Colombia has started sterilising hippos descended from animals illegally brought to the country by drug lord Pablo Escobar in the 1980s.

The hippos spread from Escobar’s estate into nearby rivers, where they flourished.

There are now more than 100 of them, prompting the government’s plan to control the population.

Two male hippos and one female have been surgically sterilised, environmental authorities said.

The government plans to sterilise 40 hippos a year, transfer others to different countries, and may euthanise some of the animals.

The hippos have no natural predators in Colombia and have been declared an invasive species that could upset the ecosystem.

Sterilisation takes time, because spotting and capturing the territorial, aggressive 3-ton animals is complicated, David Echeverry Lopez, chief of the environment office in charge of the plan, said.

The amount of rain has made things even more difficult.

More grass means “they have an oversupply of food, so baiting them to capture them becomes even more complicated,” he said.

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A group of hippos was brought in the 1980s to Escobar’s private zoo, Hacienda Napoles, which became a tourist attraction after his death in 1993.

Most of the animals live freely in rivers and reproduce without control. The government estimates there are 169 hippos in Colombia – and that if no measures are taken, there could be 1,000 by 2035.

When the plan was first announced, the environment ministry said each sterilisation costs about $9,800, and involves risks for the hippopotamus, including allergic reactions to anesthesia or death – along with dangers for the animal health personnel involved.