Surprisingly, a lot can be inferred about one’s loyalties by comparing the average testicle size in males of varying species. Since DNA evidence supports that humans are apes or, if you prefer, chimpanzees and gorillas are human, we’ll lump the men of our own species into a comparison with our distant cousins. Being a scientist and science educator, I want to first suggest that you predict a rank for the three species… humans, chimpanzees and gorillas, based on the average testicular circumference.
It is common knowledge that the testes are responsible for sperm formation. Furthermore, the larger the testicle, the more sperm produced by the owner. Interestingly, there is a tendency towards promiscuity and unfaithfulness in species where testis size are relatively large. Chimpanzees, who have the largest testicles in our comparison, are the most promiscuous and exhibit a high degree of paternal uncertainty. This uncertainty is due to the promiscuity of chimpanzee females. In order to maximize the chance of successfully producing offspring, natural selection has rewarded those male chimpanzees with the largest testicles and thus most sperm. A high sperm count increases reproductive success when females have multiple mates. In contrast, gorillas have the small testes of the three species. Male gorillas compete fiercely for a harem of females, and once the dominance has been established, the male and his females are very faithful. Without the pressure of sperm competition, paternal certainty only requires that the testes produce sperm. Interestingly, humans have an average testicle circumference that lies between chimps and gorillas. One has to wonder if this moderate testicle size is responsible for the unfaithful behaviors among our own. Recent data now support an interesting pattern observed in human males. A study published in the journal The Proceedings of the National Academy of Sciences suggest that males with smaller testes appear to be more caring fathers, while those with larger testes are less involved with their offspring. Is this a relic of our past? Small gorilla testes are equated with reproductive certainty while large chimpanzee testicles correlate with uncertainty. It appears that these findings in humans are just one more piece of evidence to suggest that our species can not escape the long arms of natural selection.
- Davies, Ella. "Promiscuous Apes Make More Sperm." BBC News. BBC, 16 Feb. 2011. Web. 05 Feb. 2015.
- Ghose, Tia. "Of Dads and Gonads: Smaller Testicles Linked with Caring Fathers." LiveScience. TechMedia Network, 09 Sept. 2013. Web. 04 Feb. 2015.
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