Why are men’s muscles so much bigger than women’s? Partly, of course, because men do the fighting and hunting. But also, perhaps, because women like men who can do these things well, and are thus attracted to muscular men. Both phenomena — competing with members of the same sex and showing off to members of the opposite — are subject to a form of evolution known as sexual selection. It is sexual selection that created the deer’s antlers and the peacock’s tail, and William Lassek of the University of Pittsburgh and Steven Gaulin of the University of California, Santa Barbara, think it explains men’s muscles as well.

The main characteristic of sexually selected features is that they are expensive to maintain. Since, whether competing or attracting, only the best will do, resources get piled into them, almost regardless of the consequences. In a study just published in Evolution and Human Behavior, Dr Lassek and Dr Gaulin show that this crucial characteristic is true for men’s muscles.

Their data came from the National Health and Nutrition Examination Survey, which followed 12,000 American men and women over the course of six years. They found that men require 50 per cent more calories than women do, even after adjusting for activity levels, and that their muscle mass is the strongest predictor of their intake of calories — stronger than their occupation or their body-mass index (a measure of obesity). And there is another cost to being muscly: men’s immune systems are less effective than those of women (which was known before), and become worse the more muscular the men are (which was not).

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