It's not just in human populations that females tend to live longer than males. The same trend has been seen in mammals such as elephants, lions, and orcas, according to a new study. Indeed, demographic data for 134 populations of 101 mammalian species shows females outlive males in 60% of cases. And not by a small margin: In human populations, where 90% of people who live to be 110 years old are female, females live 7.8% longer than males on average, reports the BBC. In those mammalian populations, however, females were found to live 18.6% longer. In species where males did live longer—horses and some bats and rabbits among them, per CNN—the difference in life span was significantly smaller, explains Jean-Francois Lemaitre of the French National Centre for Scientific Research, lead author of the study published Monday in PNAS.
A key theory for the discrepancy in life span relates to environment and "sex-specific genetics, the fact that males devote more resources towards specific functions compared with females," Lemaitre tells the BBC. While male and female bighorn sheep, for example, had little difference in life span where natural resources were plentiful, males lived much shorter lives in one particularly harsh environment. "Male bighorn sheep use lots of resources towards sexual competition, towards the growth of a large body mass, and they might be more sensitive to environmental conditions," says Lemaitre. But the study also lines up with recent research indicating an extra X chromosome might protect against destructive mutations. In bird species, the males have two X chromosomes and typically live longer than females, who have only one, as CNN points out. (Read more mammals stories.)