Why do females tend to outlive males?
Across both the human world and the animal kingdom, research consistently shows that, on average, females tend to outlive males. This widely observed phenomenon could offer valuable insights into the mechanisms of aging, though scientists still don’t fully understand why these differences exist. Experts think factors, such as vulnerability to environmental hazards (like air pollution, tobacco smoke, pesticides), patterns of parental care, the intensity of sexual selection, hormone variations, and the way chromosomes pass from parent to child might all play a part.
While almost all studied animal species have differences in how long males and females live, these differences are largely conditional and will change depending on what’s happening around them. Humans are the only species where women almost always live longer than men. However, because women tend to live longer, they are more likely to get sicker, especially in later life.
Genetic explanations
In genetic terms, most females have two X chromosomes, and most males have one X and one Y chromosome. X chromosomes contain hundreds of protein-encoding genes, which are essential for life and performing various functions in the body. In contrast, Y chromosomes carry fewer protein-coding genes, primarily focused on male sex determination and development. Among these is the SRY gene, responsible for triggering the development of testes, which produce male reproductive cells and the hormone testosterone. Other genes on the Y chromosome play vital roles in male fertility, enabling men to father biological children.
A study conducted in 2018 used genetic manipulation to generate XX and XY mice, with either testes or ovaries: the reproductive glands where the egg forms and the hormones estrogen and progesterone are produced. Their findings showed that mice with both XX chromosomes, regardless of their reproductive organs, had a normal lifespan and were less likely to succumb to early death. However, if the mice had both XX chromosomes and ovaries, they experienced extended life and outlived other mice in the study. This result implies that both genetic makeup and hormonal profile play an important role in longevity. Additionally, researchers have noticed that it is beneficial to have two X chromosomes because if one X chromosome becomes damaged, the other can take over – a process known as X-inactivation – potentially contributing to better aging outcomes.
Humans’ exclusivity
Humans are unique in the abundance of data showing that women generally live longer than men. This survival advantage for females is evident across all stages of life, from birth and maybe even slightly before. Understanding the mechanisms driving this early survival trend could offer insights into lifelong differences between the sexes.
However, as previously mentioned, females are more likely than men to suffer health limitations later in life due to their extended lifespans. This is a phenomenon known as the morbidity-mortality complex, and as of now, humans are the only species in which this phenomenon is observed.
Science has proposed several explanations for this paradox. One hypothesis is that because men die at higher rates than women throughout life, the men who survive are more likely to have superior physical health that allows them to live longer (essentially, survival of the fittest.) This could explain conditions like heart disease, where men die at a higher age-adjusted rate from the disease compared to women (mortality), but women experience higher rates of disease (morbidity).
Another explanation focuses on health issues prevalent in the later life of humans. For example, women are more prone to severe joint and bone problems, such as osteoarthritis, osteoporosis, and idiopathic back pain compared to men.
These hypotheses suggest that while men may be physically stronger and report fewer diseases, women often have more disabling conditions, highlighting the complexity of sex differences in health and function across diverse populations and age groups.
Sex-specific longevity in animals
In the animal kingdom, many species exhibit female-biased lifespans and slower aging, but the longevity differences are more complicated, and exceptions do exist. For example, American beavers have similar lifespans regardless of sex, and in some bat species, males live longer than females.
Studies like “Sex-specific aging in animals: Perspective and future directions” and “Sex Differences in Lifespan” have highlighted that these differences often depend on various factors, such as diet changes, reproductive status, stress levels, and environmental conditions. Increased survival and extended lifespan in females may be impacted by evolutionary pressures that enhance their ability to care for offspring and ensure reproductive success. Alternatively, shorter male lifespans might benefit future generations by reducing competition for resources and mates.
Environmental factors significantly influence lifespan differences between sexes. For instance, hotter environments can reduce male fertility in mammals, prompting males to prioritize reproduction over maintaining bodily functions, which accelerates aging compared to females. And, male competition in many species often leads to higher risks of injury or death, again resulting in shorter lifespans for males.
In conclusion, investigating sex differences in lifespan offers important insights into how genetics, physiology, and the environment interact to shape longevity across species. By unraveling these complexities, we can enhance our understanding of aging processes and potentially develop strategies to promote healthy aging and longevity in both humans and animals.
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ABOUT THE AUTHOR
Brooke Zarecki received her bachelor’s degree in biomedical sciences with a minor in Spanish from Colorado State University in 2024. She formerly worked as a student ambassador at CSU’s Center for Healthy Aging.