A long post-reproductive lifespan is a shared trait among genetically
distinct killer whale populations
Abstract
The extended female post-reproductive lifespan found in humans and some
toothed whales remains an evolutionary puzzle. Theory predicts
demographic patterns resulting in increased female relatedness with age
(kinship dynamics) can select for a prolonged post-reproductive lifespan
due to the combined costs of inter-generational reproductive conflict
and benefits of late-life helping. Here we test this prediction using
>40 years of longitudinal demographic data from the
sympatric yet genetically distinct killer whale ecotypes: resident and
Bigg’s killer whales. The female relatedness with age is predicted to
increase in both ecotypes, but with a less steep increase in Bigg’s due
to their different social structure. Here, we show that there is a
significant post-reproductive lifespan in both ecotypes with
>30% of adult female years being lived as
post-reproductive, supporting the general prediction that an increase in
local relatedness with age predisposes the evolution of a
post-reproductive lifespan. Differences in the magnitude of kinship
dynamics however, did not influence the timing or duration of the
post-reproductive lifespan with females in both ecotypes terminating
reproduction before their mid-40s followed by an expected
post-reproductive period of ~20 years. Our results
highlight the important role of kinship dynamics in the evolution of a
long post-reproductive lifespan in long-lived mammals, while further
implying that the timing of menopause may be a robust trait that is
persistent despite substantial variation in demographic patterns among
population.