Few things in this world are as elusive as a hippopotamus testicle—especially when you’re trying to cut it off. Zoo veterinarians have been attempting, with varying degrees of success, to castrate the 2-ton animals for more than 90 years, in hopes of controlling the size of captive populations and reducing the number of fights between the males of this aggressive species. But the hippos don’t make it easy: Hippopotamus amphibius hides its testes inside its body, where they can shift around and even retract away from a vet’s grasping forceps during surgery. Now, a team of vets working in Europe and Israel has described, in detail, a nearly foolproof method for hippo castration . After drugging the animal with a precise anesthetic cocktail, the researchers rolled the sleeping hippo on its side and hoisted up one of its hind legs with a rope. Once the animal was in exactly the right position, as shown in the illustration above, they used an ultrasound device to find its testes and then sort of … massaged them toward the point on its abdomen where they planned to make the incision. If the testes retracted during the operation, the scientists simply whipped out the ultrasound again and repeated the technique, they report in a paper in press at the veterinary reproductive medicine journal Theriogenology. The vets successfully castrated 10 out of 10 hippos with their method, losing only one of the animals to postsurgery complications. Thanks to hippos’ stunning wound-healing abilities—perhaps related to the antibacterial properties of the creepy “ red sweat ” that coats their skin—all the surviving animals were able to return to their feces-infested communal pools within hours of the surgery with no negative consequences, just a little bit nicer than when they left.

See more ScienceShots.