Female mammals live longer than males because of their sexual behavior, study suggests

By Amarachi Orie, CNN

(CNN) — Sexual behavior is one reason why female mammals typically live longer than their male counterparts, according to a new study.

Researchers from Germany, Denmark, France, Hungary and Belgium analyzed data relating to 528 mammal species and 648 bird species in zoos, in the most comprehensive study to date on sex differences in adult life expectancy in mammals and birds, published in the journal Science Advances on Wednesday.

They also analyzed data on populations in the wild for 110 species, to see if the findings would be the same in natural settings.

Among mammals, adult females lived 12% longer than males, the study found. However, the opposite trend was found among birds, with adult males living 5% longer.

The differences in longevity between the sexes were much more pronounced in the wild than in zoos for both mammals and birds, as there were fewer stressors, such as harsh climates and predators, in zoos, the study said.

Competing theories

There are several theories about why there are these sex differences in longevity. A leading theory relates to the sex chromosomes, explained study co-author Fernando Colchero, a statistical and mathematical ecologist who leads the Statistical Demography Group at Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

In this theory, known as the heterogametic sex hypothesis, it is thought that females, who have two X chromosomes, have an advantage over males, who have one X chromosome and a Y chromosome, because “if there are mutations in the X chromosome, if you only have one copy of that X chromosome, you don’t have a backup essentially. So, those mutations will eventually be harmful and reduce your longevity,” Colchero told CNN.

The same is thought to apply to birds, but with males living longer. They have two Z chromosomes, giving them an advantage over females, which have only one Z chromosome, along with one W.

However, there were exceptions to the longevity trends among some animals, leading the researchers to explore other factors that could also be causing the differences, Colchero said.

For example, some birds of prey did not conform to the male advantage among birds, with female falcons and hawks in zoos outliving their male counterparts, according to the study.

Colchero described this as “puzzling” because, among these animals, “the females are the largest, the females tend to engage more — sometimes more than the males — in territorial defenses and things like that. But, still, the females are the ones that live longer.”

Since these exceptions were found among particular families, the study authors suggested that some groups “may have evolved different strategies.”

They therefore looked at how sexual selection and the cost of reproduction relate to the sex differences in life expectancy.

Bigger, not better

One theory, the sexual selection hypothesis, suggests that some male animals spend their energy developing traits and behavior to compete for and attract mates, such as changing their physical size, developing big antlers or horns, and fighting to compete for a female. In doing so, they may be “sacrificing their survival,” Colchero said.

Another, the cost of reproduction hypothesis, suggests that carrying and delivering a child, and providing parental care, can come with a survival cost, according to the study.

The scientists found evidence in support of the sexual selection theory, with female mammals living longer than males in species that were non-monogamous — having more than one sexual partner. This suggests that spending too much energy trying to build up your size to find mates, for example, has a “cost,” Colchero said.

However, analyzing data from zoos, the researchers found the opposite trend to what they would have expected under the cost of reproduction theory.

Females with childcare responsibilities tended to live longer than males in zoos. This was the same in the wild, although the researchers said they had less evidence for this.

“This seems to be counterintuitive because, for anyone that has babies, you know that the energy that you put in care — you have to put a lot of energy. And you would imagine that that energy would come at the expense of survival,” Colchero said.

However, he added that it “does make sense” in terms of evolution, since some scientists have argued that the sex doing the caregiving needs to survive longer, especially if the offspring is highly dependent, as the caregiver has to ensure that the child can survive to adulthood and reproduce themselves.

The species in which only the females take on childcare also tend to be species that are polygamous, with the males trying to mate with many females. Therefore, it could also be that the males are at a disadvantage regardless, because they are spending more energy mating, the researchers noted.

“This study expands our knowledge of the complex interplay of factors that contribute to sex differences in adult lifespan, including heterogametic sex determination,” ecologist Zoe Xirocostas, a chancellor’s research fellow at the University of Technology Sydney in Australia, told CNN on Thursday.

While the study lends “further evidence in support of the heterogametic sex hypothesis,” it “mainly considers other evolutionary drivers of sex differences in lifespan that may be just as important, such as parental care and mating system,” Xirocostas, who was not involved in the study added, highlighting the “impressive” breadth of species covered, “including groups with limited coverage in previous studies.”

Biologist Gerald S. Wilkinson, a professor at the University of Maryland, who was not involved in the research, also said the “impressive” study “provides some additional support in favor of the heterogametic sex hypothesis for which sex is expected to have a shorter lifespan.”

“But, I think the more interesting result is that they found strong evidence indicating that the mating system and sexual size dimorphism explain much of the life expectancy variation,” Wilkinson added.

“That is somewhat surprising given that mating systems in zoos are not often typical of what happens in the wild. This result indicates that factors that influence lifespan persist even in controlled environments,” he continued.

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