Women tend to outlive men. It is one of the most consistent patterns in human mortality, observed across countries, cultures, and generations. But what if the explanation reaches far beyond human behavior, modern medicine, or lifestyle choices?

What if the answer is written, at least partly, into millions of years of evolution?

A major scientific study published in Science Advances suggests that the female longevity advantage is not uniquely human. Researchers examining 1,176 species of mammals and birds discovered a striking biological pattern: females generally live longer among mammals, while males more often have the advantage among birds.

The findings raise fascinating questions about why women live longer than men and how chromosomes, mating competition, parental responsibilities, and evolutionary pressures shape the length of life.

And perhaps the most intriguing discovery is that the difference persists even in animals living in zoos, where many dangers of the wild have been reduced.

The research suggests that longevity is not simply about avoiding risks. It is also shaped by evolutionary processes that influence how males and females survive and age.

A Massive Study Reveals a Surprising Pattern Across the Animal Kingdom

Published on October 1, 2025, the study, titled Sexual selection drives sex difference in adult life expectancy across mammals and birds, brought together an international team of 17 researchers.

Led by Johanna Staerk and Fernando Colchero, the research examined adult life expectancy across 528 mammal species and 648 bird species, using records from zoological institutions.

The data, assembled through the Species360 zoological information system, allowed scientists to compare survival patterns across an unusually diverse collection of animals.

What emerged was a broad but revealing difference between mammals and birds.

The study at a glance

Species examined

1,176

Mammal species

528

Bird species

648

Publication

Science Advances

Which sex lived longer in more species?

Mammals: females

72%

Birds: males

68%

Percentages indicate the share of studied species showing the stated direction of the lifespan difference, including some with statistically uncertain differences. Source: Staerk et al. (2025).

Female Mammals Lived About 12% Longer

Among the mammal species studied, females had longer estimated adult life expectancy in 381 of 528 species.

Across mammals, the average female advantage was approximately 12%.

This does not mean every female mammal lives precisely 12% longer than every male. The figure represents a comparative average across species, with substantial variation.

Some species show dramatic differences. Others show relatively small gaps or even the opposite pattern.

Male Birds Had an Average 5% Advantage

Birds presented an intriguing reversal.

In 438 of the 648 bird species, males had longer estimated adult life expectancy.

Across those species as a group, the average male advantage was approximately 5%.

This contrast suggests that differences in survival between males and females are influenced by biological characteristics shared across major evolutionary groups.

One possible explanation begins with the chromosomes inherited at conception.

The Chromosome Theory: Could Having Two X Chromosomes Help Females Live Longer?

One of the best-known explanations for sex differences in longevity is the heterogametic sex hypothesis.

The concept sounds complicated, but the underlying idea is straightforward.

In most mammals, including humans, typical sex chromosome combinations are:

• Females: XX
• Males: XY

The X chromosome contains many genes involved in essential cellular functions.

Females usually inherit two X chromosomes, while males usually inherit only one.

That difference may provide females with a biological advantage in certain circumstances.

A Second X Chromosome May Offer Genetic Protection

Imagine that one X chromosome contains a harmful recessive genetic variant.

In an individual with two X chromosomes, the other copy may carry a functional version of the affected gene.

This can sometimes reduce the consequences of the harmful variant.

An individual with only one X chromosome generally lacks a second X copy that might compensate in the same way.

This difference could influence susceptibility to certain inherited disorders and potentially contribute to broader patterns of survival.

However, the protective effect is not universal. X-chromosome inactivation, gene expression, and complex interactions between genes all influence the outcome.

Having two X chromosomes may provide some protection, but it does not automatically guarantee a longer life.

Why Birds Show the Opposite Longevity Pattern

Birds provide an especially interesting comparison because their usual sex chromosome system is effectively reversed.

Instead of XX and XY, birds generally possess Z and W sex chromosomes.

Their typical combinations are:

• Males: ZZ
• Females: ZW

In other words, male birds usually have two copies of the Z chromosome, while female birds have one.

If having two copies of a sex chromosome provides some protection against harmful recessive variants, male birds could experience an advantage similar to that proposed for female mammals.

The study's findings are consistent with this possibility.

Males lived longer in about 68% of the bird species examined, compared with the female advantage observed across most mammals.

Chromosomes Explain Part of the Pattern, Not Everything

The chromosome hypothesis offers an appealing explanation because it predicts the opposite longevity patterns observed in mammals and birds.

But it cannot fully explain the variation among individual species.

If chromosomes were the only important factor, researchers would expect a more uniform pattern.

Instead, some male mammals live longer than females, and some female birds outlive males.

This suggests that evolutionary behavior, reproductive strategies, hormones, and environmental conditions also matter.

Mating Competition May Help Explain Why Males Die Earlier

One of the study's strongest findings concerned sexual selection.

Sexual selection refers to evolutionary pressures associated with obtaining mates and achieving reproductive success.

In many mammalian species, males compete directly with one another for access to females.

This competition can involve fighting, territorial defense, elaborate displays, or the development of physically demanding traits.

Over evolutionary time, these pressures may favor characteristics that improve reproductive success even when they carry survival costs.

The High Cost of Competing for Mates

Consider male deer competing during the breeding season.

Large antlers, muscular bodies, and aggressive behavior can improve their chances of securing mates.

But these advantages come with potential costs.

Fighting can cause serious injuries. Maintaining a large body requires energy, while competitive behavior may increase exposure to dangerous situations.

In some species, evolutionary success may depend more heavily on reproducing during a relatively short period than on surviving for the greatest possible number of years.

This creates what biologists describe as an evolutionary trade-off.

Traits that improve mating success may sometimes reduce survival or long-term maintenance of the body.

The study found that sex differences in adult life expectancy were associated with mating systems and sexual size dimorphism, providing evidence that sexual selection contributes to longevity differences.

Why Body Size Differences Matter

Sexual size dimorphism describes differences in average body size between males and females of the same species.

In many mammals, males are larger.

This is especially pronounced in species where males compete physically for mates.

Large male body size can be advantageous during contests, but maintaining additional body mass requires resources.

The evolutionary pressures favoring size may also be associated with patterns of energy allocation, injury, reproductive effort, and mortality.

Bigger Does Not Always Mean Longer-Lived

A large, powerful animal may appear better equipped to survive.

Yet physical dominance and longevity are not necessarily the same thing.

An animal can be highly successful at securing mates while experiencing a comparatively shorter adult lifespan.

The research supports an association between reproductive competition, sex-based body size differences, and longevity.

However, these are broad comparative relationships—not proof that size itself directly causes earlier death in every species.

Why Female Longevity Persists Even in Zoos

One of the most revealing aspects of the study is where much of its information came from.

The researchers analyzed extensive records from animals living in zoological institutions.

Unlike wild animals, zoo populations typically experience reduced exposure to several major environmental hazards.

These may include:

• Predation
• Starvation
• Severe food shortages
• Some infectious diseases
• Extreme environmental exposure

Veterinary care and managed feeding also create comparatively protected conditions.

Yet sex differences in longevity remained widespread.

The researchers found that mating systems and sexual size dimorphism were still associated with life expectancy differences under these conditions.

Why That Finding Matters

If the longevity gap were entirely caused by males fighting, hunting, or encountering danger in the wild, removing many of those threats might be expected to greatly reduce the difference.

But the broad pattern persisted.

This suggests that evolutionary influences can remain relevant even when animals live in protected environments.

Importantly, zoos do not eliminate every difference in behavior, physiology, or social interaction.

The findings therefore strengthen the case for underlying evolutionary influences without proving that the observed gaps are entirely genetic.

Parenting and Reproductive Strategies May Influence Lifespan

Reproductive success is not only about attracting a mate.

For many animals, raising offspring requires substantial investments of time, energy, and protection.

Those responsibilities may also shape how natural selection acts on survival.

In species where females provide most of the parental care, surviving longer can create opportunities to raise additional offspring or support dependent young.

In other species, males contribute substantially to offspring care.

Why Parental Investment Could Matter

Imagine an animal species whose young depend on their mother for an extended period.

A female that survives long enough to successfully rear offspring may achieve greater reproductive success than one that dies early.

Across generations, this can potentially favor physiological or behavioral traits associated with survival.

The relationship is complicated because reproduction itself can also carry considerable physiological costs.

The 2025 study examined reproductive and life-history characteristics alongside mating systems, although its clearest statistical findings involved sexual selection and sexual size dimorphism.

Parental investment should therefore be understood as one possible contributing factor rather than a complete explanation for the longevity gap.

Hormones May Also Influence Aging and Survival

Sex hormones affect reproduction, metabolism, immune function, and numerous other biological processes.

Researchers have long investigated whether hormonal differences help explain why males and females sometimes experience different mortality patterns.

Two groups of hormones receive particular attention.

Estrogens and Female Physiology

Estrogens play important roles in reproductive development, bone health, cardiovascular physiology, and other body systems.

Some research suggests that estrogen signaling may influence processes associated with vascular health and cellular protection.

However, estrogen's effects depend on age, tissue, health status, and other factors.

It would be inaccurate to suggest that estrogen simply makes females live longer.

Testosterone and Male Physiology

Testosterone contributes to muscle development, reproductive function, and aspects of behavior.

In some species, reproductive hormone levels are associated with mating competition, energetic demands, and immune responses.

But testosterone is not inherently harmful, and lower testosterone does not automatically mean longer life.

The relationship between hormones and aging is complex, with both beneficial and potentially costly effects.

The comparative study does not establish that hormones independently caused its observed mortality differences.

Instead, hormonal biology remains one part of the larger evolutionary explanation.

Does This Mean Women Are Biologically Designed to Live Longer?

The findings support the idea that biological factors contribute to sex differences in longevity.

However, the phrase "biologically designed" can be misleading.

Evolution does not create organisms according to a predetermined plan.

Instead, characteristics become more or less common across generations depending on their effects on survival and reproduction.

Women, on average, live longer than men in human populations, but that does not mean any individual woman is guaranteed to outlive any individual man.

Human life expectancy also depends on factors extending well beyond chromosomes.

These include healthcare access, infectious and chronic diseases, smoking, alcohol use, occupational hazards, injuries, socioeconomic circumstances, and public health conditions.

The animal study provides valuable evolutionary context, but it cannot determine how much of the modern human lifespan gap is caused by each of these influences.

Why Some Species Break the Pattern

Not all mammals favor females in longevity, and not all birds favor males.

The researchers identified substantial exceptions across both groups.

Among the 528 mammal species, 208 showed strong statistical evidence of female-biased life expectancy, while 28 showed strong evidence favoring males.

Among 648 bird species, 122 showed robust evidence favoring males, while 26 showed strong evidence favoring females.

These stricter findings are important because an estimated difference is not always statistically convincing.

Evolution Does Not Produce One Universal Rule

Different species face different evolutionary pressures.

Some are highly competitive during mating. Others form long-term partnerships.

Some males provide extensive parental care, while in other species care falls predominantly on females.

Body sizes, reproductive schedules, social hierarchies, and environmental demands also vary.

Each species represents a distinct evolutionary history.

That is why the research is better interpreted as evidence of widespread patterns with meaningful exceptions, rather than a universal rule governing all animals.

Can Findings From Zoo Animals Tell Us About Wildlife?

The study primarily used zoological records, which provide large amounts of detailed survival information that can be difficult to collect in natural environments.

This is a major strength.

Researchers can follow animals across many years and compare species using relatively consistent data.

But there are limitations.

Captive animals may experience different diets, social structures, medical care, reproductive opportunities, and environmental conditions from their wild counterparts.

To address this, the researchers also examined available data from wild populations.

They found broadly consistent patterns, although the evidence from wild populations was weaker and mating systems appeared to have an even stronger association with longevity differences.

the University of Bath's research portal

The finding supports the relevance of the evolutionary framework beyond zoos, but more long-term wildlife data are needed.

What This Research Teaches Us About Human Aging

The study offers a useful reminder that aging does not occur independently of evolution.

Human bodies evolved within a history shaped by reproduction, survival, parental investment, competition, and genetic inheritance.

Some of those biological influences remain important today.

But humans also differ from most other animals in important ways.

Healthcare, sanitation, medical technology, education, culture, and public health policies can profoundly influence survival.

Biology Influences Lifespan, but It Is Not Destiny

A population-level biological advantage cannot predict any single person's lifespan.

Neither can an evolutionary theory replace individual medical assessment.

Men and women can both benefit from evidence-based approaches to healthy aging, including preventive healthcare, physical activity, balanced nutrition, appropriate screening, and avoiding tobacco.

Understanding biological differences may help researchers investigate mechanisms of aging and disease.

It should not lead to the assumption that one sex is naturally healthy or that another is destined for shorter lives.

Key Findings From the 2025 Science Advances Study

Source: Staerk et al., Science Advances (2025), DOI: 10.1126/sciadv.ady8433.

What Scientists Still Need to Discover

Although this research is among the broadest comparative investigations of sex differences in adult life expectancy, it does not settle every question.

Several important challenges remain.

First, scientists need to better understand how specific genes and chromosome-related mechanisms affect aging across different animal groups.

Second, researchers need more comprehensive data from wild populations, especially species that are difficult to monitor over long periods.

Third, the relationship between reproduction, parental care, hormone signaling, body size, and survival requires further investigation.

Finally, researchers must distinguish between evolutionary associations and direct biological causes.

The study demonstrates that sexual selection is an important part of the explanation.

It does not establish a single universal mechanism controlling male and female longevity.

Frequently Asked Questions

Why do women live longer than men?

Women tend to live longer because of a combination of biological, behavioral, social, and environmental factors. Chromosome differences, reproductive biology, disease patterns, and differences in risk exposure may all contribute. No single factor fully explains the gap.

Do female animals live longer than males?

In many mammal species, yes. A 2025 study of 528 mammal species found females had longer estimated adult life expectancy in approximately 72% of them. However, the pattern is not universal.

Why do male birds live longer than female birds?

One possible explanation involves sex chromosomes. Male birds typically have two Z chromosomes, while females have ZW. Differences in mating systems, reproductive effort, and evolutionary pressures may also contribute.

Does having two X chromosomes help women live longer?

Having two X chromosomes may provide some protection against harmful recessive genetic variants because one copy can sometimes compensate for a variant on the other. However, the exact contribution of this mechanism to human longevity remains uncertain.

Why Women Live Longer Than Men Is an Evolutionary Question With No Simple Answer

For centuries, the difference between male and female lifespans has invited explanations ranging from lifestyle choices to physiology and genetics.

The 2025 Science Advances study provides a broader perspective.

Across 1,176 species of mammals and birds, researchers found that longevity differences follow striking patterns associated with sex chromosomes, mating systems, and reproductive competition.

Female mammals generally live longer. Male birds more often hold the advantage.

Yet neither pattern is absolute.

The most revealing lesson may be that longevity reflects evolutionary trade-offs rather than a simple contest between male and female biology.

The traits that help an organism reproduce successfully may not always be the traits that maximize its lifespan.

Likewise, genetic characteristics that provide an advantage in one group of animals may operate differently in another.

Humans are part of that evolutionary history, but our lifespans are also shaped by medicine, society, behavior, and individual circumstances.

The research helps explain why longevity differences exist. It does not establish that anyone's lifespan is fixed at birth.

The larger discovery is that understanding why we age may require understanding why evolution has shaped males and females differently in the first place.

As scientists continue investigating the genetics and biology of aging, studies spanning hundreds of animal species may offer valuable clues about one of life's most enduring mysteries: why some of us live longer than others.