In How Men Age, Richard Bribiescas is one of the first to bring evolutionary biology into the conversation of male aging, describing how it has contributed to the evolution of the human species as a whole. The book makes fascinating reading for anyone who has wondered about the purpose of male post reproductive years. From oxidative stress to loss of hormonal plasticity, here are a few things you may not have known about male aging:
1. Compared to other animals, the human life span is much longer than one would predict. Life span is usually correlated with female reproductive life span; that is, when females cease reproducing, it is usually a signpost that mortality for the species is imminent. However, in humans about a third of female life is post-reproductive.
2. Men tend to die at higher rates at younger ages than do women from infancy, through adulthood, and into old age, regardless of culture or environmental context.
3. Sex ratios in humans at birth are biased towards males, but the reason remains a mystery. There may be an unequal number of X- and Y-bearing sperm, the fertilization process might be somehow biased, or there might be an unequal attrition during gestation, all of the above, or something else entirely.
4. In males, muscle serves two purposes unique to their sex. It augments their ability to reproduce by supporting competition and attractiveness and it is an important source of overall energy regulation. More so than other types of muscle, skeletal muscle is sexually dimorphic, which means that relative mass, form, and function differ between men and women.
5. The type of muscles men tend to have are type II, which supports quick movements and bursts of strength. The muscle tends to be in their upper body, including in their shoulders, arms, and back.
6. Men lost muscle tone as they age because of declining testosterone, lower metabolic rates, and shifts in other areas of the hypothalamic-pituitary-testicular hormone axis.
7. After a certain age, men living more urbanized, sedentary lifestyles exhibit a more notable drop in testosterone compared to other men around the world.
8. Testosterone tends to peak in the second decade of life and then slowly decline until the age of 40, after which it is pretty much stable for the rest of a man’s life.
9. As men age, their bodies become less sensitive to environmental and energetic cues and less malleable and responsive to surrounding change. This loss of hormonal plasticity causes the body to become less efficient at putting on muscle and regulating fat accumulation.
10. Oxidative stress is one contributing factor to aging. Men tend to have higher metabolic rates than women, and therefore have the capacity to generate more oxidative stress over their lifetimes. This might contribute to shorter lifespans in men.
To delve into this engaging subject further, pick up a copy of How Men Age.