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Colombia’s drug war may be over, but one strange legacy remains.

The region around drug lord Pablo Escobar’s former estate is suffering from an invasion of hippopotamuses, affecting the delicate balance of the local eco-system according to scientists.

Hippos swim languidly in the Rio Magdalena, Colombia’s principal river, which runs through the centre of the country.

The large aquatic mammal is not normally associated with Latin America.

But an estimated 80 hippopotamuses, perhaps more, live in the area around the Rio Magdalena.

They’re descendants of four hippos that were brought to the country by infamous drug lord Pablo Escobar for his personal collection.

Occasionally a hippopotamus will appear on the local soccer pitch to graze or wander down the back streets of rural Doradal, a small Colombian town a four-hour drive from Medellin

The rapid growth of their numbers has authorities worried that residents could be attacked.

The three ton animals can be aggressive and kill more people per year in Africa than any other wildlife species.

Scientists also worry that their presence threatens the area’s native flora and fauna, says Gina Serna a government veterinarian:

“We have a big problem with the hippopotami. They are an exotic introduced species that is having an accelerated growth (in Colombia). We have over 60 or 70 individual hippos in the wild. These animals aren’t controlled, they are living free. They are causing a series of ecological problems. They are effecting the ecosystems in the region of the Magdalena and they are displacing the native species.”

Locals have learnt to live with them, shops sell hippo inspired trinkets.

Shopkeeper Yordan Villegas says they are good for business.

“It’s really nice and impressive to see them in the streets of our town. For me we should maintain them because it generates more tourism and the people want to come and see these animals.”

Others are not so keen on their new neighbours.

Maria Jaramilla awoke in the middle of the night to the sound of her panicked mule.

When she looked out across her small yard, her confusion turned to shock: a hippopotamus had wandered down her driveway and was inspecting her house.

“It was a big fright for all of us,” says Jaramilla.

In the 1980s, during the height of his power, Escobar kept the hippos on his 5,500-acre (2,225-hectare) estate, Hacienda Napoles, where he kept a private zoo stocked with exotic animals such as elephants and giraffes.

Following Escobar’s death in 1993, most of the animals were taken to new homes or died.

But not the hippos. Their size and cost to transport meant they were abandoned.

But while Escobar’s estate fell into disrepair, the animals thrived.

The local conditions are ideal for the hippos. They live in the area’s large lakes and waterways, enjoy endless grass pastures to graze in and they have no natural predators.

Students at a small primary school behind Escobar’s former estate arrive for class each day by passing signs that warn of hippo danger.

“It worries us,” says Wilber Quinones who teaches at the school. “We have to lock ourselves inside with the children to try and avoid an accident.”

So far, the hippos have refrained from attacking humans, but as their numbers grow and they expand into more populated areas, experts fear an attack could be inevitable.

The hippos are also having an impact on the native flora and fauna with indications the presence of the animals is displacing some local species.

A recent study by researchers at the University of California, San Diego, found the hippos are changing the quality of the water in which they spend much of their time.

The hippos feed at night then spend the day cooling off in the water where they defecate, changing the chemistry in the lakes, resulting in the harmful algae blooms and things like red tide bacteria, say the researchers.

That has left local authorities scrambling to come up with a solution to the hippopotamus problem.

Cornare, the environmental agency that is responsible for the region where the hippos are currently located, has been tasked with finding a solution that residents are comfortable with but that also deals with the animals in a humane way.

Now a plan is underway to sterilize the animals. Serna and a group from Cornare last year conducted an in-the-wild surgical sterilization of a female, the first ever in Colombia.

It’s a complex procedure that requires luring and trapping a hippo in a corral before using sedatives to put it to sleep.

Just cutting through a hippo’s dense layers of skin, fat and muscle takes three hours. Then the animal is sterilized and stitched up and allowed to return to the wild while their recovery is monitored.

“It’s urgent,” says Serna, who works for Cornare.

“We already have a report of a family of hippopotamuses in the Magdalena river. The Magdalena connects almost all of Colombia so they could move into any part of the country.”

Later this year, Cornare will attempt multiple surgical sterilizations, as well as a chemical sterilization technique that has been successful in pigs.

But they acknowledge that won’t be enough to contain a hippopotamus population that is estimated to quadruple over the next 10 years and could eventually reach into the thousands.

For now, it’s as much as they can afford.

In the meantime, the locals are learning to live with their new neighbours.