Colombia’s “cocaine hippos” are making waves in their new home, but whether that’s a good thing or not depends on who you ask.

WHEN THE NOTORIOUS drug lord Pablo Escobar was shot dead in 1993, the Colombian government took control of his luxurious estate in northwestern Colombia, including his personal zoo. Most of the animals were shipped away, but the four hippopotamuses—of which Escobar was especially fond—were left to fend for themselves in a pond. Now, there are dozens and dozens of them.

For over a decade the Colombian government has been pondering how to best curb the growing population, a strategy largely supported by conservation experts. But not everyone is on board. Without direct evidence that the animals are doing harm, some ecologists argue that there’s no reason to cull or relocate them. Indeed, the hippos could fill in for species that humans pushed to extinction thousands of years ago—an idea known as rewilding.

When the hippos were left behind, it accidentally kicked off a rewilding experiment that’s now been running for more than 25 years. The first results of this experiment are trickling in and much like the large animals, they’re muddying the waters.

Known unknowns

The hippos have escaped Escobar’s former ranch and moved into Colombia’s main river, the Magdelena. Spread over a growing area, nobody knows exactly how many there are—but estimates indicate there may be a total population between 80 and 100, says Jonathan Shurin, an ecologist with University of California San Diego who studies the animals.

That’s at least a couple dozen higher than estimates just two years ago. Given that there were four in 1993, the population appears to be growing exponentially. “Within a couple of decades, there could be thousands of them.”

The hippos present quite a problem for the government. David Echeverri, a researcher with the Colombian government’s environmental agency Cornare, which is overseeing management of the animals, says he has no doubt they act like an invasive species. If allowed to remain unchecked, they will displace endemic animals like otters and manatees, he says. They also pose a danger to local residents since they can be territorial and aggressive, though no serious injuries or deaths have occurred as yet.

After one hippo was killed in 2009, there was a quick public outcry, quashing any plans to cull them. Instead, the government has been investigating ways to sterilize the creatures, or to move them out of the wild into captive facilities, Echeverri says. But the animals weigh thousands of pounds and aren’t exactly fond of human handling, so relocating or castrating them is both dangerous, difficult, and expensive. One juvenile hippo was successfully moved to a Colombian zoo in September 2018, but it cost 15 million pesos (about $4,500 USD).

South America lost dozens of giant herbivore species in the last 20,000 years or so, including the somewhat hippo-esque toxodons, which may have been semi-aquatic, as well as water-loving tapirs. Although several tapir species remain today, all are declining. “Hippos could likely contribute a partial restoration of these effects, likely benefitting native biodiversity overall,” Svenning says. He’d let the hippos be for now, while monitoring the creatures to ensure they don’t become a problem.

Jonathan Shurin, an ecologist with University of California San Diego who studies the animals notes that the animals may be providing a valuable service for native plants that once relied on large, now-extinct mammals to disperse their seeds. “We’re planning to look at their poop and see what’s in there,” he says.