Four hippos brought to Colombia in 1981 have multiplied into nearly 200 animals roaming the Magdalena River basin — the world’s largest invasive animal, spreading through warm rivers with abundant food and few natural checks

A sign beside a cattle pond in central Colombia warns visitors that the water belongs to the most dangerous animal on earth and that they should stay out of it. The sign is accurate. It also sits about 11,000 kilometres from the continent where that animal evolved.

Somewhere between Doradal and Puerto Triunfo, roughly halfway along the road from Bogotá to Medellín, wild African hippos have spent four decades quietly compounding.

How four became a herd

Pablo Escobar had three females and one male shipped in from a US zoo in 1981, stocking the private menagerie at his Hacienda Nápoles estate along with giraffes, zebras and a set of concrete dinosaurs. After he was shot dead in 1993, most of the collection was parcelled out to zoos, but a three-tonne territorial herbivore is nobody’s idea of an easy transfer, so the hippos were left behind. In time they wandered off the estate, into the Magdalena River system, and made themselves at home.

At home is the operative phrase. Hippo numbers in Africa get held down by drought, crocodiles, lions and the seasonal collapse of water levels, and none of that applies in the wet lowlands of Antioquia. Biologist Katerine Corrales made the same point in remarks to a Caracol news crew on one hippo-occupied river island, carried by The Bogotá Post: without Africa’s dry season to reset things, a constant climate and abundant water just mean faster breeding.

Nobody quite agrees on the number

When the sterilisation programme started, the environment ministry was working from a figure of 169 animals, as reported by CBS News, together with a warning that doing nothing could mean 1,000 hippos by 2035. A 2023 modelling paper in Scientific Reports, led by ecologist Amanda Subalusky, counted 91 in the middle Magdalena basin and put annual growth at 9.6 per cent, which gets you 230 by 2032 and more than 1,000 by 2050. A field census commissioned by the ministry, run with drones and researchers counting in person, then put the number at 181 to 215, in coverage by Scientific American.

Those figures disagree because they measure different things, one basin against one country, a projection against a headcount. What none of them dispute is the direction of travel.

The trouble is mostly digestion

Hippos graze on land at night and spend the day sitting in water, which turns them into a nutrient pump. A team led by Jonathan Shurin at UC San Diego sampled 14 small lakes near the old estate and compared the ones with hippos in them to the ones without, publishing the results in the journal Ecology. Lakes with hippos were more dominated by cyanobacteria, the blue-green algae behind harmful blooms, and showed measurably different ecosystem metabolism.

One paper is one paper, and this one caught an invasion in its early stages. Shurin’s team framed their own finding carefully, warning that continued range expansion may pose a threat to water resources rather than claiming the ponds they sampled were already ruined. Colombia’s environment ministry has since gone further, citing damage to water quality and to native species including the manatee and the river turtle when it declared hippos an invasive species in 2022.

The cheap options expired a while ago

Sterilising a wild hippo means capturing it, anaesthetising it and operating, at roughly US$9,800 a time, with real risk to the animal and to the vets. Relocation looked kinder on paper and went nowhere: officials approached seven countries and two international zoo and aquarium associations, and as Scientific American has documented, not one agreed to take a single animal. Inbreeding has left the Colombian population with genetic defects, which cooled institutional interest further, per Reuters.

Subalusky’s team also modelled the price of hesitating. Bringing the population down for good would cost at least one to two million US dollars if started promptly, and delaying by a single decade multiplies that minimum by 2.5 while putting some methods out of reach entirely.

What is meant to happen this year

Environment minister Irene Vélez announced on 13 April that the government will euthanise around 80 hippos, starting in the second half of 2026. Some 7.2 billion pesos, roughly US$2 million, has been set aside to cover the cull along with confinement and relocation. Writing in Mongabay, Naina Rao noted this amounts to the first sanctioned hunt in the country in 40 years. The plan leans on a 170-page technical report produced in 2022 by the Humboldt Institute and Universidad Nacional, and targets hotspots near Puerto Triunfo and the river island of Isla del Silencio.

It has not gone down quietly.

Senator Andrea Padilla called the decision cruel and said she would never back the killing of healthy animals that are themselves victims of state negligence. Down on the river, the objection is less abstract. Some fishing communities have lost access to water they used to work. Others built a small tourism trade on the animals, and one fisherman told The Guardian in 2023 that the hippos were born there and are Colombian now.

Vélez has budgeted for 80 animals in a basin holding around 200 that adds roughly a tenth of itself every year. Move slowly enough through the first 80 and the arithmetic just resets.