Men Have Some of the Smallest Testicles in the Animal Kingdom
Compared to many animals, human males don't have very big testicles. This is a very popular topic for research and many scientists believe it's related to our relative lack of mate competition. It has been shown across all kinds of animals, from chimps to insects, that balls tend to be bigger when females are more likely to have more than one mate. The one of the most popular hypotheses for why this is that larger testicles release more sperm.