The theory is that chimpanzees have such large testis because they are promiscuous, hence males have more competition to be successful in passing on their genes. It is called sperm warfare. In the much-physically larger gorillas, the females tend to be much more monogamous, so the much larger gorilla males have much smaller testis, because there is less competition and therefore less sperm is needed for successful breeding.

Using a concentrated laser beam, a researcher captures the sperm under a microscope to track their speed. She is testing the swimming sperm-speed of different primates (including humans). As expected, chimpanzees swim really fast, with the much more placid gorillas pulling up last place in speed, but almost first place in love.