The grooming behaviour displayed by primates is due to less rational behaviour than often thought. According to a computer model developed by scientists at the University of Groningen, one basic rule explains all possible grooming patterns: individuals will groom others if they're afraid they'll lose from them in a fight.
Primates are assumed to reconcile their conflicts by grooming each other after a fight. They are also supposed to carry out intricate trading of grooming for the receipt of help in fights. Professor and theoretical biologist Charlotte Hemelrijk shows in a computer simulation that many patterns of reconciliation and exchange surprisingly emerge simply from fear of losing a fight with another individual. 'This shows that reconciliation and exchange behaviour are not necessarily conscious behaviour', Hemelrijk -- specialist in self-organization in social systems -- states. 'It's simply a consequence of rank and of which primates are in the vicinity of the primate that wants to groom.' The results of the research conducted by the group that worked with Hemelrijk on the computer model have appeared in late December in the journal PloS Computational Biology.
'Primates are intelligent, but their intelligence is overestimated. The social behaviour of primates is explained on the basis of cognitive considerations by primates that are too sophisticated', Hemelrijk continues. 'Primates are assumed to use their intelligence continually and to be very calculating. They're supposed to reconcile fights and to do so preferably with partners that could mean a lot to them.' This would explain why primates prefer grooming partners higher in rank in order to gain more effective support in fights. Moral considerations would bring them to repay the grooming costs by grooming others.
Such behaviour patterns all presuppose a rational thought process, according to Hemelrijk: 'In order to reconcile, the primates must recall exactly which fight they last had and with whom. They must also be able to gauge the importance of each relationship. And for the reciprocity and repayment, they must keep careful track how often and from whom they have received which grooming or support 'service' in order to be able to repay it sufficiently.'
However, all these suppositions are unnecessary according to Hemelrijk: 'Our computer model GrooFiWorld shows that complex calculating behaviour is completely unnecessary. We can add the simple rule to the existing DomWorld model that an individual will begin grooming another when it expects to lose from it upon attacking the other. This in itself leads to many of the complex patterns of friendly behaviour observed in real primates.' In the DomWorld model, individuals group together and compete with their neighbours.
With the help of the computer model, Hemelrijk shows that most friendship patterns are due to the proximity of other animals. In turn, the proximity is the result of dominance interactions. The fear of losing a fight also plays an important role. 'Apparent reconciliation behaviour is the result of individuals being nearer their opponent after a fight than otherwise', the professor explains. Repaying grooming that has been received is the result of some individuals being nearer to certain others more often. Since they groom nearby primates in particular, any grooming received will automatically be repaid.'
The model and reality
That this is shown by the computer model does not mean that primates are not capable of displaying intelligent social behaviour, according to Hemelrijk. 'The resemblance of patterns of friendly behaviour in our model to those in reality means that more evidence is needed to be able to draw the conclusion that friendly relationships are based on human, calculating considerations. Our model is a 'null model' providing simple explanations which are especially useful for further research into friendly behaviour in primates, in particular into that of macaques.'
Such computer models are not only useful in analyzing primate behaviour, but also to gain insight into the social behaviour of all sorts of species that live in groups. It could for instance provide ideas for further research into the flocking behaviour of starlings. Hemelrijk: 'Simulations thus are also very important for researchers working out in the field. They can research the connection between models and reality.'
- Puga-Gonzalez et al. Emergent Patterns of Social Affiliation in Primates, a Model. PLoS Computational Biology, 2009; 5 (12): e1000630 DOI: 10.1371/journal.pcbi.1000630
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