On the Social Life and Motivational Changes of Aging Monkeys

Although nonhuman primates have been used in biomedical research to develop a better understanding of physiological aging processes, their value as models for studying age-related differences in motivation, cognition, and decision-making has only recently been appreciated. This paper reviews the state of the art, with a focus on a recent study on Barbary macaques. A number of studies reported that with increasing age, Old World monkeys spend more time resting, have fewer social partners, and/or spend less time in social interactions, though other studies found no such effects. Less was known about changes in cognitive performance and shifts in interest in the physical and social environment across age. A recent comprehensive study of motivational changes in a large age-heterogeneous population of Barbary macaques (Macaca sylvanus) living at "La Forêt des Singes" in Rocamadour explicitly tested predictions from social selectivity theory, which posits that in light of a shrinking future time perspective, humans become increasingly selective in terms of their social interactions. Given that nonhuman primates most likely have no conception of their limited lifetime, this allowed disentangling the effects of cognitive insights and basal physiological processes that contribute to changes in the valuation of different activities. The Barbary macaques under study revealed marked and differential motivational shifts with age: while they interacted with fewer social partners, they continued to attend to social information. In contrast, they revealed a marked loss of interest in novel objects in early adulthood, unless these were baited with a food reward. Some of the motivational changes observed during human aging may thus be shared with our closest living relatives. The awareness of a limited future time perspective in humans may enhance the effects of these ancestral processes, but it does not appear to be the only explanation. Future studies should employ a broader array of different cognitive tests to delineate the trajectories of different cognitive processes such as attention, memory, and behavioral flexibility more clearly. Taken together, an evolutionary developmental psychology perspective that combines life span psychology with evolutionary biology appears to be a promising avenue for investigations of age-related changes in motivation and cognition.