Faculté des sciences

Social learning and traditions in wild vervet monkeys

Van de Waal, Erica ; Bshary, Redouan (Dir.) ; Lehmann, Laurent (Codir.) ; Van Schaik, C. (Codir.)

Thèse de doctorat : Université de Neuchâtel, 2010.

Social learning is the basis for allowing the transmission of specific behaviours inside a social unit, i.e. the formation of traditions. Early field studies suggested the existence of traditions in non-human animals, while more recent laboratory experiments have demonstrated social learning abilities in a variety of species. I established a unique bridge between these perspectives by conducting... Plus

Ajouter à la liste personnelle
    Summary
    Social learning is the basis for allowing the transmission of specific behaviours inside a social unit, i.e. the formation of traditions. Early field studies suggested the existence of traditions in non-human animals, while more recent laboratory experiments have demonstrated social learning abilities in a variety of species. I established a unique bridge between these perspectives by conducting three different social learning experiments on six groups of wild vervet monkeys (Chlorocebus aethiops) at the Loskop Dam Nature Reserve, in South Africa. Using this approach, I investigated what mechanisms wild vervets use when they learn a task socially. Furthermore, theoreticians pointed out that social learning is only driven by positive selection under certain conditions. Therefore, I investigated how important the identity of a model is for the occurrence of social learning and I tried to understand why some individuals are more copied than others from a functional perspective. Finally, by analysing the stability over time of the socially acquired behaviours, I could ask whether traits acquired through social learning may turn into arbitrary traditions. First, I presented laboratory-style ‘artificial fruit’ boxes that had two doors on opposite, differently coloured ends. A dominant individual invariably monopolized the box during an initial demonstration phase, in which one door was blocked. This created consistent demonstrations of one of the two possible solutions in each of six study groups. Three groups had female models and three had male models. Following demonstrations I found a significantly higher participation rate (‘stimulus enhancement’) by other group members and significant evidence for manipulation of the same door (‘local enhancement’) in groups with female models compared to groups with male models. These differences appeared to be due to selective attention of bystanders to female model behaviour, while male and female models attracted similar numbers of bystanders and showed similar levels of aggression towards those bystanders. The results demonstrate the eminent role of dominant females as a source for directed social learning in a species with female philopatry. In this same first experiment, I analysed the proper solving of the task. During their first trial, I observed which individuals managed to open artificial fruit thus accessing the reward. This time I did not find an effect of the model sex but I found that the two groups in contact with humans were more successful than the others. This result suggests some enhanced manipulation skills due to contact with humans or their facilities. The second experiment involved a more complex artificial fruit to test for sequence imitation, where two steps were necessary to open the door: remove a bar on the top of the box which releases a rope that was blocking the door and then pull the door. Vervets largely failed to show more complex social learning abilities in this experimental setup. However monkeys in group with models touched the bar significantly more often during their first manipulation than control individuals did. This latter result implies again the use of ‘local enhancement’ as social learning mechanism in wild vervets. Finally, I conducted a food cleaning experiment that was inspired by a classic study that documented the spread of sweet potato washing in a semi-natural population of Japanese macaques. I offered the monkeys grapes covered with sand and noted if and how they cleaned the food before eating. Each group was subjected to 15 trials. Vervets either did not clean the grapes or either rubbed with their hands, rubbed on substrates, or opened the fruit with their teeth or hands to eat the inside only. I found strong variance between individuals of the same group as well as between groups with respect to the techniques used. Matrilines rather than entire groups appeared to be the key unit for social transmission, where conformity of feeding techniques could be documented. Taken together, the findings imply that in species with complex social structures, migration does not necessarily lead to an exchange of socially acquired information within populations, causing much localized traditions.