Social network shrinking is explained by active and passive effects but not increasing selectivity with age in wild macaques

Evidence of social disengagement, network narrowing, and social
selectivity with advancing age in several non-human animals challenges our
understanding of the causes of social ageing. Natural animal populations
are needed to test whether social ageing and selectivity occur under
natural predation and extrinsic mortality pressures, and longitudinal
studies are particularly valuable to disentangle the contribution of
within-individual ageing from the demographic processes that shape social
ageing at the population level. Data on wild Assamese macaques (Macaca
assamensis) were collected between 2013 and 2020 at the Phu Khieo Wildlife
Sanctuary, Thailand. We investigated the social behaviour of 61 adult
females observed for 13,270 hours to test several mechanistic hypotheses
of social ageing and evaluated the consistency between patterns from
mixed-longitudinal and within-individual analyses. With advancing age,
females reduced the size of their social network, which could not be
explained by an overall increase in the time spent alone, but by an
age-related decline in mostly active, but also passive, behaviour, best
demonstrated by within-individual analyses. A selective tendency to
approach preferred partners was maintained into old age but did not
increase. Our results contribute to our understanding of the driver of
social ageing in natural animal populations and suggest that social
disengagement and selectivity follow independent trajectories during
ageing.