It’s the testosterone, don’t you know.
Evolutionary factors are clearly at play. The question is why. What is natural selection’s deal with men? It’s a compelling academic question, for sure. But now that I’m in my 50s, I have to admit the issue of aging gets more relevant with every new gray hair.
As it turns out, shorter lifespans and higher male mortality risk are quite common in many species. Natural selection doesn’t necessarily favor traits commonly associated with health, vigor, and longevity. Instead, it promotes characteristics that provide greater lifetime reproductive success, or in the parlance of evolutionary biology, fitness. If the benefits of increased fitness are greater than the cost of a shorter lifespan or poor health, biology will prioritize those traits. In essence, sex trumps birthday candles.
In the words of a Yale evolutionary biologist, “Macho makes you sick.”
This tradeoff between longevity and reproduction takes an obvious form in women: Pregnancy, childbirth, and lactation are all physically taxing and energetically costly. Research has shown that bearing more children is associated with higher oxidative stress, which can in turn lead to accelerated aging in post-menopausal women.1 A 2006 historical study of rural Polish women, for example, found a correlation between having more children and a significantly shorter post-menopausal lifespan.2 Although more research needs to be done, it would seem that reproductive effort can literally take years off your life.
But what about men? While they obviously don’t bear the costs of pregnancy, they do still allocate a great deal of energy—also to their own detriment later in life—to improve their chances of reproduction. This “reproductive effort” takes place through engagement in riskier behavior and the accumulation of greater body mass, particularly sexually dimorphic skeletal muscle mass, the extra male-specific muscle in the shoulders, back, and arms. The metabolic costs of maintaining this muscle in men over a lifetime are comparable to the energy expenditure women experience during pregnancy and breast-feeding, but they and their associated health challenges are somewhat manageable. After all, it would be a good idea to evolve physiological mechanisms to manage the tradeoffs that result from the often conflicting needs of body functions. Hormones are one of the most vital agents in managing these tradeoffs. In men, testosterone regulates investment in muscle and reproductive behavior. But like everything else, it, too, has its price.
Testosterone is often described as the male sex hormone. Women also produce testosterone, but in much smaller amounts. Aside from its sexual effects such as stimulating beard growth and deeper voices, testosterone is an important anabolic hormone that has a significant impact on the energetic costs in men. That is, it promotes anabolism, or muscle-building, and increases metabolism, the rate at which that muscle burns calories. Testosterone also promotes the burning of fat tissue. And yes, it can also boost libido and mood. So testosterone does a lot of things that sound healthy—but it can be a double-edged sword…