Just like humans, monkeys also become less social as they age; they seek less contact with others, as researchers have found out. Accordingly, aging female macaques invest less time and energy in mutual grooming with conspecifics than younger ones. Insights into the declining social engagement of monkeys could also shed light on the origins of elderly humans’ lack of sociability.
Aging humans increasingly turn to their families and close friends. This reorientation towards fewer but particularly close relationships could explain why people interact less with former acquaintances as they age. However, the tendency to become solitary in old age is not exclusively human behavior. Although bonobos, orangutans, and others prove to be surprisingly social—for example, by showing appreciation for favors, teasing each other, and recognizing friends and relatives even after decades—there are also increasing indications of a tendency towards solitary behavior in aging primates.
Grooming as a Relationship Indicator
But why do humans and primates spend less time with conspecifics as they age? Biologists led by Baptiste Sadoughi from the Leibniz Institute for Primate Research and the University of Göttingen pursued this question. They observed the social interactions of wild Assamese macaques (Macaca assamensis), a primate species closely related to humans, in the Phu Khieo Wildlife Sanctuary in Thailand.
The researchers recorded how closely the macaque females approached each other and how often they engaged in typical grooming behavior. Mutual grooming is considered an indicator of the intensity of the relationship. “For the question of social aging, we focused on females, because they remain with their mothers, sisters and daughters in their natal group all their lives, which allowed us to track changes in behavior over their life span,” Sadoughi explains.
Overall, the team recorded more than 8,000 interactions between the macaque females. The analysis of this data provided insights into the relationship between the age of the female monkeys and their social behavior.
As the Monkeys Age, They Become Less Social
The result: Although aging female monkeys continued to interact with their close social partners, the number of these contacts decreased. Overall, they gradually withdrew from social interactions. They made fewer attempts to approach other group members and also invested less time in grooming their fur. “Older females were less willing overall to groom other females,” summarize Sadoughi and his team.
But why did the elderly female monkeys withdraw? The researchers suspect physical exhaustion as the reason for the older macaque ladies’ more withdrawn behavior. “We assume that older females try to keep up with the group at all costs, as proximity to others is one of the best protective mechanisms against predators.” “However, once they have achieved this, they lack the motivation or energy to engage socially with others,” explains Sadoughi.
Humans and Monkeys Prefer Specific Partners
Since human behavior is the result of our evolutionary past, these research findings could also provide insights into the causes of human solitary behavior in old age. Just like humans, female macaques, for example, form closer and less close relationships with certain conspecifics. Whom a female has interacted with more in the past predicts with whom she will interact more frequently later on, according to Sadoughi.
However, unlike humans, female macaques apparently do not compensate for their dwindling relationships by interacting more frequently within their close circle of acquaintances. “This tendency to be selective and prefer certain partners over others does not become stronger with age, as we see in humans, but remains the same.”Selectivity in partner choice is therefore not sufficient to explain the age-dependent reduction of the social network,” Sadoughi concludes.