Abstract
Usually animals reproduce into old age, but a few species such as humans and killer whales can live decades after their last reproduction. The grandmother hypothesis proposes that such life-history evolved through older females switching to invest in their existing (grand)offspring, thereby increasing their inclusive fitness and selection for post-reproductive lifespan. However, positive grandmother effects are also found in non-menopausal taxa, but evidence of their associated fitness effects is rare and only a few tests of the hypothesis in such species exist. Here we investigate the grandmother effects in Asian elephants. Using a multigenerational demographic dataset on semi-captive elephants in Myanmar, we found that grandcalves from young mothers (<20 years) had 8 times lower mortality risk if the grandmother resided with her grandcalf compared to grandmothers residing elsewhere. Resident grandmothers also decreased their daughters’ inter-birth intervals by one year. In contrast to the hypothesis predictions, the grandmother’s own reproductive status did not modify such grandmother benefits. That elephant grandmothers increased their inclusive fitness by enhancing their daughter’s reproductive rate and success irrespective of their own reproductive status suggests that fitness-enhancing grandmaternal effects are widespread, and challenge the view that grandmother effects alone select for menopause coupled with long post-reproductive lifespan.
Highlights
Humans, killer whales (Orcinus orca) and short-finned pilot whales (Globicephala macrorhyncus) have an exceptional life-history, with a notable proportion of all females in a given population post-reproductive and one quarter to half of the long lifespan spent as post-reproductive[1,2,3]
These questions are in pivotal role to unravel the selection pressures on post-reproductive lifespan and to elucidate whether the fitness benefits of grandmaternal effects alone lead to the evolution of menopause, or whether such effects are widespread in social species with and without menopause and extended post-reproductive lifespan
Asian elephant (Elephas maximus) females which survive to old age can have relatively long lives after last reproduction, at the population level, 17 years after age 47 and at the individual level 11 years for those females living beyond 40 years[3]
Summary
Killer whales (Orcinus orca) and short-finned pilot whales (Globicephala macrorhyncus) have an exceptional life-history, with a notable proportion of all females in a given population post-reproductive and one quarter to half of the long lifespan spent as post-reproductive[1,2,3]. Grandmothers with heavy own current reproductive costs are predicted to invest less in their grandoffspring than post-reproductive grandmothers without such costs[19,25], but whether the detected grandmaternal effects are greater for grandmothers which have ceased reproduction and whether these helping females are long-lived compared to other females in line with the predictions of the grandmother hypothesis are largely unknown These questions are in pivotal role to unravel the selection pressures on post-reproductive lifespan and to elucidate whether the fitness benefits of grandmaternal effects alone lead to the evolution of menopause, or whether such effects are widespread in social species with and without menopause and extended post-reproductive lifespan. We test this hypothesis by investigating first the effects of (grand)mother presence on the reproductive success of their daughters by comparing whether a living grandmother residing and working in the same vs another logging location as the grandoffspring (at the time of the birth) has an effect on the grandoffspring survival until their weaning age at 5 years. This study provides a much needed comparison of a long-lived non-menopausal terrestrial mammal with kin-based social groups to species with menopause and decades long post-reproductive survival, as these traits are suggested to have evolved through fitness enhancing grandmother effects
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