtedly the case for marmosets.
* Unrelated helpers can gain the benefits of living in a social group, such as communal foraging or protection against predators. This requires the unrelated individual to be accepted by the group and this usually only occurs when the incomer arrives as a potential breeding partner.
* Males that display involvement with infants are more likely to obtain subsequent mating with the female they assist. This is unlikely since the male is usually assisting in the rearing of his own offspring (so has access to the dominant female) or his own sibs. The dominant female suppresses ovulation in her daughters and thereby prevents incest in the rest of the family. “No hanky-panky please.”
UF: Social learning is something that happens in almost all animal species. It seems possible to us that this learning has shown a sort of step change in evolution in humans. This is to do with learning by deliberate teaching, a two-way process. Is this plausible? How would evolution explain such a step change?
RR: The difficulty in getting a hand-reared orphan animal to be accepted by, mate with and rear offspring with, members of their own species shows how important social learning is for survival. Evolution will favour parents who behave in such a way that offspring learn social behaviours. In New World monkey species, the fathers and older siblings contribute to the rearing of the next litter and, in captivity, offspring reared alone by just their mother are rarely able to rear offspring themselves. The difference between ‘behaving in such a way that learning in offspring occurs’ and ‘teaching offspring’ is one of intention by the parent and so the step change (if there is one) will lie in the evolution of several cognitive ‘apps’, e.g. ‘appreciation of the existence of the future’, such that intentions can develop. New ‘apps’ have emergent functions made from the amalgamation of smaller ‘apps’.
UF: I love your use of the term ‘app’ for a special cognitive adaptation!
CDF: The importance of learning by teaching is that it offers a mechanism for culture to accumulate over generations. Culture seems to be the one specialised niche for humans.
RR: Intentionality will always speed up cultural development, but animals have cultures too. Diet and food-finding practices are cultural in that they differ between groups of the same species.
The trees are down now! Good. I can go out again.
CDF & UF: Thank you, Ros. We’ll be back and quiz you more about processes in the evolution of behaviour.