Jorg J. M. Massen
Department of Animal Ecology, Utrecht University, 3584 CG Utrecht, THE NEDERLANDS.
Both in humans and group-living animals, individuals affiliate more with some individuals than others. Human friendships have long been acknowledged, and recently scientists studying animal behaviour have started using the term friendship for social bonds between animals. Yet, while biologists describe friends as social tools to enhance fitness, social scientists argue that human friendships are unconditional. Therefore, I investigate whether these different descriptions reflect true differences in human and animal friendships or whether they are a by-product of different research approaches: namely social scientist focussing on proximate and biologists on ultimate explanations. In this thesis I studied friendships of long-tailed and rhesus macaques. I defined as friends those dyads with individuals that were in close contact and groomed each other relatively more frequently than others. First, I studied exchange relations of both species. I show that macaques of both species exchange grooming and support. Moreover, I show that male rhesus macaques exchange grooming for sexual access to females. However, these exchanges are not based on immediate reciprocity, but are better explained by the long-term emotional bond between dyads. Since these macaques exchange services on an unconditional base, it is expected that they also show pro-social behaviour and that its frequency depends on the strength of such a bond, e.g., friends give more to each other than non-friends. In an experimental set-up, I tested pro-sociality in long-tailed macaques and how that is affected by relationship quality. Surprisingly, pro-social behaviour is not used by subordinates to obtain benefits from dominants, but by dominants to emphasize their dominance position, while subordinates withhold their partners access to food, and seem to prefer a more competitive strategy. When given the choice to give to either a ‘friend’ or a ‘non-friend’, only dominant individuals give to their friends, while subordinates prefer a more competitive strategy and give to the most dominant individual. Therefore, dominance rank determines which of two possible social relationships, friendship or dominance, prevails in directing pro-social behaviour. Only dominants seem to confirm their ‘friendships’ in the short-term setting of an experiment. Another assumption of friendship concerns tolerance for an imbalance within what is given and received. Yet, when tested experimentally, long-tailed macaques do show an aversion to a cost-benefit imbalance, regardless of whether they are paired with a ‘friend’ or a ‘non-friend’. Finally, I review studies on both human and animal friendships. I show that both human and animal friendships are adaptive. Yet, these friendships are not motivated by these fitness benefits, but are unconditional. Moreover, friendships are probably mediated by emotions and such a regulation of social bonds is a highly conserved system of sociality in humans and is similar in many animals. In conclusion, when biologists and social scientist describe close bond among monkeys and humans, they describe the same homologue phenomenon, and the use of the term friendship for animals, therefore, seems justified.