LUCY COOKE: So, a few years ago I went to go and investigate rumors of hippos, rogue hippos running riot in rural Colombia.

Now hippos don't naturally live in South America, they are from sub-Saharan Africa; but these hippos had been transported to Colombia by Pablo Escobar.

So Pablo Escobar, like many powerful men, fancied himself as a bit of a Noah's Ark, a kind of twisted Noah, in fact, and he wanted to build his own menagerie at Hacienda Napoles, which was his ranch where he controlled his drug empire in between Bogotá and Medellín in the Andes in Colombia.

And legend has it that he gathered together this menagerie by stealing a Russian cargo plane, flying it (or having it flown) to Africa where he loaded it up with loads of illegal wildlife, and then had to get it back to Colombia before the tranquilized animals woke up.

And amongst this cargo of creatures was one male hippo that was nicknamed el Viejo, and three females. And he transplanted these enormous beasts into a pond at his Hacienda, and they loved it. It turns out that Colombia is a hippo paradise, because actually all hippos really want is some nice shallow water to wallow around in and plenty of grass to eat, and Colombia has plenty of that. It also has no other hippos, so no other competition, and no natural predators.

So the hippos flourished in their new paradise and very quickly the four became eight, became 12, became 16. And what happens with hippos is you have a male who has a harem, so every time one of el Viejo's sons reached sexual maturity he would boot him out of the pond, and that young male hippo would then head off in search of hippo love elsewhere.

Now of course in Africa this is completely normal and hippos head off from their family group and young males will go in search of hippo love and start their own family. But of course in Colombia there are no hippos out there.

So these males, these young males are being spirited away from Hacienda Napoles in one of the many, many rivers in that part of Colombia, which acts like sort of hippo superhighways, so they're sort of pinging them out into the countryside and then they arrive and they can't find a mate. They're quite cantankerous beasts at the best of times, but these are sort of frustrated horny male hippos desperately looking for love where there is none.

I went to Colombia to investigate this story and visited one of these rogue males that had installed himself in a pond next to a kindergarten. I met the kids from the school who told me that they no longer liked to bathe in that particular pond—too right! And one of them told me about how his grandmother had been chased by the love-lorn beast for quite some time earlier in the week. So they're kind of running amok basically. So what do you do with this situation? You've got rapidly multiplying hippos—and the extraordinary thing about what's happening in Colombia is it's changing their behavior, because in Africa you have this very fierce dry season, which puts the brakes on hippo reproduction.

So I think they reach sexual maturity, I can't remember it's like five or seven years let's say, but in Colombia they were reaching sexual maturity significantly earlier and having babies every year instead of every two years. So they were multiplying at this much faster rate because they didn't have the same constraints on them that they do in Africa, so you've got a bit of a problem; you've got a massive invasive species.

Now with most invasive species the solution is to get rid of them, because basically they upset ecosystems and so they need to be removed.

But the problem with hippos though is that they star in Disney movies. So there was this sort of rogue male, and the first one that they put down there was a public outcry, and so they had to come up with a different plan. And I met with the Colombian vet who was in charge of implementing the radical plan B, which was the castration program. So Carlos Valderrama was given the onerous task of having to castrate one of these rogue males.

Now, this I'm sure you can appreciate is a job that takes a fair amount of cojones because castrating a wild male hippo is no easy task; it is actually unbelievably complicated. First of all hippos, because they've got so much fat, they're really hard to anesthetize (because the anesthetic dissolves in the fat) so even though they're enormous you have to get the dosage right.

They also when they're nervous they go into water. You don't want it to pass out in the water because it's going to drown. Then when you finally manage to anesthetize – Carlos said to me he had – and also they're really, really, really aggressive. Carlos told me that he had decoy cowboys and all sorts of shenanigans just to get through the anesthetic stage. Then the clock was on they had a certain amount of time in order to complete the castration.

Now, the thing about hippos is – the closest relative of the hippo is the whale. They have internal testicles that are buried underneath about eight inches of skin and fat. Not only that, but hippo testicles have a habit of wandering about when under threat. They can move her as much as eight centimeters. So you're looking at a blank canvas behind which there's a moving target.

Now, when you get in there they are the size of cantaloupes, but just getting to them and working out where to make the incision and then managing to grab hold of these moving melons and getting them out was quite some task. Apparently it took him about six hours and it cost well over $100,000. I think they've managed to do one more subsequently, but they're never going to - it's too difficult they're never going to be able to castrate all the males. That's just not going to happen. What I think is going to happen is we're going to have a new sub species of Colombian hippo before too long, "Hippopotamus Escobarus." Because that's how you make a new species: you put it in isolation from its parent species and it evolves into a slightly different species and that's what's going to happen.

The hippo has always been quoted as being Africa's most dangerous animal. Now I did some digging around and I've never really been able to find out the source of this fact, but I can tell you having been charged by a hippo myself they are really, really, really dangerous. I mean they do kill people. They are extremely aggressive, they move really fast. The male that we went to go and film, the rogue male in Colombia, we got a little bit too close and he was in his pond and he came toward us with such velocity that there were barrel waves coming off his nose as he hurtled through the water. It was pretty terrifying, I can tell you.

So the thing about hippos is they are they're really dangerous— so they're dangerous to humans in Colombia who don't understand how dangerous they are. Because culturally they haven't grown up alongside hippos, they've seen them in Disney films they think they're chubby and cute. Carlos told me that this is a real issue.

But also, of course, they are ecosystem engineers. One of the things about hippos is that they're big animals, they churn up a lot of vegetation. They have the power to reshape their environment and it's this reshaping of the environment that was most alarming to Carlos Valderrama, because he was worried that other native species that share the rivers like manatees, for instance, would suffer with hippos being around.