Abstract
Understanding why females of some mammalian species cease ovulation prior to the end of life is a long-standing interdisciplinary and evolutionary challenge. In humans and some species of toothed whales, females can live for decades after stopping reproduction. This unusual life history trait is thought to have evolved, in part, due to the inclusive fitness benefits that postreproductive females gain by helping kin. In humans, grandmothers gain inclusive fitness benefits by increasing their number of surviving grandoffspring, referred to as the grandmother effect. Among toothed whales, the grandmother effect has not been rigorously tested. Here, we test for the grandmother effect in killer whales, by quantifying grandoffspring survival with living or recently deceased reproductive and postreproductive grandmothers, and show that postreproductive grandmothers provide significant survival benefits to their grandoffspring above that provided by reproductive grandmothers. This provides evidence of the grandmother effect in a nonhuman menopausal species. By stopping reproduction, grandmothers avoid reproductive conflict with their daughters, and offer increased benefits to their grandoffspring. The benefits postreproductive grandmothers provide to their grandoffspring are shown to be most important in difficult times where the salmon abundance is low to moderate. The postreproductive grandmother effect we report, together with the known costs of late-life reproduction in killer whales, can help explain the long postreproductive life spans of resident killer whales.
Highlights
IntroductionReproductive senescence is unusually accelerated relative to somatic senescence in humans (Homo sapiens), shortfinned pilot whales (Globicephala macrorhynchus), belugas (Delphinapterus leucas), narwhals (Monodon monoceros), and resident-ecotype killer whales (Orcinus orca; hereafter killer whales) [4]
Female killer whales have extremely long postreproductive life spans; they stop reproducing in their late 30s and early 40s [4, 8] but can continue to live for decades thereafter [4, 5, 9]; 64% of female killer whales that survive to the age of 10 y will live to be postreproductive, at which point they will have an expected postreproductive life span of 15.78 y
We considered a number of models with a variety of terms (SI Appendix) including a general grandmother effect, and an additional effect of the grandmother being postreproductive
Summary
Reproductive senescence is unusually accelerated relative to somatic senescence in humans (Homo sapiens), shortfinned pilot whales (Globicephala macrorhynchus), belugas (Delphinapterus leucas), narwhals (Monodon monoceros), and resident-ecotype killer whales (Orcinus orca; hereafter killer whales) [4]. This manifests as a prolonged postreproductive life span: a long period of a female’s life after reproduction where she can no longer reproduce [5]. We show that grandmothers increase the survival of their grandoffspring, and these effects are greatest when grandmothers are no longer reproducing These findings can help explain why killer whales have evolved the longest postreproductive life span of all nonhuman animals
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have