You know what sounds difficult? Castrating an unwilling two-ton animal. You know what sounds more difficult? Castrating an unwilling two-ton animal that can hide its testes inside its body, "where they can shift around and even retract away from a vet's grasping forceps during surgery."p
The ability to stash and reposition one's testicles is among the many enviable skills of the hippopotamus. According to ScienceNEWS' Lizzie Wade, it's an ability that has bedeviled zoo veterinarians (who regularly castrate hippos "in hopes of controlling the size of captive populations and reducing the number of fights between the males of this aggressive species") for close to 100 years. Now, a team of vets working in Europe and Israel have published an in-depth, "nearly foolproof" method for hippo castration in the latest issue of Theriogenology that uses sonogram and careful massage to overcome what they characterize as the "spatially dynamic nature of the common hippopotamus testis." Helpfully, they have included illustrations (see above). Wade has a good lay-description over at ScienceNEWS. Do yourself a favor and check it out.