Motivational Shifts in Aging Monkeys and the Origins of Social Selectivity

Abstract

As humans age, they become more selective regarding their personal goals [1] and social partners [2]. Whereas the selectivity in goals has been attributed to losses in resources (e.g., physical strength) [3], the increasing focus on emotionally meaningful partners is, according to socioemotional selectivity theory, driven by the awareness of one's decreasing future lifetime [2]. Similar to humans, aging monkeys show physical losses [4] and reductions in social activity [2, 5-7]. To disentangle a general resource loss and the awareness of decreasing time, we combined field experiments with behavioral observations in a large age-heterogeneous population of Barbary macaques (Macaca sylvanus) at La Forêt des Singes. Novel object tests revealed a loss of interest in the nonsocial environment in early adulthood, which was modulated by the availability of a food reward. Experiments using vocal and visual representations of social partners indicated that monkeys maintained an interest in social stimuli and a preferential interest in friends and socially important individuals into old age. Old females engaged in fewer social interactions, although other group members continued to invest in relationships with them. Consequently, reductions in sociality were not due to a decrease in social interest. In conclusion, some of the motivational shifts observed in aging humans, particularly the increasing focus on social over nonsocial stimuli, may occur in the absence of a limited time perspective and are most likely deeply rooted in primate evolution. Our findings highlight the value of nonhuman primates as valuable models for understanding human aging [8, 9].