The testicular effect

Nothing like a catchy title! Some of you will be familiar with the idea that males who have to compete with other males for mates need to produce more sperm and hence larger testicles through the process of natural selection. Animals who are relatively promiscuous have compete with mates so we would expect them to have larger testicles than males who don’t need to compete (because they have monogamous relationships). It has been observed that chimpanzees have huge testicles, relative to body weight, in comparison with gorillas. And chimpanzees tend to be more promiscuous than gorillas. Human male testicles, relative to body weight, come somewhere in between – suggesting that humans have not evolved as a monogamous species (Baker and Bellis, 1995).

father-and-children-e1349466462287-200x135New research sheds further light on these testicle observations. A team of researchers from Atlanta, Georgia (Mascaro et al. 2013) have found that men with smaller testicles tend to be better fathers.

In this study testicle size was measured using MRI scans and ‘interest in children’ was measured by asking men and their partners questions about the fathers’ involvement in caring for their children. The researchers also used brain scans to assess brain activity when viewing photos of their children.

Mascaro et al. suggest that it might be that men with smaller testicles have had to find other ways to be reproductively more successful. Men with larger testicles just reproduce more; men with smaller testicles have evolved a different mechanism – being more caring.


  1. Keith E Rice October 4, 2013 at 1:55 pm #

    Thanks for this, Cara. Very interesting.

    If students score AO2 by critiquing the Baker & Bellis avenues of research for all the usual stuff about qualitative differences between humans and animals making it dangerous to generalise such research to humans, then they can score more AO2 by bringing in the Mascaro research to support the Evolutionary approach because it is with humans. Then an AO3 limitation on Mascaro because it is only a correlation.

    Hmm… Now, do I dare get out the tape measure…?

    • Cara Flanagan October 4, 2013 at 2:03 pm #

      Not sure whether a measuring tape will do. Volume can be measured using water displacement …


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