The Evolution of Prolonged Post-Reproductive Lifespan in a Non-Human Mammal

Understanding why females stop reproduction prior to the end of their lives is a key objective in the biological, medical and social sciences. In traditional human societies for example, women typically have their last child at 38 but may live for a further 20 years or so. This phenomenon is by no means restricted to humans and across many species of mammals, birds and fish, females may have a lifespan that extends far beyond their last birth. Why is this? Three possible reasons have been suggested: i) It could simply be a byproduct of females living for a long time; ii) it may benefit post-reproductive females by increasing the survival of their offspring and/or grand offspring or iii) old females may lose out to young females when competing for the food needed to support pregnancy and producing milk. In humans it seems that a combination of ii and iii have driven the evolution of menopause. Currently however, almost nothing is known about the forces that have shaped the post-reproductive lifespan in non-human animals that live in close-knit family groups. In this project we will test for the first time the current evolutionary theory for the post-reproductive lifespan in a non-human animal.

Our study will focus on two populations of killer whales Orcinus orca that live off the coast of North America. Killer whales have the longest post-reproductive lifespan of all non-human animals; females stop reproducing in their 30s-40s but can survive into their 90s. We will use data collected over the last three decades during which time more than 600 whales have been recorded. We will use information about births and deaths to examine how social factors shape fertility and survival. In particular we will ask the following questions: (1) How do post-reproductive females benefit from a post-reproductive lifespan? (2) In what ways do older females provide support to their offspring / grand offspring? (3) Do older females lose out when competing with younger females for the food needed to reproduce? (4) Can the observed benefits (question 1) and the consequences of reproductive competition (question 3) explain the evolution of the long post-reproductive lifespan in killer whales?

We will address questions 1 and 3 by using the long term data documenting births and deaths in both populations. We will use analysis techniques similar to those used by insurance companies to calculate life expectancy when deciding what premiums to charge people on their life insurance. In our analysis we will examine how survival is affected by the presence and behavior of post-reproductive females. We will address question 2 by using video and photographic records to examine social interactions between mothers and their offspring / grand offspring. We will test how important this relationship is for survival. Finally we will address question 4 by building a simulation model of the populations. We will use our observations from the whales to set the parameters in the model [e.g. the amount by which post-reproductive females increase the survival of their offspring]. The model will then simulate evolution, allowing us to examine if the effects we are seeing in the populations are sufficient to have driven the evolution of the long post-reproductive lifespan in killer whales.

This programme of research promises to advance our understanding of how natural selection has shaped life history evolution in species that live in close-knit family groups. Our work will provide the first test of the current evolutionary theory for the evolution of menopause in non-human animals and the outputs of this work will provide an informative comparison for the evolution of human life history. More generally, our work will advance our understanding of the ageing process in social species and the interplay between an individual's social relationships and its life expectancy.

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 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. More generally our work will provide a template for how to quantify the interplay between a population's social structure and fecundity and survival, which will be applicable to other endangered social species.

Wildlife ecotourism. Whale watching is a major part of the international ecotourism market. A recent report by the International Fund for Animal Welfare highlighted that more than 13 million people took whale watching tours across 119 countries in 2009, which generated an expenditure of $2.1 billion [1]. An important part of this market is based around the Southern and Northern Resident killer whale populations that are the focus of this study. For example, in 2008 the whale watching industry in Washington and British Colombia had 89 tour operators which attracted 855,600 whale watchers and generated $180 million of expenditure [1]. Across the UK the whale watching market has seen very strong growth [8% per annum], with total expenditure of $21 million in 2008 [1]. Whilst this market is based mainly around smaller cetaceans, it also includes the 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.

The General Public and Schools. Social mammals are intrinsically appealing to the general public and we aim to capitalize 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 reproductive senescence 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.