A large-scale analysis of over 1,100 mammal and bird species reveals a striking evolutionary explanation for why females generally outlive males.
The study, led by researchers at the Max Planck Institute for Evolutionary Anthropology and the University of Southern Denmark and published in Science Advances, found that lifespan differences among mammals and birds align more closely with sexual selection—such as mate competition and size dimorphism—than with XY or ZW chromosome types.
Including 528 mammals and 648 birds, this is the largest cross-species comparison to date. It shows that sexual competition, not chromosome structure, best predicts which sex lives longer.
“Our analyses provide the most phylogenetically comprehensive estimates of sex differences in ALE [adult life expectancy] in mammals and birds to date, pointing to evolutionary drivers of sex-specific survival,” researchers write. “While our results partially support the heterogametic sex hypothesis, heterogamy alone cannot explain the breadth of variation in ALE differences found here.”
Across nearly every corner of the animal kingdom, females tend to outlive males—a biological pattern so widespread it stretches from humans to whales to elephants.
For centuries, researchers have sought to explain why one sex typically lives longer, but no theory has fully resolved the debate. This new study adds unprecedented scope to that mystery, revealing that the same trend seen in humans holds true for most mammals—but not for birds.
Analyzing data from more than a thousand species, the researchers found that 72% of mammals exhibited a female life expectancy advantage, whereas 68% of birds showed the opposite trend, with males living longer than females.
At first glance, this aligns with the “heterogametic sex hypothesis,” proposing that lifespan differences stem from sex chromosome structure.
In mammals, males carry XY chromosomes, making them more vulnerable to harmful mutations on the single X chromosome. In birds, the system is reversed, with females carrying ZW chromosomes that may result in shorter lifespans.
However, when the team examined the data more closely, the genetic pattern began to break down. The expected lifespan differences predicted by chromosome type failed to hold across many species.
Some male mammals lived longer than females, while in several bird species, females defied expectations by outliving males—suggesting that genes alone don’t tell the whole story.
To understand what was truly shaping these patterns, researchers turned to behavioral analysis. More specifically, they examined sexual selection. In species where males compete fiercely for mates, such as deer, seals, or gorillas, males tend to live shorter lives. That trade-off, the authors argue, stems from investing heavily in traits such as large body size, elaborate displays, or aggression. These traits may aid in mating, but they take a toll on longevity.
By contrast, in species with monogamous pairings, competition is low. Both sexes share parental duties, and life expectancies are nearly equal. In birds, the trend reversed slightly. Monogamous males often lived longer, suggesting that evolutionary pressures differ when males care for offspring.
“Precopulatory sexual selection, specifically mating system and SSD [sexual size dimorphism], had a strong effect on adult life expectancy differences across both classes,” researchers noted. They found that female mammals in polygynous or promiscuous species enjoyed a 15% longer life expectancy than males, while monogamous species showed almost no gap at all.
Crucially, the team drew their data from both wild populations and animals living in zoos. This approach enabled them to control for environmental pressures, such as predation, starvation, and disease.
Even in these managed settings, patterns of sex-specific survival persisted. This persistence suggests that lifespan differences are evolutionary, not environmental.
“Our results from both wild and zoo populations suggest that the effect of sexual selection on the evolution of life expectancy differences may still be at play and evident even in controlled environments,” researchers conclude.
That insight challenges the idea that males die younger mainly due to external risks. Risks such as fighting, defending territory, or hunting are not the only factors that influence longevity. Instead, the findings suggest that biological costs are inherent to reproduction, representing a trade-off between reproductive success and survival.
While the general pattern was clear, the study also uncovered some exceptions. For example, lemurs, a non-monogamous primate in which females are socially dominant, showed little to no difference in life expectancy between the sexes.
Among birds, raptors such as hawks and owls displayed the opposite of the expected trend, with females often outliving males, despite being larger and more dominant in size and behavior.
These outliers, the researchers note, suggest that other evolutionary forces can override the general rule. These forces include genetic mechanisms, parental care strategies, or even ecological pressures.
In primates, for instance, greater female investment in offspring was associated with longer female lifespans, hinting that parental care itself might select for longevity in the caregiving sex.
When the authors compared their animal data to historical human records, they found that humans fit neatly into the broader pattern of mammals. Across Swedish populations dating back to the 18th century, Japanese demographics in 2012, and even hunter-gatherer groups like the Hadza and Ache, women consistently outlived men.
Yet, intriguingly, the difference was smaller than in our closest evolutionary relatives.
“The female advantage in humans is smaller than that observed in African apes, including previously studied populations of chimpanzees and gorillas in the wild,” the researchers note. “However, the case in humans does suggest the importance of environmental modifiers of sex differences in ALE.”
“For instance, in Sweden, the female ALE advantage was lower in the mid-1700s compared to the 2000s, perhaps because reductions in the risk of maternal death during childbirth have removed otherwise leveling effects on life span between men and women.”
This suggests that weaker sexual selection pressures in modern humans, combined with improved healthcare that reduces childbirth mortality, have narrowed the gap.
In other words, while biology sets the foundation, culture and technology have softened the edges.
By combining massive demographic datasets with Bayesian modeling, researchers measured how deeply sex and survival are intertwined. In mammals, female longevity increased with stronger sexual selection in males. This finding suggests that lifespan differences are an evolutionary cost of competition. In birds, the pattern flipped. This may be due to the opposite sex chromosome system or different reproductive roles.
The team’s results, spanning over a thousand species, represent the most comprehensive analysis ever of how sexual selection shapes lifespan across the animal kingdom.
Researchers conclude that chromosomes may influence longevity. However, mating strategies, size differences, and reproductive costs explain far more variance in the data. These trends also appear in environments where external threats are minimized.
“Our study, however, cannot resolve the two competing hypotheses,” researchers write. “From a comparative perspective, finding evidence in support of hypotheses on the evolution of ALE differences from data where effects are expected to be attenuated, such as those from zoo populations, warrants further investigation on the genetic basis of the evolution of sex differences in life span.”
Ultimately, the study sheds new light on a long-standing mystery: why women tend to live longer than men. The answer, it seems, may lie in the evolutionary arms race of attraction and competition, a costly pursuit that echoes throughout the animal kingdom.
“Our findings may help explain why differences in ALE between men and women are so consistent across time and cultures,” researchers conclude. “Humans conform well to the general mammalian trend of a female advantage in ALE, demonstrating that, at least in terms of sex differences in survival, our species is not unique.”
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com