The Evolution of Sex Differences in Mammalian Social Life Histories

Lead Research Organisation: University of Exeter
Department Name: Psychology


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.

Planned Impact

We expect the work to benefit, both nationally and internationally, the following key end-users:

Ecologists and conservation managers in academia, regulatory authorities and NGOs. Providing new knowledge of how social factors contribute to patterns of fecundity and mortality in natural populations will benefit ecologists studying the population dynamics of long-lived social mammals, and managers of mammalian populations that are of conservation concern. Indeed, one of the study populations (the Southern resident killer whale population) is classified as endangered in both Canada and the USA. Advancing our understanding of the relationship between population social structure and patterns of survival and mortality in the population will help inform future conservation strategies of this and other killer whale populations. Impact in this area will be facilitated by our links with NOAA and the Whale and Dolphin Conservation trust (see letters of support).

Wildlife ecotourism. Whale watching is a significant and growing part of the international ecotourism market. In 2009 for example, more than 13 million people took whale watching tours across 119 countries, which generated an expenditure of $2.1 billion [1]. An important part of this market is based around the Southern resident killer whale population, which is the focus of the behavioural work in this study. For example, the whale watching industry in Washington and British Colombia attracts almost 1 million whale watchers annually and generates $180 million of expenditure [1]. In the UK the whale watching market generates a total expenditure of $21 million per annum [1]. Whilst this market is based mainly around smaller cetaceans, it also includes killer whales in Scottish waters. The outputs of this research will both increase the awareness of potential consumers to this market and provide the ecotourism industry with key information on the biology and ecology of killer whales, which they can use to inform tourists. Impact in this area will be facilitated by our existing links with the Pacific Whale Watch Association, a group of 32 dedicated whale watching businesses that annually take approximately 500,000 visitors on whale watching trips (see letter of support).

The general public and schools. Social mammals are intrinsically appealing to the general public and we aim to capitalise on this interest to improve the public understanding of science and conservation issues. We will aim to inform the public about how social factors shape sex differences in life history in natural populations and how this helps us to understand how our own life history evolved. We will work with school children and young people who are at critical points in their education with the aim of inspiring them to follow a career path in the sciences and raising their awareness of the natural world and the threats that it faces.

[1] O'Connor, S., Campbell, R., Cortez, H., & Knowles, T. (2009). Whale Watching Worldwide: tourism numbers, expenditures and expanding economic benefits, a special report from the International Fund for Animal Welfare. Yarmouth MA, USA, prepared by Economists at Large.


10 25 50
publication icon
Franks D (2020) Calculating effect sizes in animal social network analysis in Methods in Ecology and Evolution

publication icon
Nattrass S (2019) Postreproductive killer whale grandmothers improve the survival of their grandoffspring. in Proceedings of the National Academy of Sciences of the United States of America