Scientists have called the hippos "one of the greatest challenges of invasive species in the world."
The Colombian government has had to sterilise dozens of hippos once owned by drug lord Pablo Escobar due to concerns over the impact they are having on wildlife and the local population.
Known as 'cocaine hippos,' the herd has grown from a population of 35 in 2012 to 80 this year, and is thought to be the largest herd outside of Africa.
Researchers are worried that the herd could grow as large as 1,500 in the next two decades though if they're breeding isn't controlled.
Previous plans to cull the herd received widespread opposition from the local population, so now a local environment agency has started sterilising them, using drugs from Washington.
Governmental group Cornare said it had already sterilised 24 of the animals using a contraceptive drug called GonaCon. The sterilisation of hippos is made more difficult by the fact that they have internal testicles.
Whilst most of Escobar's exotic animals were sent to zoos after his death in 1993, the hippos were left to roam on his ranch before their offspring spread to the Magdalena wetlands.
But they have toxic urine and faeces that causes algae blooms, and have the potential to wreak havoc on the local ecosystem, polluting waterways and dominating other species as they have no predators to control their numbers.
Earlier this year, scientists called for their sterilisation in a scientific paper. The lead author of the paper,Nataly Castelblanco-Martínez, of the University of Quintana Roo, previously said she believed the hippos pose “one of the greatest challenges of invasive species in the world”.
The ecologist had called for 30 hippos a year to be culled.
But local populations have grown to love the hippos, with their tourism appeal also providing livelihoods for locals who give safari tours and sell hippo-related souvenirs, according to the Independent.
Despite calling for “urgent” population control of the hippos in their study, published in the journal
Biological Conservation, the researchers
noted that “the proposal of a course of action can become controversial when the species has a charismatic value for the society, regardless of its ecological or social impact”.
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