For more than a century, scientists have been trying to explain why women consistently live longer than men—across continents, cultures, and centuries. This longevity gap has withstood revolutions in medicine, changes in lifestyle, and even global wars. The trend persists in nearly every country today, with women living, on average, about 5.4 years longer than men, according to the World Health Organization.
Now, a sweeping new study published in Science Advances reveals a deeply rooted, evolutionary basis for the divide. The research, led by evolutionary biologist Johanna Stärk at the University of Southern Denmark, analyzed life expectancy across 1,176 species of mammals and birds, both in the wild and in captivity. The results provide compelling evidence that the gender gap in longevity is not a modern phenomenon—or even a human one. It’s embedded in biology itself.
The team found that sex-based lifespan differences were widespread across the animal kingdom. In mammals, females lived around 12–13% longer than males on average. In birds, the trend often reversed: males outlived females by about 5%. These patterns held true regardless of diet, body size, or environment—suggesting something more fundamental is at work.
The X Factor: Why Chromosomes Matter
The first clue lies in the structure of sex chromosomes. In mammals, females carry two X chromosomes, while males carry one X and one Y. This gives females a genetic “backup”—if one X carries a harmful mutation, the other often compensates. Males lack that redundancy. As a result, male mammals are more vulnerable to genetic disorders and age-related damage.
Birds operate on a different genetic system. There, females are the heterogametic sex, with ZW chromosomes, while males have ZZ. The trend flips accordingly: female birds often live shorter lives, consistent with their greater genetic vulnerability.
“Even when you remove environmental pressures, such as disease or predators, the sex gap remains,” said Stärk, according to Science Advances. “That strongly suggests the difference is biological—not cultural or behavioral.”
Data from zoos, where animals are shielded from external threats and receive veterinary care, confirmed that the pattern persists in controlled conditions. Despite near-equal access to resources, female mammals still outlived males, while male birds generally lived longer. The conclusion: nature—not nurture—drives much of the disparity.
Battle Scars of Reproduction
Another powerful factor is sexual selection. In many mammal species, males compete aggressively for mates—through displays, territory defense, or direct combat. Think of the antlers of deer, the manes of lions, or the battles among bighorn sheep. These traits help males reproduce, but they come at a biological cost.
“Evolution tends to prioritize reproductive success over long-term survival,” the study notes. Traits that improve mating odds—like larger body size or showy features—can also accelerate aging, increase stress, and raise the risk of injury. The more intense the mating competition, the shorter male lifespans tend to be.
In contrast, many bird species form monogamous pairs, sharing caregiving and facing less intense competition. These species often show longer male lifespans, suggesting that lower reproductive stress leads to greater longevity.
A notable exception comes from birds of prey, such as eagles and hawks, where females are larger and more dominant. In these species, the female often lives longer—flipping the expected trend and highlighting how reproductive roles and biology interact in complex ways.
Parenting Power and Immune Advantage
Longer female lifespans also correlate with maternal investment. In mammals especially, females tend to do most of the caregiving, from gestation to feeding to protection. Evolution appears to have favored longevity in females because the survival of offspring depends heavily on the mother’s well-being.
In species where females provide extended care—like primates and elephants—they tend to live even longer. The survival of the young increases dramatically if the mother lives longer, making extended female lifespans an evolutionary advantage.
There’s also the immune system to consider. Females generally mount stronger immune responses, which help them resist infections more effectively. But this comes with trade-offs: higher rates of autoimmune disease. Males, with weaker immune systems, are more prone to infections but less likely to develop autoimmune conditions.
This biological balancing act plays out across species and contributes to the consistent survival advantage observed in females.
A Legacy That Outlasts Medicine
One of the study’s most compelling revelations is that even in modern societies with advanced healthcare and reduced maternal mortality, the female survival edge remains. In Sweden, for example, the gap in life expectancy widened from two years in the 1700s to over five years today, largely due to fewer early male deaths and improved maternal care.
The pattern is consistent globally. Data from the World Health Organization and Our World in Data confirm that women outlive men in nearly every country. Medicine can reduce the disparity—but cannot erase it.
Even in our closest relatives—chimpanzees and gorillas—females live longer, despite stark differences in environment and lifestyle. Human populations from hunter-gatherers to industrialized nations all follow the same evolutionary script.