| Summary: | In a handful of mammals, females show an extended post-reproductive lifespan (PRLS), leading to questions over why they spend a substantial portion of their lifespan non-reproductive. Theoretical and empirical studies suggest that PRLS may evolve when (1) demographic patterns lead to increasing local relatedness as females age, and (2) females come into reproductive competition with their daughters, as these conditions lead to high relative benefits of helping kin versus reproducing in later life. However, evolutionary pathways to PRLS are poorly understood and empirical studies are scarce. Here, we use a dataset of 1522 individuals comprising 22 pods to investigate patterns of reproduction and relatedness in long-finned pilot whales Globicephala melas; a toothed whale without species-wide PRLS. We find a similar relatedness structure to whales with PRLS: pods appear composed of related matrilines, and relatedness of females to their pod increases with age, suggesting that this species could benefit from late-life help. However, females with a large number of philopatric adult daughters (but not sons) are less likely to reproduce, implying intergenerational reproductive competition between females. This suggests that individuals may display a plastic cessation of reproduction, switching to investing in existing offspring when they come into competition with their daughters. To the best of our knowledge, this is the first time such a relationship has been described in relation to PRLS, and it raises questions about whether this represents a step towards evolving PRLS or is a stable alternative strategy to widespread post-reproductive periods. Nichols et al pilot whale female reproduction dataset, and relatedness dataset Dataset containing female reproductive data: female identity, social group (pod) identity, female age (years), number of offspring genetically assigned to the female, the number of genetically assigned adult offspring aged 10+, number of foetuses (whether or not the focal female was pregnant at ...
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