Why It’s Nearly Impossible to Castrate a Hippo

By Elizabeth Preston | December 23, 2013 1:29 pm

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

  • http://www.blogger.com/profile/00336081873435545376 G Wolf

    Why not just give them something systemic to control it, like we do with humans? Put it in their water or their food or something.

  • Anonymous

    Okay, I have to ask, where and when do we just give humans something systemic to control their aggression?

  • Anonymous

    Lol, in the nut house?(pun intended)

  • Anonymous

    Floride should do it.

  • Anonymous

    > Okay, I have to ask, where and when do we just give humans something systemic to control their aggression?

    I think G Wolf means something to control the breeding, not the agression. Something like the pill.

  • Anonymous


  • Anonymous

    Here's another really radical idea; how about…..eating them?

  • Anonymous

    Why not leave the males alone and give the females birth control pills? Much easier I would think!

    • concerned citizen

      Because That’s not the issue. They have no problem with their breeding it’s the aggressiveness of the unnuetered male. I think they should give them a pill that lowers their testosterone.

  • Anonymous

    “Fixed” ? Surely you mean broken.

  • http://www.blogger.com/profile/07099418442691239669 Unknown

    Well, first they'd have to figure out the proper dosage, and make sure there are no adverse side-effects. It's possible that some of the hippos will be perfectly content on the medication, while others will have adverse side-effects–what then? Even assuming all that works, the keepers then have to keep track of the menstrual cycles of all the female hippos, and give them the right pill on the right day. And on that note–how do you make a hippo eat a pill? (Bearing in mind that you can't let it spit the pill out, especially if that means another hippo might eat the pill instead.) None of those are insurmountable problems, but maintaining a group of animals on a regimen of pills isn't exactly simple.

  • http://www.blogger.com/profile/07099418442691239669 Unknown

    Also, regulating female hormones doesn't do anything to curb aggressive males. So if that's the real problem a particular zoo is facing, then removing those testosterone factories is a much better solution.

    • concerned citizen

      I completely agree!!

  • Pingback: Waving Hello with Many Arms - Inkfish : Inkfish()

  • Pingback: Wednesday Morning! | Gerry Canavan()

  • sabakunogaaraai

    hm. maybe they should start marketing hippo sweat…

  • concerned citizen

    This is awful. One of them died because of it?? Why not test on already dead animals first until you actually know a little about what you are doing? This is torture.

  • http://www.hiperactivos.com Gerardo Ochoa-Vargas

    “Theriogenology (that’s reproductive science for vets)”

    No, not really. Theriology is the science of mammals, specifically. The journal is devoted to the biology of reproduction of mammals and no other animal class.

  • Michael Sandell

    “categorized under: ‘posts that will bring me creepy internet traffic’.” Yep.



Like the wily and many-armed cephalopod, Inkfish reaches into the far corners of science news and brings you back surprises (and the occasional sea creature). The ink is virtual but the research is real.

About Elizabeth Preston

Elizabeth Preston is a science writer whose articles have appeared in publications including Slate, Nautilus, and National Geographic. She's also the former editor of the children's science magazine Muse, where she still writes in the voice of a know-it-all bovine. She lives in Massachusetts. Read more and see her other writing here.


See More

@Inkfish on Twitter


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

Collapse bottom bar