If you watched the video from the previous blog post, you may have some questions about a certain part of the male chimpanzee anatomy. Think of this post as Everything You Wanted to Know About Chimp Testicles, But Were Afraid to Ask.

Burrito is not unique – male chimpanzees have rather large testicles compared to other apes. And there’s a very interesting explanation for this. Most people are familiar with the concept of males competing for access to females. For example, male gorillas will compete for control over a harem of females – whoever controls the harem has primary mating access. But female chimpanzees will regularly mate with multiple males during estrus (see this recent article for an interesting take on chimp mating behavior). Therefore, much of the competition between males takes place between their sperm within the female’s reproductive tract.

If you think of mating as a lottery, with the prize being a fertilized egg, then whoever can produce the most sperm is essentially buying the most lottery tickets. And whoever has more tickets has a better chance of winning. There are also other forms of sperm competition besides this sperm “arms race”. Chimpanzees also produce sperm plugs (also called copulatory plugs). After mating, some of the semen will coagulate to plug up the cervix, so that sperm from future mating (with other males) will be less likely to reach the egg.

So for chimpanzees, it pays to produce a lot of semen, and that requires large testicles. Now you know!