Macaques become socially isolated with old age, like humans

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An elderly female Assam macaque (Macaca assamensis) near the sea research Phu Khieo Wildlife in Thailand.

Agence France-Presse

With old age, macaques restrict the circle of their relationships while maintaining exchanges with their most loyal peers, shows a German study. And this isolation is not simply suffered, but also desired.

The phenomenon is observable in many primates, and in particular in the ;human, even if the latter knows how to compensate for it, recall the authors of the study published in the Proceedings B (New window) of the British Royal Society (in English).

These researchers note that the subject is of even more interest as the human population is aging rapidly, with 2 billion humans aged over 60 by 2050, according to figures from the UN.

Research, which assumes that the preservation of social ties in old age is a factor of good health, still debates the reasons explaining this disengagement. The observation is that with aging, we restrict ourselves to a smaller circle of people, ethologist Baptiste Sadoughi, first author of the study, explains to AFP.

Individuals would thus favor the quality of relationships rather than their number, according to the theory of socio-emotional selectivity developed in the 1990s, specifies this doctor in behavioral ecology at the& #x27;University of Göttingen and the Leibniz Institute for Primate Research (DPZ).

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The problem with humans is that behavioral studies are based on cross-sectional samples, that is to say mixing age classes at the same time. A method introducing unavoidable biases.

The interest of the study carried out by the DPZ team was to follow a population of primates, Assam macaques, longitudinally, i.e. over time: the behavior of 61 females, aged 4 to 30 years, were observed over several years between 2013 and 2021, living freely in a Thai reserve.

With several findings without appeal. Social isolation increases with age first, with a halving of the size of the average social network between a ten-year-old female and a twenty-year-old female, according to Mr. Sadoughi.

This measurement, in the macaque, is carried out by evaluating mutual grooming practices, the preferred mode of relationship in this species of primates, in the absence of master bridge or bingo.

The observation ruled out that this isolation is the result of spatial segregation, since even old macaques maintain physical proximity with their conspecifics.

Their relative isolation is partly deliberate: the aging individual is responsible for a large part of these changes, because he approaches others less and initiates fewer body-to-body interactions.

An isolation also suffered, since with age, the female macaque will be deloused by fewer and fewer individuals. She will remain so through her loyal friends, notes the ethologist, by interacting most with these same individuals with whom she has always interacted more often or better.

The macaques of Assam would thus be subject to a phenomenon of social selectivity, but with a significant difference with humans, since the latter compensate for the shrinking of their circle of relationships with more extensive exchanges with their faithful friends.

This latest phenomenon of social selectivity has been observed to date, via a 2022 longitudinal study, in a population of rhesus macaques focusing their socialization efforts on one limited number of partners.

This 2022 study, the only downside of which was that it focused on a semi-free population, concluded that social selectivity would be thus deeply anchored in the evolution of primates.

For his part, Baptiste Sadoughi, imagines that this social selectivity would not be the the prerogative of humans alone, but a strategy for coping with aging, which has perhaps always existed since we were primates, humans and macaques alike.