The difference in demography observed between the resident and Bigg’s ecotypes are predicted to generate different patterns of kinship dynamics (age-specific changes in relatedness) which can be illustrated using the theoretical model presented by Johnstone & Cant (2010)4. Previous theoretical work on kinship dynamics in resident killer whales predicted an increase in local relatedness with female age4 which has been confirmed using individual-based demographic and social data from resident killer whales21. Here, we use this established modelling framework to predict the patterns of age-related changes in kinship for female Bigg’s killer whales allowing us to directly compare the predicted kinship dynamics between the resident and Bigg’s ecotype. Using this approach it is predicted that female local relatedness will increase with age for both killer whale ecotypes, albeit with a weaker relationship under the demographic conditions of Bigg’s killer whales (Fig. 1)4. This increase in female local relatedness is opposite to the pattern observed in most mammals (male dispersal and local mating), and it predicts that there will be selection for a post-reproductive female lifespan in both killer whale ecotypes. We hypothesise however, that the difference in the strength of the kinship dynamics will lead to a lower potential for inclusive fitness benefits from helping kin in late life and inclusive fitness costs of reproductive conflict with younger females for Bigg’s females21. Given the predicted differences in kinship dynamics, and assuming the costs and benefits of reproduction with age are equal and that these don’t change with changing dispersal, we predict that selection for a post-reproductive lifespan will be weaker in Bigg’s killer whales compared to resident killer whales4 and we further hypothesise that this will be reflected by an older age at last reproduction, and a shorter post-reproductive lifespan in female Bigg’s killer whales.
Here we test for the presence of a post-reproductive period in Bigg’s killer whales and compare female post-reproductive lifespan between the sympatric resident and Bigg’s killer whale ecotypes. Using over 40 years of individual-based demographic data, we model survival trajectories using a Bayesian hierarchical framework42 and calculate the post-reproductive representation (PrR)43to compare the presence of a long post-reproductive lifespan in both resident and Bigg’s killer whale ecotypes, as well as the timing of a potential post-reproductive lifespan. We show that females in both ecotypes have a prolonged post-reproductive lifespan. However, in contrast to our predictions, the timing and duration of the female post-reproductive lifespan did not appear to differ between ecotypes.