The Evolutionary Reasons For Why Women Live Longer Than Men

An expanded phylogenetic tree showing all of the mammal species that the researchers studied, with red bars representing species where females lived longer and blue bars representing species where males lived longer.

You may have noticed how women often outlive men by a considerable margin. In fact, according to the Centers For Disease Control and Prevention (CDC), the average lifespan, in 2022, for American men was 74.8 years, while for women, it was 80.2 years, putting the difference at 5.4 years. Expectedly, by age 85, Harvard Health states that 67% are women. This trend of females living longer than males isn’t just present in humans; it’s present across most mammalian species, and up until recently, scientists didn’t fully understand why this is.

A new study by Johanna Stärk and her team, which was published in Science Advances, took a look at adult life expectancy (ALE) across a total of 528 mammal species and 648 bird species in order to understand the evolutionary and biological mechanisms behind sex differences in lifespan. The researchers included both zoo and wild population data to account for environmental influences. Starting at the age of first reproduction, they measured ALE, thus allowing the researchers to focus on adult survival patterns rather than mortality in early life, which is often affected by predation, disease, or starvation. Given below is the metric the study used to quantify the sex difference:

δe = (ef - em) ÷ max(ef, em)

In this equation, “ef” and “em” represent the ALE in females and males respectively. “δe” represents the proportional difference in adult life expectancy between females and males. Positive values of δe indicate female advantage, while negative values indicate male advantage. Using this standardized measure, the researchers were able to compare differences across species that vary widely in body size, lifespan, and reproductive strategies.

The results in mammals showed that across all species, 72% of them had a female advantage in ALE, with an average female advantage of about 12% in zoo populations. In wild populations, the female advantage was even more pronounced, at an average of 19%. When looking at birds, on the other hand, the pattern was pretty much the opposite, as 68% of the species showed a male advantage. In comparison to females, the males, on average, lived 5% longer in zoos and 27% longer in the wild. Because the wild populations showed consistent but larger differences than zoo populations, the researchers emphasized that while environmental pressures amplify the sex-based patterns, they do not actually create them. According to them, “The average ALE differences were more pronounced in the wild than in zoos, although with much greater variability.”

One of the key hypotheses that the researchers looked into as part of this study is the heterogametic sex hypothesis. In mammals, as you may have learned in your biology class, males are XY (heterogametic), while females are XX (homogametic). Since males carry only one X chromosome, deleterious recessive mutations on the X are automatically expressed, whereas females are partially protected by a second X chromosome. This leads to males having a shorter lifespan than females. In birds, however, the situation is reversed, as females are ZW (heterogametic) and males are ZZ (homogametic). The researchers noted that with this reversed situation, the pattern in life expectancy was also reversed, where, unlike in mammals, the males were the ones that tended to live longer than their female counterparts.

However, the chromosomes being different, on their own, do not completely explain the differences in longevity between males and females. The study also evaluated something called sexual selection, which refers to the pressures that come from competing for mates. In mammals, species with nonmonogamous mating systems, where one male has more than one mate, and high male-biased sexual size dimorphism (SSD), where males are much larger than females, show stronger female longevity advantages. Simply put, when males invest heavily in competitive traits such as larger body size, weaponry, or energetic displays, which are common among many mammalian males, it increases reproductive success but reduces survival.

A photo showing the vibrant appearance of a male Painted Bunting.

In birds, mating systems and SSD are generally less extreme compared to those observed in mammals. Most birds are monogamous, and male-biased size dimorphism is smaller, which contributes to the smaller or reversed sex differences in adult lifespan. Even in bird species with strong male competition, the male survival cost is less pronounced than in mammals, resulting in male advantages or reduced female advantages for them.

The role of parental care was examined as well during the study. Across mammals, the fact is that females typically provide a lot more parental investment through gestation, lactation, and extended care. The authors found that species with female-only care had a larger female advantage in ALE. This is not surprising, since in such species, the longer the females live for, the higher the chance that the offspring reach independence and reproductive maturity, thereby helping secure the future of the offspring and the species as a whole. In some bird species, where males contribute more to parental care or defense, males tend to live longer, further supporting the link between caregiving and longevity.

This longevity gap persists even in low-risk zoo environments. While life expectancy in absolute terms is higher in captivity due to the reduction of predation, disease, and food scarcity, the relative sex differences remain. This indicates that intrinsic biological factors like genetic composition, hormonal regulation, and life-history trade-offs are the primary determinants of adult lifespan differences. For example, male mammals in highly competitive species often have higher testosterone levels, which increase aggression and reproductive success but also elevate mortality risk. Meanwhile, females invest more in somatic maintenance, which supports both reproduction and longevity.

The study also highlights exceptions to general patterns. Certain species, particularly some birds that live off of hunting other birds, deviate from expected outcomes. In these species, females are both larger and longer-lived than males, despite males still being heterogametic (ZW): “Some species showed the opposite of the expected pattern. For example, in many birds of prey, females are both larger and longer-lived than males.” The authors attribute this to local ecological pressures, such as hunting behavior, nesting requirements, or territorial risks, which may override general evolutionary trends. Exceptions like these demonstrate that although genetics and sexual selection play major roles, the interactions between other factors like ecology, behavior, and phylogeny can also influence adult lifespan.

Another important finding in the study relates the magnitude of the sex difference in wild versus captive populations. In both mammals and birds, regardless of sex, wild populations displayed a larger difference in advantages than those in the zoo, therefore suggesting that environmental pressures, which includes predation and resource scarcity, amplify the survival costs of competitive males and the protective benefits of caregiving females. Nonetheless, because the direction of sex differences persist in a consistent manner across both captive and wild contexts, it is evident that intrinsic, evolutionarily embedded mechanisms underlie these patterns.

The study serves to show that humans are part of the broader patterns discussed in this article. The female longevity advantage observed today, with it being 5.4 years on average in the United States, has a lot to do with the deep evolutionary mechanisms humans share with other fellow mammals. Genetic factors, including X-chromosome redundancy, the survival costs of male competition, and selective pressures favoring maternal longevity, are embedded in our biology. If interested, you can check out the original study published in Science Advances, which goes over, in depth, even more of the patterns the researchers observed and their takeaways from them.