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Female Asian elephants stop reproducing towards the end of their lives, putting them among a very small number of species that experience something akin to the menopause.

Only humans, orcas, narwhals, beluga whales and short-finned pilot whales are known to exhibit what biologists call extended post-reproductive lifespan. The existence of such a stage is an evolutionary riddle: why should individuals give up trying to leave more descendants?

Simon Chapman at the University of Turku, Finland, and his colleagues studied records of 3802 female Asian elephants that worked in timber camps in Myanmar between 1940 and 2018. This population is considered semi-captive as humans don鈥檛 intervene in their reproduction and their mortality and fertility patterns are similar those of to wild elephants.

The oldest elephants in this group live into their 70s, but most have had their last calf by age 55. The researchers calculate that the proportion of years lived by females in a post-reproductive phase is 16 per cent: some way short of humans鈥� 43 per cent but considerably more than African elephants鈥� 4 per cent.

Although their fertility declines in the later stages of life, there is no cut-off point at which reproduction becomes impossible, as it does in humans. In elephants, the cessation of reproduction may be determined by behaviour, rather than physiology, says Chapman. 鈥淲e think it鈥檚 more of a social thing.鈥� It could be that this behavioural trait is a first step towards evolving a true menopause.

One prominent idea to explain why females might stop having offspring some time before the end of their lives is known as the grandmother hypothesis. If older females help care for their grandchildren, the theory says, they end up leaving more descendants than if they have more children of their own. There is some supporting evidence from studies of human populations before people had access to modern medicine.

in the same group of elephants found that the presence of grandmothers is beneficial for the survival of young elephants, particularly if the mother is young, and that adult females reproduce more quickly if their mothers are around.

However, this was true whether or not the grandmother was still having offspring of her own, so it doesn鈥檛 appear that this can explain why older females would stop reproducing. 鈥淕randmothering could have a role in the evolution of post-reproductive lifespan, but by itself can鈥檛 be driving it,鈥� says Chapman.

For now, the riddle remains unsolved. 鈥淲e鈥檙e filling in pieces and getting closer, but the answer is just eluding us,鈥� he says.