The Colombian government has sterilised dozens of hippos once owned by notorious drug lord Pablo Escobar, amid concern over their impact on wildlife and the local population.
So far, the authorities have sterilised about 24 of 80 creatures. The drug lord Pablo Escobar was killed by police in 1993 had smuggled 'exotic animals, including a male and a female hippo', reports BBC News.
Since then, a growing population has been taking over the countryside near his former ranch, Hacienda Nápoles.
The governmental group Cornare said it has already sterilised 24 of the animals, using the drug GonaCon, a contraceptive developed by the US Department of Agriculture, which donated 55 doses to help the effort.
Eleven hippos had previously been sterilised "in the traditional way", according to Cornare.
Colombian environmentalists say the hippos, believed to be the biggest herd outside Africa, are an invasive species and have pushed away the native fauna.
Many have campaigned for the animals to be culled or sterilised.
Escobar – a notorious criminal responsible for kidnappings and death on a vast scale, once thought to be one of the world's richest men – had purchased various breeds of exotic animals to live on his ranch, such as flamingos, giraffes, zebras and kangaroos.
After his death in a shootout 1993, all of the animals were sold to zoos but not the hippos.
Earlier this year, Columbian Biologist Nataly Castelblanco told BBC News that the authorities had kept the creatures in the zoo as they believed that the animals would 'die'. However, the population of Hippo has continued to increase over time.
According to experts, they started spreading through one of the country's main waterways - the River Magdalena.
Scientists studying the hippos' environmental impact say the animals could affect the local ecosystem in a number of ways: from displacing native species already under threat of extinction, like the manatee, to altering the chemical compositions of waterways, which could endanger fisheries - though other studies suggest they might help the environment too.