Why do females live longer than males?

The mystery of lifespan between the sexes could be solved, says scientists.

According to research, recently published in the journal Proceedings of the National Academy of Sciences (PNAS), on average - female mammals live 18% longer than males. 

Scientists looked at the lifespan of 101 different species - from sheep to elephant and found females had a longer lifespan in over 60 of the species.

When it comes to humans, women tend to live 7.8% longer than men.

It was unclear from the data as to why females survive longer than males, however the researchers suggest that it could be due to complex interactions between the local environmental conditions and sex-specific costs of reproduction.

"It was already known that women live longer than men, but we were surprised to find that these differences in life expectancy are even more pronounced in wild mammals," explains Tamás Székely of the Milner Centre for Evolution at the University of Bath. 

"This could be because women are naturally able to live longer or because female mortality is lower compared to that of men,” he continued. 

In the case of lions - lionesses live 50% longer than their male counterparts. It was previously thought this was mostly due to sexual selection, because males fight with each other to gain access to females, but the data did not support this theory. 

"Lionesses tend to live in groups: sisters, mothers and daughters hunt together and look after each other, while adult male lions often live alone or with their siblings and therefore do not have the same support network," explains the researcher, although this explanation is only a hypothesis that needs to be tested.

"Another possible explanation would be that female survival increases when males assume some of the parental care, as is the case with birds. Giving birth and caring for the young implies a very high health cost for the females, and therefore this cost is reduced if both parents collaborate in raising the cubs," explains Székely.

The researchers next plan is to compare the data on wild animals with that of captive zoo animals, which do not have to deal with predators or competition for food or mates. 

This will allow them to measure the extent to which biological differences between the sexes have an impact on life expectancy.

The impact of disease, viruses and other harsh environmental conditions is thought to have an impact on sex-differences in lifespan, the researchers say. They hope with further research a clearer understanding of why these differences are so prevalent will be established. 

Reference: Lemaître et al. (2020) "Sex differences in adult lifespan and aging rates of mortality across wild mammals", Proceedings of the Academy of Natural Sciences, March 23, 2020 DOI: 10.1073/pnas.1911999117