Social ageing: exploring the drivers of late-life changes in social behaviour in mammals

Abstract

Social interactions help group-living organisms cope with socio-environmental challenges and are central to survival and reproductive success. Recent research has shown that social behaviour and relationships can change across the lifespan, a phenomenon referred to as 'social ageing'. Given the importance of social integration for health and well-being, age-dependent changes in social behaviour can modulate how fitness changes with age and may be an important source of unexplained variation in individual patterns of senescence. However, integrating social behaviour into ageing research requires a deeper understanding of the causes and consequences of age-based changes in social behaviour. Here, we provide an overview of the drivers of late-life changes in sociality. We suggest that explanations for social ageing can be categorized into three groups: changes in sociality that (a) occur as a result of senescence; (b) result from adaptations to ameliorate the negative effects of senescence; and/or (c) result from positive effects of age and demographic changes. Quantifying the relative contribution of these processes to late-life changes in sociality will allow us to move towards a more holistic understanding of how and why these patterns emerge and will provide important insights into the potential for social ageing to delay or accelerate other patterns of senescence.

Keywords: ageing; evolution; mammals; senescence; social behaviour.

Figure 1.

Challenges to disentangling explanations for age-related changes in adult social behaviour. Here, we make example predictions for how sociality might be expected to change with age according to each of the seven explanations outlined in the text, using a hypothetical mammal species that is group-living, with low levels of fission–fusion, where one sex is philopatric and the other disperses, and where both sexes experience declines in reproductive value with age. Explanations resulting from senescent decline have green shading, explanations that are secondary adaptations to senescence have purple shading, and explanations resulting from the positive effects of age and demographic changes have blue shading. In the predictions under ‘kinship dynamics’, dotted lines represent philopatric individuals, while dashed lines represent dispersers. We have chosen three measures of prosocial behaviour that have commonly been measured in the social ageing literature (e.g. [10,14,16,35]), although there are likely to be many additional metrics of interest, including centrality, clustering and betweenness, among others. We have included supporting references that either provide some empirical evidence that a given explanation might be driving social ageing and/or offer theoretical support for the predictions shown. We acknowledge that the predictions outlined here may change depending on the study system in question, requiring a clear understanding of an organism's ecology and life history. Regardless, our example predictions clearly show that it is likely to be challenging to quantify the relative contribution of these explanations by studying behavioural outcomes alone because predictions are similar for many explanations. This, in turn, indicates that integrative approaches involving longitudinal data, physiological markers and/or experiments are needed (see box 1).