Aging and its impact on sociality in Barbary macaques (Macaca sylvanus)

With the aging of the human population, studies on the causes and consequences and, particularly, age-associated diseases, are attracting substantial attention. But how does aging affect peoples’ motivation to engage with the environment, and their social lives? Throughout adulthood humans’ openness to experience, social activity, and the sizes of their social networks decrease. Several models of successful aging have been put forward to explain changes in older peoples’ social lives, of which the socio-emotional selectivity theory receives the greatest support. According to this theory, the awareness of a shrinking future time perspective drives shifts in human preferences, resulting in a focus on a circle of emotionally close persons over gaining new experiences. Yet, physiological changes may also contribute to motivational shifts. To explore the evolutionary origins of human behavior, researchers have frequently turned to our closest relatives – the nonhuman primates. Macaques exhibit an aging phenotype similar to humans, but most likely lack an insight into their limited lifetime. My dissertation study set out to test core components derived from models of successful aging by investigating age-related variation across social and cognitive domains in Barbary macaques (Macaca sylvanus). Specifically, I studied a large age-heterogeneous population living in the outdoor enclosure La Foret des Singes in Rocamadour, France. These monkeys are food-provisioned and do not experience any predation, resulting in a relatively large number of older individuals in the population. This provided me with the opportunity to study the motivational and social consequences in an aging non-human primate population, in the absence of cognitively sophisticated processes and cultural conventions that accompany human aging. I used an experimental approach to determine Barbary macaques’ interest in social and non-social stimuli and assessed the behavioral patterns of females. I found that the motivation to engage in novel object tasks declined in early adulthood. In contrast, I did not observe any age-related changes in interest in social information. Similar to younger monkeys, older monkeys were more interested in portrait photographs of important social partners compared to other group members. Irrespective of age, females responded more strongly to playbacks of their main female affiliation partner compared to females with which they do not have an affiliative relationship. Finally, age had no effect on the frequency female’s emitted vocalizations when observing social interactions between third parties. However, older females engaged less in grooming and groomed fewer partners themselves. Average grooming bout length increased with age, suggesting that faster tiring might not account for the decrease in grooming patterns and that the older females still seem to be interested in physical affiliative interaction. I observed that older females tended to be more spatially reclusive and the frequency of events in which females were the target of aggression declined with age. However, other group members still groomed the older females at similar rates. Thus, the aging females themselves seem to be responsible for the shrinking network size. Finally, I did not find evidence for a decrease in active aggression rates, but the proportion of low-level aggression increased with age. Thus, older females appear to avoid situations with uncertain outcome, and particularly potentially harmful ones. In sum, monkeys became overall more selective with age regarding interactions with their environment, in terms of shifting the focus on social rather than non-social stimuli. Moreover, female subjects became more socially selective with age. This observation does not seem to be due to physical deterioration, but older females’ social life does seem to be characterized by a greater avoidance of situations with uncertain outcomes and risky situations. In conclusion, the findings reported in this dissertation suggest that at least some of the motivational changes observed across the human lifespan are a shared trait with nonhuman primates. This does not rule out that, in humans, consciousness about a limited life span contributes to shifting preferences with age, but it does suggest that physiological changes may account for some of the observed age-related variation. My dissertation study paves the ground for further research into the predictors of successful aging in primates, as well as the potential benefits of social selectivity. As such, my dissertation study contributes to the growing field of evolutionary developmental psychology. Future studies should focus on the potential age-related avoidance of negative situations, as it appears to be an important explanatory variable contributing to similarities in old humans’ and monkeys’ behavioral patterns. Moreover, combining psychological and biological methods might facilitate the understanding of the similarities and dissimilarities that occur in the progress of aging across the primate order.