Abstract
The extended female postreproductive life span 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 postreproductive life span due to the combined costs of intergenerational 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 postreproductive life span in both ecotypes with >30% of adult female years being lived as postreproductive, supporting the general prediction that an increase in local relatedness with age predisposes the evolution of a postreproductive life span. Differences in the magnitude of kinship dynamics however did not influence the timing or duration of the postreproductive life span with females in both ecotypes terminating reproduction before their mid‐40s followed by an expected postreproductive period of about 20 years. Our results highlight the important role of kinship dynamics in the evolution of a long postreproductive life span 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 populations.
Highlights
The evolution of an extended female postreproductive life span is extremely rare in nature and is at present only known in five species of wild mammals (Croft et al, 2015; Ellis, Franks, Nattrass, Cant, Bradley, et al, 2017; Ellis et al, 2018)
When taken together with previous work, our findings support the hypothesis that kinship dynamics play a key role in the evolution of a prolonged postreproductive life span in killer whales
Contrary to our predictions, the timing and expected duration of the postreproductive life span did not vary with the dispersal pattern from the natal group, which likely represents different costs and benefits of helping and harming in the two ecotypes
Summary
The evolution of an extended female postreproductive life span is extremely rare in nature and is at present only known in five species of wild mammals (Croft et al, 2015; Ellis, Franks, Nattrass, Cant, Bradley, et al, 2017; Ellis et al, 2018). Outside of the prolonged postreproductive life span seen in humans the only other species of mammals in which females have evolved early cessation of reproduction are toothed whales: short-finned pilot whales (Globicephala macrorhynchus), narwhals (Monodon monoceros), belugas (Delphinapterus leucas), and resident-ecotype killer whales (Orcinus orca) (Ellis, Franks, Nattrass, Cant, Bradley, et al, 2017; Johnstone & Cant, 2010). Grandmother benefits appear to be key for the evolution of a long postreproductive life span (Hawkes, 2004; Hawkes & Coxworth, 2013) and recent work in resident killer whales provides support for both the mother and grandmother hypothesis with the presence of both mothers and postreproductive grandmothers having a positive impact on the survival of their adult offspring and grandofffspring (Foster et al, 2012; Nattrass et al, 2019). Recent work has shown that kin-selected costs, as well as benefits, are important for the evolution of extended postreproductive life spans (Cant & Johnstone, 2008; Johnstone & Cant, 2010)
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