"Highly Mobile" Testicles Frustrate Effort to Calm Hippos in Captivity: Scientific American
Geege Schuman stashed this in Hippos
Why the hippo has evolved a set of retractable testicles is not exactly known, but it is possibly a defense mechanism. “One of the theories is that when male hippos really fight – not just the display of bravado when they yawn and stretch their mouths open – they will go for the testicles and try and crush them with their teeth,” said Walzer. “If you can destroy your rival’s testicles, then that’s a evolutionary, reproductive advantage.” The ability to yank them more than a foot further into the body is certainly one defence against the probing of hippos' extremely long, self-sharpening teeth.