Bulging biceps might make men more likely to appeal to the opposite sex, but extra muscle may take its toll on their sperm count.
Men who look physically stronger tend to produce fewer sperm than their weaker-looking counterparts, according to a study of 118 men carried out at the University of Western Australia. The study highlights a curious evolutionary trade-off between pre and post-mating selection. Large, toned muscles may well attract more females, but they also make males less likely to be able to father offspring when they mate.
In the study, published in the journal Animal Behaviour, the men each provided a headshot, a full-body picture and a sample of ejaculate, which was measured for sperm count, motility and shape. A group of heterosexual women used the pictures to rate the men on how attractive they were. Another group of both men and women rated the men on how physically strong they looked.
The men who were rated as strongest also scored the highest for attractiveness. However, these men also tended to have lower sperm counts. Their sperm motility and morphology, however, weren't significantly different from the weaker-looking men.
'Our findings indicate that despite the benefits to premating sexual selection, having greater strength may come at a cost to sperm competitiveness,' the authors write in the study.
The study did not assess why more attractive men produce fewer sperm. However, previous studies have suggested that it could be a way of attractive males maximising their chances of mating with as many women as possible, by reserving some sperm in case of another mating opportunity on the horizon. In contrast, less attractive males will invest more sperm while mating, because for them such opportunities are fewer and further between.
'This can be a real advantage, depending on the environment,' Dr Jennifer Kotler, an evolutionary biologist at Harvard University who was not involved in the Australian study, told Smithsonian Magazine. 'Sometimes, it's better to be a bit smaller or weaker, but mate really well only once.'
However, another theory is that expending more energy in building muscle can alter sex hormone levels, with a detrimental effect on sperm production. The study authors plan to explore this hypothesis further in future research, by seeing whether a session in the gym affects sperm production.
As a result, men concerned with their sperm count need not abandon their exercise regime. Dr Mollie Manier, a biologist at George Washington University who researches sperm and sexual competition, emphasised that the link between muscle mass and sperm quality was not one-to-one.
'There are going to be people who are good at both, and people who are bad at both,' said Dr Manier. 'Pragmatically, no one needs to be alarmed [because of their muscle mass] about their sperm quality.'