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Technical Challenges to Castrate Hippos

The hippopotamus is one of the world’s largest creatures on land, only elephants and some species of rhino are larger. Hippos are usually very aggressive – they are legendarily bad tempered and totally unafraid of humans. In Africa, they are responsible for the majority of wildlife deaths.

Although hippos are listed as vulnerable in the wild by the IUCN, they breed well in captivity. Rather too well in some zoos, as they are large and expensive animals to keep; an adult female might have probably 25 offspring over a 40 year lifespan. Therefore, it is necessary for some zoos to manually restrict hippos’ reproduction, for example, castrate them.

A recent study by Chris Walzer of the University of Veterinary Medicine in Vienna, described a tricky method to castrate hippos. Unlike a quick visit to the vet to have your pet spayed, castrating a one-and-half ton animal with powerful jaws and thick, rubbery hide is not easy.


Cartoon character of hippos are often charmingly naïve, but in reality they are devastating jumbos. *Image source: shutterstock.


The first technical challenge is the anaesthetic procedure.  “Hippos are too dangerous and no one wants to work with them. The problem was delivering the right amount through their skin to keep them down,” said Walzer. “That’s why we developed the new anaesthesia protocol.”

Another great challenge is that hippos’ testicles move around, making it difficult to target them. According to Walzer, hippos have “highly mobile testicles”: Unlike humans, hippos’ testicles are not external nor tucked inside the abdomen. Instead, they are actually located inside the inguinal canal, a space in the lower front part of the body. However, it is hard to predict their exact location in the canal. “Hippo testicles are retractable, and can vary in depth by about 40cm, which makes them quite difficult to find,” Walzer said, adding that previous studies pointed out the difficulty in locating hippos’ testicles, and at least one paper declared that it was “not known” where they are.

Castrate Hippo

Schematic of hippo castration. *Image source: Chris Walzer et al. (2013) Theriogenology.

The castration technique developed by Walzer includes initial anaestheic procedure and the following castration. Ultrasound scanning is adopted to locate the testicles. However, the operator still has to be careful: after the hippo is down, the location of hippos’ rear legs is very important for targeting hippos’ testicles (Figure A). If this leg is lifted too high, the testicles will retract into inguinal folds and therefore become impossible to locate (Figure B). Since hippos’ testicles are moveable in both vertical and horizontal directions, scientists have to decide the incision location by the ultrasound scanning results (Figure C). If the testicles retract too much, the researchers need to use a sterile probe to detect the specific location of the testicles. Currently, the team has performed the surgery on 16 hippos across Europe, and with the publication of the procedure now encourages zoo staff to accomplish the job by themselves.

By far, scientists are not clear about why the hippo has evolved a set of retractable testicles, but it’s 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 mouth open – they will go for the opponent’s testicles and try to crush them with their teeth,” said Walzer. “If you are able to destroy your rival’s testicles, then that is an evolutionary, reproductive advantage.” The ability to yank the testicles more than a foot further into the body is definitely a great defense against the probing of hippos’ extremely long, self-sharpening teeth.

In the wild, hippos rarely fight to the death, with the weakling backing off once the stronger has demonstrated his dominance. However, in captivity, the restricted space alters their behavior. Walzer said: “It is important that young hippos are castrated before they become breeding adults, since otherwise two adult males will kill each other.” Thus, in addition to cutting down the unwanted hippo calves, castration also leaves males much more placid, meaning they can share spaces with others.


  1. Chris Walzer et al. (2013) Surgical castration of the male common hippopotamus (Hippopotamus amphibius).Theriogenology.

Source: theconversation.com