And researchers at The College of Wooster in Ohio hope their findings will help to improve treatments for fertility Quantity, movement and structure all aid sperm health.
Writing in the journal Evolutionary Psychological Science, researchers said they believe these traits may change for the better, with new sexual partners. 'Our findings are the first to demonstrate that men's ejaculate behaviour and composition change in response to novel female stimulus,' the team led by Paul Joseph said.
The study involved 21 participants, all heterosexual men aged between 18 and 23 years old. The men each provided seven ejaculate samples over the course of 15 days.
Those recruited to take part were only enrolled if they had no history of sexual dysfunction, no conditions affecting testicular health and no sexually transmitted infections. In addition, they were not taking any medication and didn't smoke.
Researchers used clips from sexually explicit films, involving one actor and one actress. Each participant watched the clips in the same private room at roughly the same time of day every 48 to 72 hours. The clips were three minutes long and were played on repeat until a man ejaculated.
Six film clips featured the same man and woman but differed in the sexual acts performed. Meanwhile a seventh clip featured the same man while the female differed distinctly, with different facial and body features, hair colour and tattoos.
Each man was asked to record the time he started watching the film clips, and when he ejaculated. Researchers then analysed the men's samples, to ascertain sperm health. The authors wrote: 'In our study, men produced higher quality ejaculates when exposed to novel, rather than familiar women.
'Additionally, men ejaculated more quickly when viewing a new woman after being exposed to the same woman repeatedly.' They said their findings suggest that men 'preferentially invest more' into new sexual situations with unfamiliar partners.
The study's authors said one reason for producing 'better' sperm with a new partner is down to sperm competition and an evolutionary desire to secure an heir.
'An increase in the total number of motile sperm may result in higher likelihood of fertilisation and greater ability to compete with other male’s sperm, whereas a decrease in the time to ejaculation may decrease the likelihood of an extra-pair copulation (with a partner that is not your own) being detected,' they wrote.
They added the results could have an impact on fertility treatments, warning male infertility could be being under-diagnosed. This is because 'ejaculate samples used to test for infertility are often collected with the use of images depicting women other than the man’s partner', the researchers said.
'Our results have important implications for understanding selective pressures on male reproductive patterns, the plasticity of ejaculate allocation, and diagnosis and treatment in the context of male fertility,' they added.
They suggest that further studies would be beneficial to help assist medical professionals in devising improved strategies for male infertility diagnosis.