While people are living longer than ever, women almost always have the higher average life expectancy than men\u2014and this is not exclusively a human phenomenon. Mammalian lifespans usually favor females\u2014with some exceptions, they tend to live about 12 percent longer than males. But the opposite is seen in many species of birds, insects, and reptiles, where males often outlive females. <\/p>\n
Genetics may be the reason behind this. It is though that the two X chromosomes of female mammals\u2014which make them monogametic<\/a>\u2014may actually protect them from detrimental genetic mutations that heterogametic males do not have as much of a defense against, as they only have one X and one Y chromosome. This reverses in birds, where males are the monogametic sex and females are heterogametic.<\/p>\n But genetics are not necessarily the only reason female mammals (and male birds) have an advantage. To try and solve the rest of the puzzle, an international team of researchers\u2014led by Johanna St\u00e4rk from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany\u2014investigated what factors can lengthen or shorten male and female lifespans. Zoo records from over a thousand bird and mammal species appeared to support what is known as the heterogametic sex hypothesis<\/a>, and so did data from species in the wild. But it could still not explain exceptions, such as female birds of prey being longer-lived than males. <\/p>\n Related Story<\/p>\n As it turns out, sexual selection<\/a> might be what determines who gets to live longer\u2014one sex of a species may ultimately drain potential lifespan by using too many resources to develop and maintain traits that give them an edge in attracting mates. It is also possible that reproduction itself can take years off the life of one sex. Survival rates can be reduced by the stresses of gestation, giving birth or laying eggs<\/a>, and caring for offspring, though this doesn\u2019t seem to affect the average lifespans of human females. St\u00e4rk\u2019s team focused mostly on data from zoo<\/a> populations because animals living in a zoo are not exposed to the added pressures of predators, illness, injury, or starvation. <\/p>\n \u201cSpecies often live considerably longer in zoos than in the wild, and abundant resources and managed reproduction in zoos may lower individual survival costs associated with growth and reproduction,\u201d the team said in a study recently published in the journal Science Advances<\/a>. \u201cFor example, the influence of sexual size dimorphism on male survival tends to be lower, and the individual cost of producing offspring appears to have no effect on female survival in zoo settings.\u201d<\/p>\n In zoos, female mammals had an even greater lifespan advantage of 16 percent, with male birds at 6 percent. But it was even more surprising that male birds lived an average of 5 times longer in the wild, and female mammals lived 1.5 times longer. There were still some mammals that defied the odds\u2014about 5 percent of male mammals in zoos and 7 percent in the wild outlived females, and the same went for 4 percent of female birds in zoos and slightly over 2 percent in the wild that outlived the males of their species. The female advantage was especially high in ungulates such as hippos, rhinos<\/a>, antelope, and zebras<\/a>, while there was more of a male advantage in carnivorous mammals.<\/p>\n Monogamy and polygamy may also have something to do with how long males and females live. Males of polygamous species of mammals often experience intense competition for a mate. As a result, a larger body size and built-in weapons like teeth<\/a> and claws may give them the advantage with females, but they also may use up enough energy and other body resources to shorten their lifespans. More birds tend to be monogamous (compared to mammals), and without as much pressure to show off for mates, male birds that are not sexually dimorphic<\/a> often have longer lives. <\/p>\n Related Story<\/p>\n There are some outliers, however. Male birds<\/a> of paradise have flamboyant plumage (never mind elaborate mating dances), and most species are polygamous,\u2014all factors which may lower lifespan. But males are still living longer overall across bird species, while females have the longer lifespans among mammals.<\/p>\n
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