Social aging in dolphins

Kelley Caroline Meehan , Barbara Class , Shinichi Nakagawa , Meredith MacQueeney, Vivienne Foroughirad, Janet Mann, Celine Frere

Recent work has unearthed strong relationships between aging and average sociability. Clear patterns of decreases in average sociability are observed across taxa; many of these are sex-specific. Individuals, however, generally deviate from population averages, and discounting individual variance in behaviour could disguise mechanisms of adaptation, selection, and developmental stability. Using forty years of behavioural data from Indo-Pacific bottlenose dolphins, we provide new insights into social aging. We achieve this by simultaneously measuring both mean (repeatability) and variance (predictability) in group size—a trait reflecting social interaction preference—across the population and throughout individual lifespans. Double hierarchical location-scale modelling reveals a multidimensional response; at both the population and individual level, the mean trait response (plasticity) and variance (malleability) of group size changes with age. At a population level and for most individuals, average group size increases then declines in later life. This decline is more pronounced in males, though males are consistently in larger and more predictable groups than females at all life stages. Population predictability of group size increases throughout life, consistent with our prediction that social preferences strengthen with age. Individuals which are highly sociable are also highly predictable, and increased sociability and predictability both appear correlated with longevity. The positive correlation between increasing predictability and longevity leads us to hypothesise that individuals develop social competence by refining their social experiences. This process likely involves increasing social selectivity and optimizing social choices to achieve a net fitness benefit. These findings provide novel insights into social aging and illustrate how studying variance in social traits can capture processes of social competence, selectivity, and adaptation of social phenotypes occurring within an individuals’ social environment.