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].

On the Social Life and Motivational Changes of Aging Monkeys

“…We were able to contrast changes in social behavior with performance in a variety of cognitive tests that assessed interests in the physical and the social environment, a procedure which allowed us to evaluate whether the subjects became increasingly selective in their activities, and whether these changes were indicative of motivational shifts rather than of diminishing competence [7] .…”

“…Almeling et al [7] recently set out to test the predictions of the SST in nonhuman primates by comparing variation in age across different social and cognitive domains in a large age-heterogeneous population of Barbary macaques [7] living at "La Forêt des Singes" [29] . This enclosure is a 20-ha tourist attraction in which the visitors are confined to a park, while the monkeys roam freely.…”

“…We first presented male and female subjects with different types of novel objects: animal toys and a plastic cube, and a tube that was baited with a food reward, allowing us to explore the effect of the food on the eagerness to explore -and the persistence in examining -the novel object [7] . The tube was closed with soft tissue at both ends, so that the monkeys had to figure out how to retrieve the reward.…”

“…About one-third of the subjects were older than 20 years; the oldest subject in the park was 30 years old during the time of the study. Females from the age of 4 years and males from the age of 5 years and older were included in the study [7] .…”

“…I will then discuss in more detail the results of a recent study of motivational changes in aging Barbary macaques ( Macaca sylvanus ), which was explicitly designed to test predictions from psychological theory [7] , and will conclude with some ideas for future research.…”