Mother theory? Why female gorillas live long after their last birth

Berlin (dpa) – Unusual for animals from a biological point of view, female mountain gorillas often live many years after last giving birth – just like people. This may be to care for their young, according to scientists from the University of Turku in Finland and the Max Planck Institute for Evolutionary Anthropology in Leipzig. They weighed three decades of life history and behavioural data from 25 wild mountain gorillas in Bwindi Impenetrable National Park in Uganda, for a study published in the journal Proceedings of the National Academy of Sciences (PNAS). Seven of the females examined lived more than 10 years after their last birth – more than twice as long as the average interval between two births, the researchers found. Six of the animals were older than 35, the highest reproductive age observed in mountain gorillas to date. Wild female gorillas rarely reach the age of 50, so these post-reproductive years, meaning years after fertility, account for at least a quarter of their adult life, the researchers say. How is this possible? There are several possible explanations for why female gorillas continue to live so long without reproducing, even though this does not seem to offer any evolutionary advantage for the species. The grandmother hypothesis assumes that females live longer to help their grandchildren by protecting them or passing on knowledge – a theory also applied to whales and humans. The grandmother effect in humans is probably due to the fact that grandmothers took care of their grandchildren early on in human history, scientists say. Childhood lasts relatively long in humans because the nervous system needs a long time to mature. Under these conditions, the theory is that the chances of survival for offspring were better if grandmothers could care for their grandchildren for as long as possible, relieving their daughters. Grandmother or mother theory? But such intergenerational assistance is unlikely among gorillas, as both females and males often leave their birth groups before they reproduce and thus live with relatives less frequently, according to the researchers. The mother hypothesis seems more likely – in which older females stop reproducing in order to save energy and other resources and instead invest in their existing offspring. Observations show that mothers also play an important role for adult animals and that females that no longer reproduce are often in better physical condition than older, reproducing females. But the scientists also say the long lifespan after reproduction in gorillas may not be a targeted evolutionary trait, but a by-product of longer life expectancy. In the wild, many animals die early, meaning there is no selection against genes that only cause problems in old age. But with animals living longer, thanks to fewer predators or better conditions, such effects become apparent, such as continuing to live long after the end of reproduction. The following information is not intended for publication dpa ram xxde arw coh

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