Within-individual changes reveal increasing social selectivity with age in rhesus macaques

Abstract

Accumulating evidence in humans and other mammals suggests older individuals tend to have smaller social networks. Uncovering the cause of these declines is important as it can inform how changes in social relationships with age might affect health and fitness in later life. Smaller social networks might be detrimental, but may also be the result of greater selectivity in partner choice, reflecting an adaptive solution to physical or physiological limitations imposed by age. While greater selectivity with age has been shown in humans, the extent to which active 'social selectivity' within an individual's lifetime occurs across the animal kingdom remains an open question. Using 8 years of longitudinal data from a population of free-ranging rhesus macaques we provide the first evidence in a non-human animal for within-individual increases in social selectivity with age. Going beyond previous cross-sectional studies, our within-individual analyses revealed that adult female macaques actively reduced the size of their networks as they aged and focused on partners previously linked to fitness benefits, including kin and partners to whom they were strongly and consistently connected earlier in life. Females spent similar amounts of time socializing as they aged, suggesting that network shrinkage does not result from lack of motivation or ability to engage. Furthermore, females remained attractive companions and were not isolated by withdrawal of social partners. Taken together, our results provide rare empirical evidence for social selectivity in non-humans, suggesting patterns of social aging in humans may be deeply rooted in primate evolution and may have adaptive value.

Ageing in a collective: The impact of ageing individuals on social network structure

“…In our dataset, the median age of adult females was 10-years. To capture later-life changes in indirect connectedness we thus focused our analyses on individuals aged 10 and older (in line with previous ageing studies in this system [6]. Females ranged between 10 and 28 years old.…”

“…The copyright holder for this preprint this version posted August 13, 2022. ; https://doi.org/10.1101/2022.08.10.503309 doi: bioRxiv preprint females was 10-years. To capture later-life changes in indirect connectedness we thus focused our analyses on individuals aged 10 and older (in line with previous ageing studies in this system [6]. Females ranged between 10 and 28 years old.…”

“…We were particularly interested in the potential effects of the age-dependencies observed in the rhesus macaque system on Cayo Santiago [6]. As mentioned above, female rhesus macaques have been observed to change two aspects of their sociality with age: their number of social partners, and the proportion of partners that are kin [6]. We therefore included kinship as an attribute of individuals in the model, and parameterised the model based on our empirical More specifically, the model simulates social networks of varying age distributions, in which the probability that two nodes (i.e., individuals) have an edge (i.e., a grooming link) depends on age and kinship of the individuals.…”

“…Patterns of social interactions and social attention vary from adulthood into old age [6,32,54]. Critically, as female rhesus macaques grow older, they reduce the size of their social networks, and focus their social effort on a few important partners including close kin [6]. Despite this, females do not reduce the rate at which they engage in social interactions as they age, indicating that although their ego-networks get smaller, older females continue to invest the same amount of time into fewer relationships [6].…”

“…Drawing on this idea that underlying processes can predict an individual’s position in the network, and on the age-based changes in social behaviour we have documented in rhesus macaque females, we made predictions for four common measures of indirect connectedness: eigenvector centrality, betweenness, closeness, and clustering coefficient. Adult female rhesus macaques maintain their strength of grooming ties to others as they age [6]. We therefore predicted that weighted eigenvector centrality would also remain stable with age.…”