Article

Postreproductive female killer whales reduce socially inflicted injuries in their male offspring

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Abstract

Understanding the evolution of menopause presents a long-standing scientific challenge1,2,3-why should females cease ovulation prior to the end of their natural lifespan? In human societies, intergenerational resource transfers, for example, food sharing and caregiving, are thought to have played a key role in the evolution of menopause, providing a pathway by which postreproductive females can boost the fitness of their kin.4,5,6 To date however, other late-life contributions that postreproductive females may provide their kin have not been well studied. Here, we test the hypothesis that postreproductive female resident killer whales (Orcinus orca) provide social support to their offspring by reducing the socially inflicted injuries they experience. We found that socially inflicted injuries, as quantified by tooth rake marks, are lower for male offspring in the presence of their postreproductive mother. In contrast, we find no evidence that postreproductive mothers reduce rake marking in their daughters. Similarly, we find no evidence that either reproductive mothers or grandmothers (reproductive or postreproductive) reduce socially inflicted injuries in their offspring and grandoffspring, respectively. Moreover, we find that postreproductive females have no effect on reducing the rake marks for whales in their social unit who are not their offspring. Taken together, our results highlight that directing late-life support may be a key pathway by which postreproductive females transfer social benefits to their male offspring.

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... As long as the benefit to overall reproductive fitness of the lineage outweighs the cost of the female producing no further progeny at late ages, then the menopause can be selectively advantageous. Consistent with the Grandmother hypothesis, female reproductive cessation and postreproductive caregiving has now been observed in multiple species, including humans and killer whales (Hawkes and Coxworth, 2013;Croft et al., 2017;Grimes et al., 2023;Nielsen et al., 2023). It is possible to consider this mechanism in terms of an AB replicator model. ...
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... Understanding the taxonomic prevalence of long periods of post-reproductive life (relative to lifespan) is vital so that we can identify other species that could be studied to help understand how extended post-reproductive periods could have evolved and been maintained. Long-term individual-based datasets on wild mammal populations, such as killer whales (Orcinus orca) that have comparable post-reproductive life to humans, has allowed us to test the generality of the evolutionary framework proposed to explain the evolution of menopause in humans and provides a very rare and informative comparison for human life history evolution [33][34][35][36][37][38][39] . While observing that females of the captive population can have artificially long lifespans and post-reproductive life may open up opportunities for new work to study the physiology of the ageing process, captive populations can tell us little about how menopause has evolved. ...
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Reproductive senescence is widespread across mammals, but only a small number of species have physiological reproductive cessation and an extended post-reproductive lifespan. A recent commentary in Cell by Winkler & Goncalves (2023) suggests that menopause is actually a widespread trait of mammals, which would change our understanding of senescence and have implications for the study of menopause in humans. Here, we identify three main issues with the methodology of this commentary: the use of captive populations, the use of maximal lifespan, and misinterpretations of the data sources. We show that this methodology does not support the conclusions at the species-level, and conclude that, in line with the predictions of classic life-history theory, menopause is indeed a rare trait.
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