Age, sex, and social environmental effects on immune cell composition in a free-ranging non-human primate

Proc. Natl. Acad. Sci. U.S.A.

Valle

Phillips

et al. 2022

Self Cite

Accumulating evidence in humans and other mammals suggests older individuals tend to have smaller social networks. Uncovering the cause of these declines can inform how changes in social relationships with age affect health and fitness in later life. While age-based declines in social networks have been thought to be detrimental, physical and physiological limitations associated with age may lead older individuals to adjust their social behavior and be more selective in partner choice. Greater selectivity with age has been shown in humans, but the extent to which this phenomenon occurs across the animal kingdom remains an open question. Using longitudinal data from a population of rhesus macaques on Cayo Santiago, we provide compelling evidence in a nonhuman animal for within-individual increases in social selectivity with age. Our 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, nor was this narrowing driven by the deaths of social partners. 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 nonhumans, suggesting that patterns of increasing selectivity with age may be deeply rooted in primate evolution.

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Little Evidence Of Between-individual Variation In the Rate ...mentioning

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

Proc. Natl. Acad. Sci. U.S.A.

Valle

Phillips

et al. 2022

Self Cite

“…In this population, older low-ranking females are more likely to be injured, which is strongly associated with increased mortality risk (41). Furthermore, low-ranking individuals exhibit greater increases in inflammatory cells with age (42). Thus, it is possible that older low-ranking females reduce their social integration more strongly with age to mitigate injury risk and the associated costs given their immune-compromised state.…”

Section: Discussionmentioning

confidence: 99%

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

Valle

Phillips

et al. 2022

Preprint

Self Cite

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

Pereira

Brask

et al. 2022

Preprint

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Ageing affects many phenotypic traits, but its consequences for social behaviour have only recently become apparent. Social networks emerge from associations between individuals. The changes in sociality that occur as individuals get older are thus likely to impact network structure, yet this remains unstudied. Here we use empirical data from free-ranging rhesus macaques and an agent-based model to test two things: (1) how age-based changes in social behaviour influence an individual's level of indirect connectedness in their network, and (2) how social ageing feeds up to affect overall network structure. Our empirical analyses revealed that female macaques became less indirectly connected as they aged for some, but not all measures examined, suggesting that ageing animals can remain well integrated in some social contexts. Our agent-based model showed that aged-based changes in social behaviour can scale up to the network level. Networks with a higher proportion of old individuals were less connected and less cohesive. We did not find similar effects of age distribution on the structure of empirical networks, which may reflect limited age variation in our data. Overall, our results suggest a potentially important and underappreciated role of age in the structure and function of animal collectives.