The Evolution of Sex Differences in Mammalian Social Life Histories

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Understanding why social behaviour and life histories have diverged between the sexes in long-lived social mammals, sometimes to an extreme degree, is a key objective in the biological, medical and social sciences. We propose that differences between the sexes in how males and females interact with related individuals across the lifespan are a major force driving the evolution of sex differences in both social behaviour and life history. In social species related individuals (kin) often live together in close knit family groups and individuals can influence the survival and reproductive success of their relatives both by their behaviour (cooperative and competitive) and reproductive decisions (if an individual reproduces it will use resources that may negatively impact on the survival and reproductive success of kin). Such interactions between kin are a strong evolutionary force, with individuals gaining indirect benefits (through the genes they share with relatives) by increasing the survival and reproductive success of their kin. For example, kin selection can favour individual strategies that increase the reproductive success of kin, even if this comes at a cost to an individual's survival and reproductive success. The opportunity for evolution to be shaped by kin selection is dependent on how and when related individuals interact. Our pilot work suggests that in many species males and females experience very different patterns of local relatedness across their lifespans (kinship dynamics) due to patterns of dispersal and mating. For example, for some species such as killer whales, females become more related to their local group with age whereas males, in contrast, become less related. We hypothesise that sex differences in kinship dynamics will be a major force driving the evolution of sex differences in both (i) social behaviour and (ii) life history. We will determine the role of kinship dynamics in driving the evolution of sex differences in social behaviour and life history evolution using a combination of theoretical modelling and empirical data analysis. We will develop a general theory of kinship dynamics and develop new models to predict both the patterns of kinship dynamics and their consequences for the evolution of social life histories in both males and females. Our model will make predictions for patterns of helping and harming in social groups including both behavioural traits (cooperation and conflict) and reproductive traits (e.g. age at first or last reproduction). We will provide the first test of the behavioural predictions of our new theory by collecting new data on patterns of helping (e.g. babysitting and food sharing) and harming (aggression) in resident killer whales using unmanned aerial vehicles (drones). Resident killer whales are ideally suited to testing the behavioural predictions of the model - they have extreme sex differences in kinship dynamics and have unexplained sex differences in life history. We will test the life history predictions of the theoretical framework by comparing patterns of kinship dynamics and sex differences in life history evolution (e.g. patterns of growth, age at first reproduction, age at last reproduction, reproductive investment and longevity) across social mammals which will allow us to determine the role of kinship dynamics in driving life history evolution and the divergence of life histories between the sexes.