Estratégias de forrageio e rede social em primatas de vida livre: uma abordagem experimental

Abstract

Primates, including humans, have a large brain size and complex cognitive abilities that enable them to store, recover, and integrate information to overcome challenges. Two hypotheses attempt to explain the cerebral evolution in primates, the ecological hypothesis and the social hypothesis. However, this may be a false distinction, since by living in stable groups and inhabiting dynamic environments, primates must be able to use both ecological information, such as the availability of resources in time and space, and social information such as the identity, behavior, and social relationships among group members to make decisions. In this context, social life can offer a number of advantages and disadvantages for animals living in groups. When it comes to searching and obtaining food, living in a group may incur costs associated with an increased competition for resources, as well as benefits related to an enhanced localization, access, and defense of these resources. Therefore, in this research, we used the common marmoset (Callithrix jacchus), a cohesive and cooperative primate species, as a model to investigate how different ecological contexts of food availability, and social factors such as rank, age and sex, affect individual foraging strategies, feeding success, and social networks among group members during social foraging. To do so, we conducted a series of field experiments in which the distribution (concentrated or scattered), productivity (high, medium or low quantity), and type of resource (fruit or insect) were manipulated to simulate different food conditions naturally found in the wild. By investigating the use of foraging strategies and the feeding success of individuals under different experimental conditions we found that except for the breeding female, rank and aggressions (contest competition) were not strong predictors of feeding success on marmosets. In each study group, the breeding female was the highest ranked individual and obtained the higher feeding success. However, the other group members, including adults and juveniles, obtained relatively similar feeding success among them. Our results indicate that this was achieved through a balance in the use of strategies related with contest competition (mainly by the breeding female), scramble competition (non-aggressive forms of competition associated with a finder’s advantage), and tolerance on feeding sites. To examine the social network of marmosets during foraging, we assess the foraging associations between group members (number of individuals and time spent sharing a feeding site) and further investigate marmosets’ levels of social tolerance under different experimental conditions. We found stronger associations when food was concentrated on a single feeding site. Juveniles shared platforms with more individuals and for longer time than adults did. We found stronger associations between dyads composed by individuals of closer rank, of different sexes (female-male), and of different ages (adult-juveniles). However, foraging associations among partners (dyads) varied between experimental conditions, being stronger when food was clumped, regardless of the amount of food. Therefore, this study showed that common marmosets are able to use and integrate both ecological and social information to make decisions during social foraging, by adjusting their behaviors and associations with each other to obtain access to resources, maximize their feeding success, and maintain group cohesion necessary for cooperation. In conclusion, this thesis contributed in demonstrating that considering the social characteristics of the studied species (cooperation and a pyramidal social structure), common marmosets do not fit the socioecological model regarding intra-group food competition. In addition, we showed that within a group, individuals build affiliative relationships with all other group members by associating during foraging, resulting in cohesive and tolerant groups during a potentially competitive activity. We stress that future studies and models should incorporate both cost and benefits of social life to individuals and to the group as a social unit. Finally, we highlight the importance of using field experiments to systematically investigate the socioecology of primates inhabiting their natural environment.