The Evolutionary Ecology of Social Ageing

The quantity and quality of our friendships can profoundly impact our health and lifespan. The benefits of social relationships are not unique to humans; across species, individuals with strong social connections, or “friendships”, generally experience greater reproductive success and longevity. However, despite these benefits, a perplexing phenomenon is that social connectivity - the number and strength of social ties - often declines with age. Understanding the causes and consequences of this phenomenon, termed social senescence, is a critical interdisciplinary challenge. Social senescence has generally been attributed to biological ageing processes. Age-related declines in cognition, mobility, and sensory perception may hinder individuals' ability to form and maintain social bonds in old age. However, significant variation in social ageing patterns exists across species, sexes, and populations. Indeed, individuals sometimes maintain or even increase their social engagement in later life. These observations challenge the prevailing view that social senescence is merely a byproduct of biological ageing. This project will develop an innovative, adaptive framework to understand why social ageing occurs and why patterns of social ageing differ between individuals, the sexes and species. We propose that social senescence is an adaptive process shaped by evolutionary trade-offs in the costs and benefits of forming and maintaining social relationships across the lifespan. Building and sustaining social ties early in life can provide long-term benefits, including improved reproductive success and survival. However, as individuals age, the returns on these investments diminish, potentially leading to a reduction in social investment and social connectivity. Despite its potential significance, such an adaptive framework for social ageing remains unexplored. We predict that adaptive patterns of social ageing will depend on the degree of relatedness between social partners. Kin relationships offer indirect benefits - by helping relatives survive and reproduce, individuals can promote the transmission of shared genes. Thus, we predict that the patterns of social interactions with kin across the lifespan will shape the evolution of social ageing. Our previous work reveals substantial variation across species and the sexes in how social interactions with kin change across the lifespan. We predict that these species and sex differences in kinship dynamics will explain variation in social ageing patterns. This research will develop a novel theoretical framework to predict how individuals adaptively allocate social effort (forming and maintaining relationships) over their lifespan and how age-related changes in these investments drive social ageing. Using this framework, we will predict the evolutionary consequences of varying patterns of interactions with relatives across the lifespan for social ageing. These predictions will be tested using over 50 years of longitudinal data on Southern Resident killer whales - a population characterised by strong kinship structures and well-documented social behaviour - and comparative datasets on social ageing across mammal societies. This research is inherently interdisciplinary, spanning biology, ecology, social sciences, health and psychology. It will advance our understanding of social ageing by providing an adaptive framework grounded in evolutionary theory. Practically, the findings will support conservation efforts for the endangered Southern Resident killer whales, shedding light on how shifts in population age structure influence social dynamics and population resilience. More broadly, as the number of people over 65 is projected to double by 2025, understanding how and why social relationships adaptively change with age has the potential to shape societal perspectives on ageing and help foster more age-aware communities.