Demography, life history and the evolution of age‐dependent social behaviour

Abstract

Since the inception of modern social evolution theory, a vast majority of studies have sought to explain cooperation using relatedness-driven hypotheses. Natural populations, however, show a substantial amount of variation in social behaviour that is uncorrelated with relatedness. Age offers a major alternative explanation for variation in behaviour that remains unaccounted for. Most natural populations are structured into age-classes, with ageing being a nearly universal feature of most major taxa, including eukaryotic and prokaryotic organisms. Despite this, the theoretical underpinnings of age-dependent social behaviour remain limited. Here, I investigate how group age-composition, demography and life history shape trajectories of age-dependent behaviours that are expressed conditionally on an actor and recipient's age. I show that demography introduces novel age-dependent selective pressures acting on social phenotypes. Furthermore, I find that life history traits influence the costs and benefits of cooperation directly, but also indirectly. Life history has a strong impact not only on the genetic structure of the population but also on the distribution of group age-compositions, with both of these processes influencing the expression of age-dependent cooperation. Age of peak reproductive performance, in particular, is of chief importance for the evolution of cooperation, as this will largely determine the age and relatedness of social partners. Moreover, my results suggest that later-life reproductive senescence may occur because of demographic effects alone, which opens new vistas on the evolution of menopause and related phenomena.

Kinship dynamics: patterns and consequences of changes in local relatedness

“…Kinship dynamics mean that the strength and direction of kin selection can change systematically over time [11,12,16,63]. To date however, very few studies have incorporated kinship dynamics into models of social evolution.…”

Resource availability and adjustment of social behaviour influence patterns of inequality and productivity across societies

“…Future work might focus on particular species and study the influence of species-specific factors on social behaviour and wealth transfers among group members. For instance, one might consider a model with overlapping generations in which age mediates the reproductive value and dominance-rank of individuals (e.g., Rodrigues, 2018 ). In addition, one might also consider a model where group size is mediated by resource availability, and where the dispersal rates of group members co-evolve with cooperation (e.g., Rodrigues & Gardner, 2016 ).…”

Senescence: why and where selection gradients might not decline with age

“…For example, helpers can provide load-lightening benefits to breeders [80] that may delay breeder senescence and extend lifespan [90–92]. Helping tendencies may also change with age [93], due, for example, to changing relatedness to its group as an individual helper ages [94], or to changing benefits of independent reproduction [95]. Age trajectories of how individuals could affect both helper and breeder senescence profiles, and lead to plastic senescence patterns within a species.…”