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Skinny men have new reason to celebrate. Well, kind of. Beefcakes may be able to attract women by rippling their muscles, but the downside of all that brawn is a poor immune system and an increased appetite, a new study finds.

Such evolutionary costs could explain why males of our species do not all look like He-Man, according to William Lassek, an evolutionary psychologist at the University of Pittsburgh, Pennsylvania, who led the new study. "In some respects I was surprised at how big the costs were… something I hadn't anticipated," he says.

Sex boost

Many other studies have shown that women tend to prefer more toned men, and muscle-bound men tend to have more sexual partners than slender men, when other factors are controlled for.

Previous research has also suggested that musculature comes with a cost. Testosterone, a hormone that promotes secondary muscle growth, suppresses the immune system of all animals, including people.

Yet no one had examined both the positives and negatives of big muscles in a single population, says Lassek, who analysed data from more than 5000 men, aged 18 to 49, who participated in the National Health and Nutrition Examination Survey, between 1988 and 1994.

The beefier the man – measured by total fat-free mass, or arm and leg muscle mass – the more sexual partners he had, Lassek confirmed. The study also showed that more muscled men tended to lose their virginity at a younger age, compared to skinny men.

Hunger pangs

Yet in the same men, muscles didn't come free. Muscle mass did a better job of predicting caloric intake than body mass index (BMI), age, or activity levels. A larger appetite may not seem like a cost in modern western societies, with a restaurant or grocer on every corner. But ancient humans struggled to get all the calories their bodies needed at times, Lassek says, which could have meant hunky men going hungry more often.

Compared to skinnies, muscular men also tended to produce fewer infection-fighting white blood cells and less of an important immune molecule called C-reactive protein, which helps destroy pathogens.

Together, the pluses of sexual benefits and minuses of the energetic costs of being beefy may even out in the end, explaining why men of all shapes and sizes thrive, Lassek contends.

Journal reference: Evolution and Human Behavior (DOI: 10.1016/j.evolhumbehav.2009.04.002)