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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. A 2006 historical study of rural Polish women, for example, found a correlation between having more children and a significantly shorter post-menopausal lifespan. 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.
FATAL ATTRACTION: Male quolls (above) experience a dramatic one-time rise in testosterone that triggers intense bouts of mating—and very high mortality. Wildlife Explorer / Wikimedia Commons
Burning fat may make you look better in the mirror, for instance, but in the wild, less fat makes you more vulnerable to food shortfalls and infection. This is apparent in many organisms, whose acute rises in testosterone signal an increase in reproductive effort, only to cause challenges to other physiological demands related to well-being. Take the northern quoll (Dasyurus hallucatus), a medium-sized Australian marsupial. Male quolls experience a dramatic one-time rise in testosterone that triggers intense bouts of mating—and very high mortality due to male/male aggression and fat depletion. Females live up to three years, whereas males are lucky to make it a year. As ecologist Jaime Heiniger so eloquently states, “It could likely be that they males shag themselves to death.”
