HACIENDA NÁPOLES, Colombia-Castrating hippos wasn't part of Cristina Buitrago's veterinary training, especially since the natural habitat of the semiaquatic beasts is roughly 7,000 miles away.
But she has gotten a crash course in the procedure now that the Colombian government has declared the African natives an invasive species.
Her six-person team lures the target into a corral with 180 pounds of carrots, bringing it down with enough sedative-filled darts to fell three horses. Rolling the hippo comes next. Ms. Buitrago then extracts the testicles, located under thick folds of skin. The operation can take five hours under the tropical sun and cost up to $17,000 in a country that struggles to finance healthcare for humans.
"It's dirty. There's mud everywhere. You're soaked in sweat," says Ms. Buitrago, a veterinarian for Cornare, a state-sponsored environmental group. "This is not a practical way to solve the problem."
Sterilization, hunting, and other schemes have failed to winnow the hippos, which have grown to a wild herd of more than 140 since drug lord Pablo Escobar imported a handful of the animals to Colombia 43 years ago for his private zoo.
Years after his 1993 death, some escaped and got busy breeding in the lakes and marshes of this broiling swath of northern Colombia, where they eat industrial quantities of grass, poop prodigiously and, in two instances, have mauled people.
Now, Colombian authorities are teaming up with a Mexican animal lover to fly them out of the country.
Ernesto Zazueta runs Ostok, a private animal sanctuary in Culiacán, Mexico, that's home to macaws, pumas, jaguars, two hippos named Fredy and Kathy, and an elephant that goes by Big Boy who was rescued from a circus. His likeness is tattooed on Mr. Zazueta's left forearm.
Mr. Zazueta points out that African hippos have been classified as either endangered or at-risk animals, which is why he opposes hunting or sterilizing them in Colombia. His plan involves trapping breeding-age hippos, placing them in custom-built metal crates and then transporting them to his reserve or to a large sanctuary in India.
"Depending on their size, we could get 15 to 20 on a single aircraft," says Mr. Zazueta, a gravelly voiced 59-year-old who heads Mexico's association of aquarium and zoo keepers. "This is a huge opportunity to help the species."
Aníbal Gaviria, who is the governor of Antioquia province and recently flew with Mr. Zazueta in a helicopter to take in the full scope of the hippo problem, has signed off on the transfer as long as private donors pick up the $5 million tab for the cages and airfare. Only half of the herd would be moved, but it appears to be the best of Colombia's limited options.
"We can't just stand by as they reproduce at will," says David Echeverri, a Cornare biologist who has been wrestling with the hippo issue for 15 years. "If someone is killed in an attack, the scandal is going to be tremendous."
The hippos first arrived here in 1980 when Mr. Escobar, the billionaire head of the Medellín cocaine cartel, stocked his vast Hacienda Nápoles ranch with elephants, giraffes, zebras and other exotic species. After police gunned him down with U.S. assistance, most of the animals died or were donated to zoos.
But some of the hippos went feral by working their way into the nearby Magdalena River, the Mississippi of Colombia that runs through the middle of the country.
In Africa-where hippos live in sub-Saharan countries from Kenya to South Africa-the very young can be killed by crocodiles and lions. But the main predator for the 130,000 hippos that remain are poachers out for their meat and ivory teeth.
In Colombia, with no natural predators, they have colonized a 120-mile stretch of the river and are now breeding faster than their African relatives. Twenty years ago, there were 10 of them. But hippos live long lives-more than 40 years in the wild-and females can give birth to a calf every two years.
By 2039, Colombia's hippo population could reach 1,500, according to the scientific journal Biological Conservation.
Last year, Colombia's Environment Ministry declared that hippos, which can tip the scales at 9,000 pounds, are an invasive species. The decree notes that they displace manatees, otters and capybaras. They also degrade water quality, compact soil and create canals that alter the water flow of Magdalena tributaries, which could cause fish kills.
In Africa, hippos kill up to 500 people a year. Yet many people here, including tourists who visit the region, view them as lovable, Disney-like creatures.
At Hacienda Nápoles, which is now a theme park, hippos in artificial lakes are a major draw. They include a female named Vanesa who waddles out of the water to munch carrots tossed to her by onlookers from behind a fence.
"They're good for business," says Nestor Orozco, who runs a hotel next to the theme park and sells hippo souvenirs just feet from a sign reading: "Danger: Presence of Hippopotamus."
After dark when the heat has lifted in the Magdalena River town of Doradal, hippos wander through neighborhoods like stray dogs. "Those animals look so pretty passing by," says 76-year-old Luis Enrique Isaza.
Reports and videos on social media have surfaced of people who've adopted baby hippos, raising them on infant formula and treating them like big dogs.
For others here, the animals are nothing but a menace.
They have destroyed so many barbed-wire fences that ranchers build special wooden gates to allow them across pastures. In November, a man on a motorcycle slammed into a hippo, destroying his front wheel and fracturing an arm. Though motorists flash headlights to shoo them off roads, a car on a recent day smashed into a hippo, killing the animal.
"From a distance, they are beautiful. but if you get near them, they are aggressive" says Flor Marina Andrade, whose 14-year-old granddaughter attends a country school next to a hippo-filled lagoon.
A teacher at the school, Yorlin Cuesta, says her pint-size charges find the hippos endlessly fascinating and have drawn them for school art assignments. But she worries.
"It's a delicate situation, and we're right in the middle," she said. "Look at these little kids. Can you imagine a hippo coming in? I'd start running."
Hippos here have charged out of the water to attack people, including farmworker Javier Díaz, who was hospitalized for a month with three cracked ribs and a broken leg and collarbone. He still hobbles around on crutches two years later.
His employer, cattle rancher Juan Cadavid, says: "The hippo tossed him in the air as if he were a ball."
Fisherman Álvaro Molina recalls colliding with a hippo on the Magdalena River when the animal suddenly rose up out of the water. His small wooden boat flipped and his motor sank but he managed to swim to shore. "It was like a giant pig," he says.
Colombian officials have long sought to avoid such encounters. In 2009, they hired a big-game sharpshooter who killed one of Mr. Escobar's original hippos. The animal went by the name of Pepe.
But the hunt backfired when a photo emerged of soldiers posing triumphantly with the dead animal. Then, in a macabre twist, Pepe's severed head was found buried in the Andes mountains 200 miles away. Following street protests and a save-the-hippos campaign, a Colombian judge in 2012 banned hunting the animals.
Mr. Zazueta's hippo airlift is seen as a more humane solution. But if that doesn't work, sterilization will remain Colombia's main line of defense.
Veterinarians have so far "fixed" 11 males and two females. Previously, the largest animal Ms. Buitrago had operated on was a horse, the smallest a bird weighing less than an ounce. Now, she's getting used to hippos. But given the dimension of the problems they have caused, hippo gonads strike her as rather modest.
"They're about the size of chicken eggs," she says. "Maybe a little smaller."