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.