The sociality–health–fitness nexus: synthesis, conclusions and future directions

Abstract

This theme issue has highlighted the links between sociality, health and fitness in a broad range of organisms, and with approaches that include field and captive studies of animals, comparative and meta-analyses, theoretical modelling and clinical and psychological studies of humans. In this concluding chapter, we synthesize the results of these diverse studies into some of the key concepts discussed in this issue, focusing on risks of infectious disease through social contact, the effects of competition in groups on susceptibility to disease, and the integration of sociality into research on life-history trade-offs. Interestingly, the studies in this issue both support pre-existing hypotheses, and in other ways challenge those hypotheses. We focus on unexpected results, including a lack of association between ectoparasites and fitness and weak results from a meta-analysis of the links between dominance rank and immune function, and place these results in a broader context. We also review relevant topics that were not covered fully in this theme issue, including self-medication and sickness behaviours, society-level defences against infectious disease, sexual selection, evolutionary medicine, implications for conservation biology and selective pressures on parasite traits. We conclude by identifying general open questions to stimulate and guide future research on the links between sociality, health and fitness.

Disease implications of animal social network structure: A synthesis across social systems

“…Previous studies have typically focused on group size as the key parameter impacting disease transmission and group living costs. However, the expectation of higher disease costs of group living has yielded mixed results (Arnold & Anja, ; Patterson & Ruckstuhl, ; Rifkin et al., ), which can be explained in part by the presence of group‐level behavioural (Meunier, ; Schaller, Murray, Bangerter, & Schaller, ) and physiological defence (Habig, Archie, & Habig, ) against infection spread, as well as the presence of chronic social stress (Kappeler, Cremer, & Nunn, ; Nunn et al., ). While group size might be an easy parameter to measure, it does not capture the complex spatio‐temporal dynamics of most animal societies.…”

Competing pressures on populations: long-term dynamics of food availability, food quality, disease, stress and animal abundance

“…However, the short-term analysis of parasites does not support an effect of group-size on parasitism (C. A. Chapman 2006-2014, unpublished data). Larger groups were associated with fewer parasites, perhaps an effect of behavioural mechanisms such as cliques in social networks that reduce parasite transmission in larger groups [90][91][92][93].…”

“…This begs the questions: is disease/parasitism offsetting the benefits of increased habitat-wide food (quantity and quality as indicated by the habitat-wide assessment), or is the population responding in compensatory ways to the increased risk of disease, possibly via behavioural flexibility (e.g. avoidance of infected individuals or increased clique formation within social networks) [90,92,93,95]? As the number of studies on different populations of the same species accumulate, there is increasing recognition that there can be considerable flexibility in behaviour and demographic structure, which presumably can influence the numeric response of a species to a given environmental change [96][97][98][99].…”

Primate reinfection with gastrointestinal parasites: behavioural and physiological predictors of parasite acquisition

“…In humans and other social animals, variation in behaviour and physiology can alter the risk of exposure to, and infection by, pathogens, ultimately affecting host fitness (Kappeler, Cremer, & Nunn, 2015; Nunn, Craft, Gillespie, Schaller, & Kappeler, 2015; Silk, 2014). Variation in parasitism often has direct links to host social behaviour, such that infection-related costs of sociality are considered important selective forces in human and animal evolution (Altizer et al, 2003; Kappeler et al, 2015; Møller, Dufval, & Allander, 1993).…”