Men who view pornographic images of two men and a woman produce better-quality sperm than men viewing pornographic images of just women, an Australian study reveals.
The finding suggests that humans may be capable of subconsciously increasing semen quality when faced with the possibility that their sperm will have to outrun those of other men in a woman’s reproductive tract.
In the study, zoologists Leigh Simmons and Sarah Kilgallon of the University of Western Australia in Perth asked 52 heterosexual men aged between 18 and 35 years to ejaculate into a container after viewing the two types of image.
The volunteers had previously abstained from sexual activity for two to six days. In samples from men who viewed the images containing the two men and a woman – the “sperm-competition” images – 52% of the sperm were motile. This compared with 49% sperm motility in the men who viewed the images of women only – a difference that was statistically significant after taking into account lifestyle factors such as cigarette smoking and alcohol consumption.
But there was also a seemingly contradictory finding. Men who viewed the sperm-competition images had fewer sperm in their ejaculate: 61 million per millilitre compared to 77 million per millilitre for the men who viewed the female-only images. More studies are needed to explain this finding.
“It’s a fascinating study. The effect is obviously immediate. This suggests that something [in the body] can be adjusted very, very quickly,” says Jon Evans of the University of New South Wales in Sydney, who studies sperm competition in guppy fish.
The findings might suggest ways to improve the quality of sperm during fertility treatment.
The postcoital struggle between sperm is well known in species in which females may mate with more than one partner. For example, male chickens allocate more sperm to an attractive hen with a large comb than an unattractive one, upping the chances that one of their sperm will get to the egg before those of other contenders. Bulls and boars used for artificial insemination by the farming industry produce better-quality semen if they are allowed to view other animals mating.
However, Simmons is not suggesting that humans regularly indulge in multiple matings. “We need to step away from that in 2005. The risk of sperm competition is very low nowadays, but in the lineage of primates that resulted in humans there was probably sperm competition,” he says
Previous studies have found that men who look at pornographic images depicting groups prefer the sorts of sperm-competition images used in the current study. Men may simply have evolved to find them more erotic so that they can respond appropriately to sperm competition, says Simmons.
The study, which examined the role of lifestyle factors, also suggested that carrying cellphones might be associated with lower sperm counts and a lower percentage of motile sperm. But previous studies in this area have been equivocal.