Parts of Pablo Escobar’s legacy were clear even before he was gunned down on the rooftop of one of his safe houses in 1993 – he was the drug lord who manufactured and shipped massive amounts of cocaine, the murderer who executed untold numbers of people on Colombia’s streets, the terrorist who murdered 110 people when he blew up a commercial airplane.

But one of his most enduring legacies was probably not as apparent.

Hippos.

In the 1980s, Escobar smuggled several into his Colombian estate, Hacienda Nápoles, along with many other exotic species, to create a private zoo. After seizing the property, authorities sold off the animals but left the four hippos.

“It was logistically difficult to move them around, so the authorities just left them there, probably thinking the animals would die,” Nataly Castelblanco-Martínez, a Colombian ecologist working at the University of Quintana Roo in Mexico, told the BBC earlier this year.

Instead, the hippos flourished. In the 27 years since Escobar’s death, the group of four has swelled to between 80 and 120. Researchers recently estimated their numbers will skyrocket to more than 1,400 by 2039 if left alone.

By then, the hippos will have done irreversible damage to the environment, and their numbers will be impossible to control, researchers said. Authorities this year have intervened, using a chemical contraceptive to sterilize the animals without the blowback that would come from exterminating what has grown to become “the town pet.” Developed by the U.S. Department of Agriculture, the drug GonaCon inhibits production of an animal’s sex hormones, such as estrogen and testosterone, putting it in “a nonreproductive state.” The USDA donated 55 doses of the drug to Columbian wildlife officials.

Colombia has proved to be a “hippo paradise.” In the hippos’ native Africa, seasonal droughts keep their population tamped down by making them vulnerable to disease and predators. Without that natural check, their numbers exploded in Colombia, where water is plentiful year round, food is abundant and there are no predators big enough to threaten them.

Officials have tried to make the case over the years that hippos are bad for the country. Ecologists say they wreak havoc on the environment. Nutrients in the hippos’ feces fuel algae blooms, which reduce oxygen levels in the water. That can kill fish, kneecapping local industry. Hippos can also hurt people. Last year, one bit a rancher’s leg, breaking his leg, hip and several ribs.

As a result, scientists argued for their sterilization in a research paper published in January. Castelblanco-Martínez, the paper’s lead author, has said the hippos pose “one of the greatest challenges of invasive species in the world.” Euthanizing 30 each year is “the only efficient strategy to deal with the invasion,” Castelblanco-Martínez and her colleagues wrote in their paper.

“It is obvious that we feel sorry for these animals, but as scientists we need to be honest,” Castelblanco-Martínez told the BBC. “Hippos are an invasive species in Colombia and if we do not kill a part of their population now, the situation could be out of control in just 10 or 20 years.”

But officials haven’t sold the public on the argument that hippos are bad. In fact, some Colombians have grown fond of the African transplants over the years, The Washington Post reported earlier this year. They’ve even stoked a budding tourism industry. Residents give visitors safari tours and sell hippo-related souvenirs. Gift shops in a nearby town sell hippo T-shirts and keychains. At the amusement park built on the ruins of Hacienda Nápoles, tourists check out the lake where dozens of hippos now live.

“The hippopotamus is the town pet,” one resident, Claudia Patricia Camacho, said in a 2018 piece by the news program Noticias Caracol.

That affection has stopped officials from taking Castelblanco-Martínez and choosing the easiest, most efficient option of stopping the hippos’ spread and environmental destruction: killing them. After three hippos escaped Hacienda Nápoles in 2009 and reportedly terrorized local farms, Colombia’s environmental agency sent hunters to kill the animal. When a photo emerged showing the hunters posing with the carcass of one of the adults that had been killed – a male named Pepe – animal rights activists howled. A judge quickly suspended further hunts.

“Some people in Colombia can get very angry when they talk about the hippos,” Castelblanco-Martínez said. “People tend to understand much more about invasive species when we talk about plants or smaller creatures, instead of a massive mammal that many may find cute.”

Officials fell back on less drastic measures. One option they tried was traditional sterilization, but that took too much time and money. Scientists spent three months tracking one male. Even after they found it and doped it with a powerful elephant tranquilizer, they struggled to find its reproductive organs. Plus, castration can cost around $50,000.

“It was horrible,” recalled David Echeverri López, a researcher at the regional environmental agency Cornare, which led the 2013 sterilization effort.

Cornare and Echeverri López are also leading the most recent project to sterilize the hippos through chemical contraception. In a statement, they said officials plan to keep sterilizing the main group of hippos that grew out of the ashes of the Escobar drug empire.