Temporal dynamics of mother-offspring relationships in Bigg’s killer whales: opportunities for kin-directed help by post-reproductive females

Age-related changes in the patterns of local relatedness (kinship
dynamics) can be a significant selective force for shaping the evolution
of life history and social behaviour. In humans and some species of
toothed whales, average female relatedness increases with age which can
select for a prolonged post-reproductive lifespan in older females due to
both the costs of reproductive conflict and the benefits of late-life
helping of kin. Killer whales (Orcinus orca) provide a valuable system for
exploring social dynamics related to such costs and benefits in a mammal
with an extended post-reproductive female lifespan. We use >40
years of demographic data to investigate the opportunities for helping and
harming in the mammal-eating Bigg’s killer whale by quantifying how
mother-offspring social relationships change with offspring age. Our
results suggest a high degree of male philopatry and female-biased budding
dispersal in Bigg’s killer whales, with some variability in the dispersal
rate for both sexes. These patterns of dispersal provide opportunities for
late-life helping of particularly adult sons, while partly mitigating the
costs of mother-daughter reproductive conflict. This is an important step
towards understanding the evolution of menopause in the few species it
occurs.