There is increasing evidence from non-human animals that males adjust their ejaculate expenditure according to the risk of sperm competition. In this study we show that, after controlling for lifestyle factors known to influence semen quality, human males viewing images depicting sperm competition had a higher percentage of motile sperm in their ejaculates…
These adjustments conform to the expectations of game theoretical models that male fitness will be maximized when subject to the competition that arises between sperm when females mate with more than one male (; ). Thus, males ejaculate more sperm, or sperm of better quality, when the risk of sperm competition (the probability that a female will mate with more than one male) is high ( ; ; ) but reduce the number of sperm ejaculated when the intensity of sperm competition (the number of males competing for a given set of ova) increases beyond two ( ; ).
I don’t think most women trying to get pregnant would find it thrilling that their husband wanted to watch porn with two guys having sex with one woman before they attempted to conceive… but apparently it would help.
Other interesting findings:
Many lifestyle variables were confirmed to influence semen quality, including the recent suggestion that storage of mobile phones close to the testes can decrease semen quality… men who carried their mobile phone in their hip pocket or on their belt had lower sperm motility (49.3±8.2%) than men who did not carry a mobile phone or who carried their mobile phone elsewhere on the body (55.4±7.4%; F1,23=33.28, p<0.0001).
Men who spent more time seated per day had lower sperm counts (see also Figa-Talamanca et al. 1996) and sperm concentration increased with increasing testes size (see table A2 in the Electronic Appendix). Interestingly, moderate consumption of caffeine was associated with higher sperm concentration…
Source: “Image content influences men’s semen quality” from Biol Lett. 2005 September 22; 1(3): 253–255.
Hat tip: @anibalmastobiza
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