Pictures of men with women drive male viewers to produce better-swimming sperm. Credit: © Getty

It might sound unlikely, but men looking at explicit pictures of two naked men with a naked woman have been shown to produce higher-quality sperm than those watching pornographic images featuring women only.

Although this seems to go against common perceptions about male sexual preferences, it is consistent with the theory of sperm competition, says study leader Leigh Simmons of the University of Western Australia, Perth. This states that males (of many species, including humans) should produce better sperm when faced with a female who has other mates, because this stimulates them to boost their chance of procreation.

The findings may help fertility clinics to obtain the best possible sperm samples from their clients, by providing specialized images of intercourse for men to view. This might help prospective fathers maximize their fertility, Simmons suggests. Though he adds that some women may disapprove of their partner viewing such material.

The report, published online by_ Biology Letters_1, also suggests that men who keep their mobile telephone near to their testes may be harming the quality of their sperm. Before viewing the explicit photos, volunteers in the study were asked to complete a lifestyle questionnaire including details of their alcohol intake, smoking and telephone use. Those who kept a phone in a pocket or clipped to their belt seemed to show lower levels of sperm motility, the researchers note. But experts caution that it is hard to interpret this information, because the study was not designed to look at this effect.

Animal behaviour

The sperm competition theory has previously been tested in animals such as sticklebacks, which produce more sperm after watching fellow males court a female. But studies in humans are difficult to carry out because of the ethical difficulty of evaluating sexual situations, Simmons says.

He and his colleague Sarah Kilgallon recruited 52 heterosexual men for their study, and supplied them with a set of explicit pictures featuring either two men and a woman, or three women. They asked the men to watch the pictures privately, and collect a sperm sample for analysis.

The researchers found no consistent difference in sperm number between men who viewed the different images. But sperm quality did differ. After controlling for factors such as smoking and drinking, the researchers found that 90% of the difference in sperm motility, a key measure of fertility, could be explained on the basis of which pictures the men were given. "There was a real change in response to the images," says Simmons.

Some theorists, such as Robin Baker in his book Sperm Wars, have argued that such images improve sperm quality because the viewer would expect to encounter sperm competition if he were actually part of the situation depicted.

It is an intriguing result, says Tim Birkhead, who studies sperm competition at the University of Sheffield, UK. "The results now need to be checked with a much larger sample," he adds.

Simmons says that further experiments and animal studies should be carried out to investigate the effects of mobile phones on fertility. At least one previous study has shown a possible link between phones and sperm count. But experts have cautioned that confounding factors, such as the fact that phone-users may have more stressful lives, make the results difficult to analyse.