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

Some facts no doubt borne in mind during the work performed by a team of veterinary researchers who have developed a method for the delicate operation of castrating a hippo. The problem is that, unlike a quick visit to the vet to have a pet spayed, castrating a one-and-a-half tonne animal with powerful jaws and thick, rubbery hide is not easy. Their recent paper has revealed how this tricky task is complicated by the fact that hippos testicles are not only hard to find, but actually move around. Hippos are blessed with, as lead author Chris Walzer of the University of Veterinary Medicine in Vienna put it, “highly mobile testicles.”

Unlike humans, hippos’ testicles are not external, nor are they tucked inside the abdomen. Instead they are located inside the inguinal canal, a space in the lower front part of the body. But their exact location in the canal varies widely, sometimes minute by minute. “Hippo testicles are retractable, and can vary in depth by around 40cm, which makes them quite hard to find,” Walzer said, adding that there had been in the past several documented efforts that tried and failed to locate them, and at least one paper that declared that it was “not known” where they are.

Eva Polsterer/Chris Walzer

The team led by Walzer, veterinary professor of wildlife population health at the university’s Research Institute for Wildlife Ecology, developed a technique for applying the initial anaesthetic, and for the castration itself. Ultrasound scanning is used to locate the testicles, and if they are retracted the scanner is wrapped in a sterile bag and inserted into the incision in order to get a better reading from the inside on where they’ve gone.

The anaesthetic procedure was also difficult. “The thing about hippos is that no one wants to work with them as they’re so dangerous, which is why we developed the anaesthesia protocol. The problem was delivering the right amount through their skin to keep them down,” Walzer said.

The team has performed the surgery on 16 hippos across Europe, and with the publication of the procedure now encourages zoo staff to tackle the job themselves.

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Why the hippo has evolved a set of retractable testicles is not exactly known, but it is possibly a defence mechanism. “One of the theories is that when male hippos really fight – not just the display of bravado when they yawn and stretch their mouths open – they will go for the testicles and try and crush them with their teeth,” said Walzer. “If you can destroy your rival’s testicles, then that’s a evolutionary, reproductive advantage.” The ability to yank them more than a foot further into the body is certainly one defence against the probing of hippos’ extremely long, self-sharpening teeth.

While listed as vulnerable in the wild by the IUCN, hippos breed well in captivity. Rather too well for many zoos, as they are large, expensive animals to keep; an adult female hippo may have perhaps 25 offspring over a 40 year lifespan.

Hippos in the wild rarely fight to the death, with the weaker animal backing off once the stronger hippo has demonstrated his dominance. But in captivity, the restricted space changes their behaviour. Walzer said: “It’s important that young bulls are castrated before they become breeding adults, because otherwise two adult males will kill each other.” So in addition to cutting down unwanted hippo calves, castration leaves males much more placid, meaning they can share space with others.