Obvious Effects of Cold Temperatures on Male Anatomy Aside, Low Temperatures Enhance, rather than Attenuate, the Physical Differences between the Women and Men.

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It is no secret that men and women’s bodies are different, and that this goes beyond sexual organs or facial hair. Body composition, defined as the percentages of bone, fat and muscle that make up the body, also varies a great deal, where men tend to have more lean mass (muscle tissue) relative to women, who, themselves tend to have more fatty mass relative to men. Although these physical differences between the sexes have long been established, the factors that influence the magnitude of these differences are only just beginning to be determined.

Recently, a study, published in the American Journal of Human Biology, demonstrated that the degree of sexual dimorphism in humans, defined as the physical differences between two sexes of the same species, is not just a result of a random match of chromosomes, but  is rather the result of a combination of genetics and ecological factors, such as annual temperature and proxies for food energy supply.

In this study, Dr. Jonathan K. Wells, a British scientist, used data from a previously gathered database that contained the age, weight, stature and various skinfold thickness measures of men and women belonging to 96 non-industrialized populations and calculated their lean mass and fat mass.

He used non-industrialized populations because these populations would, most likely, not have the luxury of mitigating the effects of temperature through heating or air conditioning. In addition, industrialized populations like ours, have high levels of obesity, which would also have confounded the relationships between climate and adiposity (fat mass).

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Wells found that adiposity (fat mass), used as a proxy for short-term energy supply, was an important ecological factor. In populations where there was more readily available food sources, women had greater excess fat mass relative to men and men had greater lean mass relative to women.

He also found that the difference between men and women’s body composition increased with decreasing temperature, in both lean mass and adiposity. So, at low temperatures, women invest much more in fatty tissues, while men invest more in lean tissues. This indicates a fundamental difference in the ways in which energy is allocated between the sexes and that this difference is accentuated in more hazardous climates.

Wells concludes that the degree of sexual dimorphism in human populations is affected by food availability and temperature, where the large difference in sexual dimorphism that normally occurs in areas where there is a lot of available food may be either mitigated by hotter temperatures or accentuated by lower temperatures.

This study is limited, however, by the fact that the author used proxies of body composition in order to reach his conclusions. In addition, given the nature of his data, Wells was unable to differentiate between the effects of mortality and energy supply on stature.

Scientists believe that sexual dimorphism is the result of sexual selection. Previous studies have found that lean mass is an important factor in how easy it is for men to find a mate, or many mates for that matter (Hughes and Gallup, 2003; Lassek and Gaulin, 2009). As for women, some studies have suggested that fat mass in women is an indicator of attractiveness, but these results remain controversial (Brown and Konner, 1987). Most agree that excess fat mass in women provides fuel for the tough task of bringing new life into the world.

It is interesting to note that the magnitude of the difference in energy allocation diminishes in old age. This change is largely due to a shift in women’s in energetic target, where the bulk of their energy is no longer used for procreation, but rather for survival (Wells et al., 2010, 2011). So, for those in heterosexual relationships who are worried about keeping up with their partners, I would suggest exercising a little patience because you might just meet your other half in the middle.

Wells, J. (2012). Sexual dimorphism in body composition across human populations: Associations with climate and proxies for short- and long-term energy supply American Journal of Human Biology DOI: 10.1002/ajhb.22223

This work by Arielle Duhaime-Ross is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported