Female chimpanzees live for longer than their male counterparts. jindrich_pavelka/ShutterstockI’ve long been fascinated by one of the most stubborn patterns in biology: males and females rarely live the same length of time. In humans, women nearly always outlive men, with an average advantage of about five years worldwide today. This pattern holds across history, from 18th-century Swedes to contemporary Japanese – though the magnitude of this sex-difference can vary considerably.Humans are not alone in showing such sex biases in longevity. Across the Tree of Life, sex differences in longevity are widespread, but they are neither uniform nor always female-biased. And a new study study led by evolution researcher Johanna Staerk, published in Science Advances, conducted the most comprehensive analysis of sex differences in longevity in birds and mammals. The study offered new insight into why these differences evolved and found these differences are more pronounced in the wild than in zoos. Among mammals, females generally live longer. The recent comparative study examined 528 mammal species in zoos and found that 72% showed a female life expectancy advantage, averaging 12% longer lives for females. In the wild, where environmental pressures are stronger, the female advantage was even greater: around 19%. In my own work with the Compadre animal demographic database and with wild populations, I’ve seen the same female advantage emerge time and again. Wild female African elephants often live into their 60s, while males rarely exceed their late 40s. This is in part because males spend much of adulthood in risky solitary ranging and competing through combat. Similarly, in moose, females live twice as long (17-22 years) as males.Genetics plays a key role. Male mammals are the heterogametic sex (meaning males have an X and a Y sex chromosome), so males are more likely to inherit recessive x mutations. Also high levels of testosterone can suppress immune function. However, the clearest evidence points to sexual selection. In polygynous mammals (males compete for access to harems of females) such as red deer or lions, males grow large bodies and weaponry, and fight for mates, which reduces their survival chances. Those magnificent antlers come with a cost. HMD_93/Shutterstock Birds: the male advantageThe reversal in birds still surprises students when I teach life-history theory. Biology rarely hands us neat rules. Across the 648 avian species studied in zoos by Staerk and colleagues, in 68% of them males outlived females, with an average 5% male advantage. In the wild, the gap widened to over 25%. Female birds are the heterogametic sex (ZW chromosomes), which may expose them to greater genetic risks. More importantly, many female birds pay heavy reproductive costs. Egg production, incubation and chick rearing demand enormous energy. In some species such as ducks and songbirds, this burden translates into shorter female lifespans.The exceptions are illuminating. Raptors like buzzards and eagles often show a female advantage. In the wild, female tawny owls live longer. But in zoos the advantage shifts to males. The reason is not yet known. Beyond fur and feathersPrevious research has shown insects have vivid contrasts in longevity. In many moths and mayflies, females live only a few hours or days as adults, exhausting themselves in egg laying, while males persist days or weeks longer. Insects who live in highly organised colonies reverse this pattern: ant and bee queens can live decades, far outlasting short-lived male drones.Here, the colony shields queens from many ecological risks, such as the need to evade predators when out looking for food, illustrating how social organisation can radically alter the relationship between sex and survival.Amphibians and reptiles are known to showcase mixed patterns. Male frogs often die younger due to the costs of calling and combat at breeding sites, whereas female frogs sometimes pay higher survival costs through egg production. Fish often show flexibility in sex roles, alongside variability in female and male lifespans. In stickleback fish, males provide sole parental care, defending nests at great cost. They often die shortly after the breeding season, while females survive to reproduce again. Conversely, in species where females produce enormous clutches, their shorter lives balance the equation. Humans in contextAcross cultures and history, women live longer than men. In 21st-century Japan, female life expectancy exceeds 87 years, compared to 81 for men. Among Hadza hunter–gatherers, who live in Tanzania, women live longer than men too.Social and medical advances, such as better maternal care, have widened the human female edge in modern populations.Intriguingly, the human “female advantage” is smaller than in apes, probably because sexual selection is weaker in humans. Female chimpanzees and gorillas live substantially longer than males, often by more than a decade. Indeed, men face fewer risks from mate competition than chimpanzees. Why does sex-based longevity differ so much?There are two main hypotheses. First is the heterogametic sex hypothesis, as mentioned above, which predicts that the sex with two different sex chromosomes (XY in mammals, ZW in birds) suffers shorter lives. However, this fails to explain the exceptions, such as long-lived female raptors.Second are life history and sexual selection trade-offs. Traits that increase reproductive success often reduce survival. Among mammals, males die younger when they invest heavily in competition, size or weaponry. Among birds, females pay with their lives for egg production and parental care. The new study supported this explanation. Non-monogamous mammals with significantly larger males show the largest female advantage.Who wants to live forever anywayA longer life does not necessarily translate into better quality of life. In humans, although women live longer almost everywhere, they often spend more years in poor health compared to men, burdened by chronic conditions such as osteoporosis, dementia or autoimmune disorders. Similarly, in some nonhuman animals, females survive longer but may experience prolonged periods of reduced reproduction or physical performance. A “female advantage” in survival may therefore come with hidden costs.So, do females live longer than males across the Tree of Life? Often, yes, but not because of a universal law. The patterns are the messy product of chromosomes, hormones, competition, care and chance. That’s what makes it such an interesting question to study.Rob Salguero-Gomez receives funding from NERC.