Across the animal kingdom, females generally enjoy longer lives than males. In humans, our mammalian kin, birds, amphibians, fish, and even insects, the pattern broadly holds, suggesting that there’s some unknown biological reason females live so much longer. An investigation published Wednesday in Science Advances uncovers an explanation in balmy Orchid Island, off the coast of Taiwan, where territorial snakes revealed how aggression can shorten a lifespan.
Wen-san Huang, Ph.D., chair of the Department of Biology at Taiwan’s National Museum of Natural Science, is part of a team that spent many years following an “ideal snake system” on Orchid Island (also known as Lanyu Island), where kukri snakes collect each summer to feed on the eggs of sea turtles nesting on the gravelly beaches. Since their whereabouts are so reliable, Huang tells Inverse, “we can recapture them very often” — which was necessary for the team to determine how many of them survived year to year.
A major hypothesis explaining why females live longer posits that males in the wild have shorter lives because they are always competing for mates — so aggressively that they die early because of injury or stress. Aggression, he and his colleagues posit, is a major reason for sex-specific differences in mortality.
The team supports this idea with data showing that the same pattern can be seen in an opposite situation, where females are the ones who must compete aggressively. Kukri females don’t fight over mates, but they do fight over food. Somewhat unusually, the females are in charge of defending and competing for eggs in this species. As the team found, female kukris who must be territorial to protect their access to turtle eggs will indeed be very aggressive, and their survival suffers as a result.
A huge storm on Orchid Island in 2001 created the perfect environment for the team to conduct a “natural experiment.” Before the storm, they had already been monitoring female snake survival and injury rates at two sites on the island, Little Paiday and Tungching. They continued after the storm, which had created one big change in the Tungching site: It eroded the beach, so the sea turtles could no longer lay their eggs there.
Before the storm, the survival rates of females on both sites were about the same, but the storm created a major difference between the sites: The female snakes at egg-free Tungching seemed to be fighting less, as the lower injury odds compared to those on Little Paiday revealed. Since there were no turtle eggs to get territorial over, the females at Tungching didn’t have much of a reason to fight. On Little Paiday, territorial fights continued as usual.
The most interesting insights came from comparing the data on males and females. When females had a reason to be territorial and aggressive, they had lower survival and decreased longevity rates compared to the males. But when females were non-territorial, their survival and longevity were about the same.
“Thus, we can conclude that the presence of a high-value resource, and the associated territorial behavior, was the likely driver of the difference in longevity between males and females,” write the researchers.
Huang’s study presents female competition over territory as a parallel situation to male competition over mates. Both forms of competition incite aggression, which leads to injuries and shorter lifespans. And aggression is not sex-specific, at least not in this case — as the equal survival and longevity of males and non-territorial females showed.
What does appear to be sex-specific, at least in the wild, is the reason that animals become aggressive in the first place. This brings us back to the hypothesis that the reason males don’t live as long is because they’re always fighting each other over mates. Ultimately, this research suggests, longevity boils down to aggression, regardless of what incites it: it’s just that competition for mates seems to be the prevailing dynamic in the wild.
The kukri snakes support this particular hypothesis, but there are “so many” other existing hypotheses, says Huang. There’s one nicknamed “Mother’s curse,” which suggests that shortened lifespan is the result of deleterious mutations in the mitochondrial DNA (mtDNA), which is passed down from the mother and appears “more likely to affect males.” Then there’s the “hormone hypothesis,” which posits that males are naturally more exposed to anabolic-androgenic steroids, thus shortening their lifespan by weakening the immune system. Huang notes that male humans and animals have a high concentration of androgen that makes them more aggressive than women, though in humans a number of other socioeconomic reasons must be factored into differences in longevity.
For now, it remains unclear whether one hypothesis, or some combination of them, will prevail. This paper stands by the aggression hypothesis, but regardless of what underlies it — androgens, mtDNA, competition — there’ll be one big caveat, jokes Huang: “When man sees an angry woman, you had better escape as soon as possible, otherwise…”
Phylogenetic analysis has shown that males’ propensity to engage in aggressive encounters is associated with females having greater longevity. Here, we confirm the causal link between aggression and reduced longevity by looking at an egg-eating snake (Oligodon formosanus) in which females defend territories in the presence of sea turtle eggs. We monitored aggressiveness and survival at two sites: a control site with a stable supply of turtle eggs, and a second site where we collected data before and after a storm that eroded the beach on which turtles nested, thus leading to a loss of territoriality. We show that territoriality was the driver behind higher injury rates in females. Territorial females also had lower survival and decreased longevity compared with the nonterritorial males, but these differences disappeared when females were not territorial. Our study demonstrates how resource availability can influence the evolution of sex-specific patterns of survival across vertebrates.