Why Do Females Live Longer than Males?

In wild mammals, females tend to live significantly longer than their male counterparts—just like in humans. In a study published yesterday in PNAS, researchers investigated why.

After looking at the lifespans of 101 different species, the researchers found that females lived an average of 18% longer than males in more than 60% of the species studied. In humans, females tend to live around 7.8% longer. This lifespan difference is not due to the sexes aging at different rates but rather to females having an average lower risk of mortality in adulthood than males.

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

“We’ve known for a long time that women generally live longer than men, but were surprised to find that the differences in lifespan between the sexes was even more pronounced in wild mammals than in humans,” says coauthor Tamás Székely of the University of Bath. “This could be either because females are naturally able to live longer, or that female mortality drops compared with males.”

One example that Székely gives is that lionesses live at least 50% longer in the wild than male lions. Székely’s original thought was that this was due to sexual selection—males fighting males to overtake a pride and access females—but the data did not support this theory.

“Female lions live together in a pride, where sisters, mothers, and daughters hunt together and look after each other, whereas adult male lions often live alone or with their brother and therefore don’t have the same support network,” Székely suggests. “Another possible explanation for the sex difference is that female survival increases when males provide some or all of the parental care. This is also true in birds. Giving birth and caring for young becomes a significant health cost for females and so this cost is reduced if both parents work together to bring up their offspring.”

The researchers plan 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 effect on life expectancy.