Chapter 1: The Gender Gap in Lifespan

During my doctoral studies on the evolutionary patterns of men in a secluded indigenous community of South American hunter-gatherers, I encountered a man wearing an old baseball cap with the slogan, "There are three stages to a man's life: Stud, Dud, Thud." It's a humorous yet sobering encapsulation of life's journey, especially in light of the research I was conducting.

It's well-established that young men, particularly those in their late teens and early twenties, face higher mortality rates due to accidents and risky behaviors. Insurance companies have long recognized this phenomenon. Moreover, men generally have shorter lifespans than women, irrespective of their living conditions or lifestyle choices. They are also more likely to develop certain cancers and heart disease earlier in life. In fact, men are disproportionately affected by the primary causes of mortality in the U.S., which together account for nearly 80% of all deaths.

This raises the intriguing question: what role does natural selection play in male mortality? As I approach my 50s, the implications of aging resonate more deeply with me.

Notably, shorter lifespans and elevated mortality risk among males are observed across various species. Natural selection appears to prioritize traits that enhance reproductive success over those that contribute to health and longevity. Essentially, if the advantages of reproductive success outweigh the downsides of reduced lifespan or health, evolution will favor those traits. In simpler terms, reproductive capability often takes precedence over longevity.

As a Yale evolutionary biologist aptly noted, "Macho makes you sick."

The trade-off between longevity and reproduction is particularly evident in women, who undergo physically demanding processes such as pregnancy, childbirth, and lactation. Research indicates that having more children correlates with increased oxidative stress, potentially leading to accelerated aging in post-menopausal women. For instance, a 2006 study of rural Polish women revealed that those with a higher number of offspring had significantly shorter post-menopausal lifespans. While further studies are needed, it suggests that the energy exerted in reproduction may indeed shorten one’s life.

But what about men? Although they do not face the physical toll of childbirth, they still expend considerable energy—often at their own expense—on reproductive activities. This "reproductive effort" manifests through engaging in high-risk behaviors and developing greater muscle mass, particularly in the shoulders, back, and arms. The metabolic costs of sustaining this muscle over a lifetime are comparable to the energy demands women face during pregnancy and breastfeeding. Therefore, it’s beneficial for evolution to create mechanisms that can balance the conflicting needs of bodily functions. Hormones play a critical role here; in men, testosterone governs muscle development and reproductive behaviors. However, like many things in biology, it comes with its own costs.

Testosterone is often labeled as the male hormone, though women produce it in smaller quantities. Beyond its sexual attributes, such as promoting beard growth and deeper voices, testosterone is crucial for muscle building and elevating metabolism, thus aiding in fat burning and even enhancing mood. While these effects seem advantageous, they can also be detrimental.

The first video, titled "Why do men have a lower life expectancy than women?", delves into the biological and social factors contributing to men's shorter lifespans.

Interestingly, while burning fat can enhance physical appearance, it also leaves individuals more susceptible to food shortages and infections. This phenomenon is observable in various species where spikes in testosterone lead to increased reproductive efforts, often resulting in health challenges. For instance, male northern quolls (Dasyurus hallucatus), a medium-sized Australian marsupial, experience a significant testosterone surge that triggers intense mating periods but leads to high mortality rates due to male aggression and depletion of fat reserves. Females may live up to three years, while males often don’t survive past one year. As ecologist Jaime Heiniger poignantly remarks, "It could likely be that they [males] shag themselves to death."

The implications of testosterone on aging and longevity in humans are complex and not easily quantifiable. Since it’s unethical to manipulate testosterone levels in men to observe effects on lifespan directly, researchers rely on historical data to find clues. For example, in late 19th-century China and the Ottoman Empire, certain religious males underwent castration, including complete genital removal. Eunuchs were also prevalent in preindustrial Korean courts and among boys' choirs in Europe during the 17th and 18th centuries. While these instances provide some insight into longevity, they cannot conclusively determine the effects of testosterone due to other influencing factors like nutrition and socioeconomic conditions.

To gain a clearer understanding, researchers have explored the effects of testosterone supplementation in intact males. Studies show that increasing testosterone levels can enhance a male bird's ability to establish multiple nests, fend off rivals, and sire more offspring. Males with naturally high testosterone levels exhibit similar advantages. However, if high testosterone is beneficial for reproductive success, why don’t all males maintain elevated levels? The answer lies in the associated costs. Testosterone-supplemented male birds may outperform their counterparts in reproduction but often face lower survival rates due to decreased fat reserves and challenges during breeding seasons.

As testosterone supplementation becomes more popular among healthy men, the potential implications for the balance between reproductive effort and longevity are worth exploring. Although conclusive evidence on whether men on testosterone have shorter lifespans is lacking, emerging research suggests a correlation. A study from 2014 indicated that older men receiving testosterone prescriptions were at a higher risk of experiencing acute, non-fatal heart attacks shortly after starting treatment. Thus, while higher testosterone levels may support muscle growth, they could place significant stress on other organs in older men.

Testosterone's effects extend beyond metabolic changes; it also has substantial impacts on the immune system throughout a man's life. As Yale evolutionary biologist Stephen Stearns states, "Macho makes you sick." Men often face greater challenges than women in combating infections. Several factors may contribute to this disparity, including increased exposure to infections or a biological disadvantage in immune response. Testosterone has been shown to suppress immune function, while estradiol—the primary female sex hormone—supports it. This dynamic can lead to increased severity of infections and higher mortality rates in males, as seen in various wildlife populations. Although the direct applicability to humans requires further investigation, data from men in high-infection-risk regions suggest a link between testosterone levels and infection outcomes.

In addition to infections, men must also contend with cancer risks associated with testosterone and other sex hormones, particularly prostate cancer. Higher testosterone levels are often linked to increased prostate cancer incidence.

The second video, titled "Men on average don't live as long as women. Experts say these steps can increase life expectancy," outlines actionable advice for men to potentially improve their longevity.

So, why do men accept the negative consequences of elevated testosterone levels? From a Darwinian perspective, the potential reproductive benefits for males are significantly greater than for females. A male capable of mating with numerous females can produce a multitude of offspring, while the same cannot be said for females. This disparity is evident in the prevalence of polygyny in mammals, other primates, and many human cultures. While women can enhance their reproductive success through additional mating opportunities, they cannot do so by simply increasing offspring. Consequently, males are inclined to invest in costly hormones like testosterone, engage in risky behaviors, and develop costly physical traits due to the high potential reproductive rewards.

This rationale may have been advantageous for early hominids living in the Pleistocene, but is it still relevant today? While cultural influences are profound, the fundamental principles of natural selection—trait variation, heritability, and differential reproductive success—are difficult to escape.

However, men can evolve alternative reproductive strategies. Despite their tendency towards risky behavior and life-shortening traits, men have developed paternal investment—a rare reproductive strategy among primates and mammals. For paternal care to evolve, males must ensure their survival to support their offspring, which may necessitate a shift away from risk-taking and towards behaviors that promote better health and longevity. Research indicates that men may experience declines in testosterone and increased body mass when they become fathers and engage in caregiving. Thus, fatherhood might actually confer health benefits.

I believe that natural selection is not finished with men—or humans in general. While we may continue to face shorter lifespans and poorer health due to our evolutionary past, evolution is characterized by change over time. Humans are inherently adaptable, which likely explains the evolution of traits that define our species: large brains, extended lifespans, prolonged childhoods, and offspring requiring significant care. This adaptability may also be a factor in the current global population exceeding 7 billion—a testament to reproductive fitness. While men have developed innovative reproductive strategies like paternal care that contribute to their evolutionary success, they still rely on testosterone, which brings with it a host of health and longevity challenges. Though it may be a harsh reality, it’s certainly preferable to the fate of a northern quoll.