Pablo Escobar's hippos

In the late 1980s, Colombian drug lord Pablo Escobar kept four hippos in a private menagerie at his residence in Hacienda Nápoles, 100 kilometres (62 mi) east of Medellín, Colombia. They were deemed too difficult to seize and move after Escobar's death, and hence left on the untended estate. By 2007, the animals had multiplied to 16 and had taken to roaming the area for food in the nearby Magdalena River.[1][2] In 2013, the National Geographic Channel produced a documentary about them titled Cocaine Hippos.[3] As of early 2014, 40 hippos were reported to exist in Puerto Triunfo, Antioquia, from the original four belonging to Escobar,[4] and in 2018 the growing population was estimated at 50–70.[5] In December 2019, it was estimated that there were 65–80 individuals with their range covering almost 2,000 km² (770 sq mi) in Antioquia, Bolívar, Boyacá and Cundinamarca; in December 2020, the estimated population of hippos was around 90-120, with their range covering around 2,250 km² (870 sq mi) and now extending into Santander; it is expected that the population will almost certainly increase to more than 150 individuals within a decade and could reach up to more than 200 hippos, while the range eventually could cover more than 13,500 km² (5,200 sq mi).[6] Population projections estimate that there could be thousands within a few decades.[7]

Hippo at São Paulo Zoo

Being non-native introductions, most conservationists considered them problematic and invasive in Colombia, as they have the potential to change the ecosystems, feeding heavily on plants and displacing native species like the West Indian manatee, Neotropical otter, spectacled caiman and turtles.[6][8][9] The critically endangered Dahl's toad-headed turtle and Magdalena River turtle are largely restricted to the Magdalena River basin,[10] as are many threatened fish.[11] In 2020, a study showed that there was an increase in the nutrient levels and cyanobacteria in Colombian lakes inhabited by hippos. Cyanobacteria can cause toxic algae blooms and die-offs of aquatic fauna. Despite the limited magnitude of the observed change, it was noticeable since the species' population was still quite small.[8][9] Additionally, they can represent a serious threat to fishers and other locals. There have been attacks on humans, but as of 2017 nobody had been killed or seriously injured by the Colombian hippos.[12]

In contrast to the opposition by most conservationists, some ecologists have argued that they should remain and might even have a positive effect on the local environment. It has been suggested that the nutrients they introduce to the water and the occasional fish kills caused by them are overall positive,[8] but this was based on a study in their native Africa.[13] Alternatively, the introduced hippos could be a form of Pleistocene rewilding project, replacing species like Toxodon that became extinct in prehistoric times,[8] but Pleistocene rewilding itself is highly controversial.[14] Others have argued that the Colombian hippos should be regarded as a safe population, isolated from the threats faced by African hippos, and that they could be beneficial to the local ecotourism industry.[8]

In 2009, two adults and one calf left their herd and, after attacking humans and killing cattle, one of the adults (called "Pepe") was killed by hunters under authorisation of the local authorities.[2] When a photo of the dead hippo became public, it caused considerable controversy among animal rights groups both within the country and abroad, and further plans of culling ceased. Alternative methods have been considered, but they are unproven, or difficult and expensive. A wild male hippo was caught, castrated and released again, but it cost about US$50,000.[12] As of 2020, there were no plans by the local government on managing the population, but further studies on their effect on the habitat have been initiated.[8] Because of the fast-growing population, conservationists have recommended that a management plan needs to be rapidly developed.[6][7]