Chances are you've never wondered how difficult it is to remove the testes of a hippopotamus. Other people have been thinking hard about it, though, because in fact it's almost impossible. Before sitting down to emasculate a common hippopotamus, Hippopotamus amphibius, it would be reasonable to ask why. They're a threatened species, so usually conservationists try to make more baby hippos—not fewer. But in zoos, hippos turn out to be prolific baby-makers. Females can live for 40 years and may birth 25 calves in that time. This would be great news in the wild, but zookeepers don't always have someplace to store a new two-ton animal. Male hippos can also be aggressive toward each other, at least while they have all their man parts. For both of these reasons, zoos may want to have their male hippos fixed. But there are a few factors working against them, explains a new paper in the journal Theriogenology (that's reproductive science for vets) by an international group of authors. The first challenge is that hippopotamuses hide their genitals. The testes are inside the body, instead of outside in a scrotum. (Other mammals in the internal-testes club, since you asked, include the armadillo, sloth, whale, and platypus.) This makes the hippo's testes totally invisible from the outside. Combined with a penis that the paper's authors describe as "discreet," it means it's hard to tell males from females at a distance. Another problem is that testes aren't in the same place from one hippo to the next, and they may "retract" even farther during surgery. Hippopotamuses are also difficult to safely put to sleep. "In the past, hippopotamus anesthesia has been fraught with serious complications," the authors explain. After moving past the anesthesia problem (they used an apparently safer blend of drugs, delivered via a dart to the hippo's ear), the researchers turned to the anatomical problems. Their answer was ultrasound. Once they had positioned the animal, they used ultrasound imaging to find the testes—then used it again after cutting into the hippo, if the testis they were looking for had scooted farther away from them.

Even after finding the sneaky organs, the procedure wasn't simple. The depth of the testes' hiding places varied by as much as 16 inches from one hippo to the next. Everything had to be done deep inside the animal's body, making it hard to see what was going on. "Grasping the testicle with forceps proved laborious" in most of the animals, the authors write. They also mention using a "two-handed technique" and "moderate traction." The whole hour-and-a-half procedure, based on a method for castrating horses, is described in detail for anyone who wants to try it themselves. All ten hippos in the study were successfully castrated, though one died shortly afterward, following a complication from a unknown pre-existing condition. Over the next six months, the authors checked in with the zoos housing the hippos to see whether their behavior or interaction with other animals had changed. There were four cases where zoos wanted their hippos fixed to ease aggression between males; in all four, the problem seemed better. (One zoo, though, reported that castrated males were harassed more by females.) Overall, the authors think their technique will help zoos take better care of their hippos. The final challenge to hippopotamus surgery—what should be a challenge, anyway—is that the animals spend most of their time in a pool of water packed with feces. The animals in the study lowered their stitched-up bellies into this infectious slurry as soon as they had a chance. Yet all of them healed from surgery without trouble. Hippos in general seem to be especially good healers, the authors write. A possible explanation for the animals' healing superpower is the "red sweat" or "blood sweat" that oozes from their skin. It's not really sweat and it doesn't contain blood, though it is red. The pigments in this skin secretion have been found to absorb UV light, making the "sweat" a potential sunscreen. The pigments can also keep bacteria from growing. So a built-in antibiotic may be what keeps hippos from getting infections after they tussle and bite each other (or after meddling vets come and cut out their manhood). However the red sweat works, it shows that a hippo's secrets don't end with the location of his testicles.

Images: Charlesjsharp (via Wikimedia Commons); drawings by Eva Polsterer (from Walzer et al.).

Walzer C, Petit T, Stalder GL, Horowitz I, Saragusty J, & Hermes R (2013). Surgical castration of the male common hippopotamus (Hippopotamus amphibius). Theriogenology PMID: 24246424