Why do females live longer in mammals but males in birds?

By Vijay Kumar MalesuReviewed by Lauren HardakerOct 7 2025

A sweeping global study reveals that who lives longer, male or female, is written not in the chromosomes but in the battles for mates, reshaping how scientists understand aging across the animal kingdom.

Study: Sexual selection drives sex difference in adult life expectancy across mammals and birds. Image credit: Robirensi/Shutterstock.com

In a recent study published in Science Advances, researchers tested whether sexual selection, beyond sex chromosomes, explains sex differences in adult life expectancy (ALE) across mammals and birds using harmonized zoo and wild datasets.

Background 

Across cultures and centuries, women outlive men by about 5.4 years on average, a gap that persists despite changing diets, medicine, and lifestyles. Similar sex gaps appear across animals, yet not always in the same direction: many mammals favor females, while many birds favor males.

Two ideas compete to explain this: the influence of sex chromosomes in the heterogametic sex, and the survival costs linked to sexual selection and reproduction. Understanding which forces have the stronger effect is important for public health, wildlife management, and aging science. The longevity gap also affects families through caregiving, retirement, and long-term planning. More research is needed to connect these evolutionary drivers to the biological mechanisms and policies that shape lifespan differences.

About the study

The team assembled individual life records from the Species360 Zoological Information Management System (ZIMS) for 528 mammal and 648 bird species and added wild-population data for 110 species (69 mammals and 41 birds).

They estimated ALE from the age at first reproduction using Bayesian survival trajectory analysis (BaSTA) with Siler mortality models and Markov Chain Monte Carlo (MCMC). Sex differences were summarized as delta-e (δ_e) = (female Adult Life Expectancy − male Adult Life Expectancy (e_f − e_m)/max(e_f,e_m)), interpreted as proportional female or male advantage, and statistical support used a two-sided “zero-overlap” test on posterior densities.

To test evolutionary drivers, they ran weighted Bayesian Phylogenetic Generalized Least Squares (BPGLS), with predictors for precopulatory sexual selection- Sexual Size Dimorphism (SSD), social mating system (monogamy vs nonmonogamy), and, in birds, plumage dichromatism, and postcopulatory sexual selection (relative testis mass).

Annual female productivity and parental care tactics captured the costs of reproduction. Model choice used the Deviance Information Criterion (DIC); uncertainty is shown as credible intervals. Species coverage spanned most mammalian and avian orders. Analyses required at least 35 individuals per sex, with records primarily from 1980 to 2024.

For birds lacking ages at first reproduction, imputed values were used for 134 species. Phylogeny was modeled explicitly (Pagel’s lambda), and uncertainty was summarized as posterior means and standard deviations.

Study results

In zoos, mammals showed a mean 12% female ALE advantage, whereas birds showed a mean 5% male advantage; using only the most precise estimates, these were increased to about 16% and 6%, respectively. In the wild, gaps were larger and more variable: mammals averaged a roughly 19% female advantage and birds an approximately 27% male advantage, yet the direction matched zoo and wild for most species checked.

Contrary to a simple sex-chromosome rule, exceptions existed: some mammals had male-biased ALE, and some birds had female-biased ALE. Within mammals, ungulates, bats, and marsupials showed pronounced female advantages; primates, rodents, and carnivores showed smaller or mixed gaps.

In primates, female advantage predominated in Old World monkeys and great apes, while some night monkeys showed male advantage; humans showed a smaller female advantage than chimpanzees and gorillas, based on populations from Japan (2012), Sweden (1750 and 2012), and the Hadza and Ache hunter-gatherers.

Among birds, songbirds, parrots, pigeons and doves, galliforms, and waterfowl tended to favor males, yet raptors and some owls often favored females. Evolutionary drivers pointed strongly to precopulatory sexual selection: across classes, non-monogamous systems and greater male-biased SSD aligned with a larger female advantage in mammals; in birds, monogamy was associated with a male advantage, consistent with background costs of heterogamy in females.

Postcopulatory sexual selection showed little association overall, aside from some artiodactyl signals via relative testis mass. Against expectations from simple reproductive-cost models, female-only parental care correlated with a greater female advantage, particularly in primates, possibly reflecting selection for higher survival in the caregiving sex or confounding with polygyny.

The effects of hunting were not detected overall, though an interaction suggested that trophy hunting could amplify gaps in some clades. Even with reduced predation and controlled diets in zoos, the imprint of sexual selection remained visible, implying deep evolutionary roots. In zoos, 72% of mammal species were female-biased and 68% of bird species were male-biased, though many cases had weak support. Order contrasts stood out: ungulates showed large female advantages (25% in even-toed, 18% in odd-toed), bats and marsupials were female-biased, while carnivores, primates, and rodents were mixed with many near-ties.

Rodent examples ranged from female-advantaged capybaras to male-advantaged naked mole rats. Birds of prey bucked the rule, with female-biased life expectancy despite reversed size dimorphism. Directions were matched between zoos and wild animals in two-thirds mammals and over half birds, reinforcing that patterns reflect intrinsic life-history trade-offs, not only local hazards. The δ_e metric expressed gaps as intuitive percentages, clarifying class-level means and variability across clades and environments.

Conclusions 

Sex differences in ALE are not governed by sex chromosomes alone. Across 1,176 species, mating systems and sexually selected size differences consistently aligned with who lives longer, with stronger female advantages in mammals and male advantages in birds, and notable clade-specific exceptions.

These patterns persist even in protected zoo environments, suggesting evolutionary forces. The message for human health and wildlife policy is clear: to understand and narrow sex gaps in longevity, investigate how competition, care, and life-history strategies trade off against survival, and probe the genetic and ecological levers that modulate those trade-offs in diverse ecological contexts.

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