Date of Award
Doctor of Philosophy (PhD)
Michael L. Platt
Across primates, socially integrated individuals exhibit improved genetic fitness compared to their peripheral conspecifics. However, the mechanisms through which this disparity operates is unclear. One hypothesis is that social bonds mitigate the stressors imparted by environmental instability. To date, this relationship has gone critically unexamined, owing to the inability to anticipate and account for instability in prospective research design. In this dissertation, I evaluate this hypothesis within a population of free-ranging rhesus macaques (M. mulatta). I employ a long-term behavioral data set comprised of 691 unique individuals across 6 groups, followed from 1 to 8 years. I employ the tool-kit of social network analysis to integrate social phenotypes at the individual, dyadic, and group-level. I examine two destabilizing events. First, I examine how affiliation patterns precede a matrilineal overthrow, an outburst of aggression which resulted in group dissolution. As a prelude to this analysis, I investigate the manifestation of rank instability more generally, and, in concert with other dimensions of social life, examine its relationship to psychosocial stress. Second, I examine affiliation in the wake of environmental disaster: here, that of Hurricane Maria, which in 2017 had caused near-complete deforestation. I find that instability leaves an enduring signature in expressions of psychosocial stress. In the context of instability, monkeys show critical transformations in their affiliative patterns — in frequency, variability, and direction — but in ways highly dependent on the type of instability experienced. Such transformations are not experienced uniformly across social units, but instead are concentrated upon those most likely to suffer (or endure) the costs of that instability. This analysis indicates that rhesus monkeys exhibit considerable social flexibility in response to environmental instability and that social bonds, rather than static, are continually renegotiated. Destabilizing events are rare but can impart instantaneously brutal fitness consequences. It is critical, then, that we accumulate case studies of their effects —messy though they may be — to avoid blind spots in our understanding of the evolution of social bonds.
Larson, Sam, "The Affiliative Social Relations Of Free-Ranging Rhesus Macaques In The Context Of Instability" (2019). Publicly Accessible Penn Dissertations. 3261.