A handful of hippos brought to Colombia in the 1980s for Escobar's private zoo has grown to a population of 80, prompting biologists to express concern over their environmental impact and threat to human safety. In January, a study published in Biological Conservation called for the growing herd -- now ranging wild in the Antioquia department -- to be culled.

The regional government has instead tried neutering the enormous beasts, but traditional surgical sterilization is too dangerous and demanding to scale up. Only 11 of the hippos have so far been sterilized this way, according to local authorities.

On Friday, however, the regional environmental agency Cornare announced its efforts to control the hippo population had led to 24 more hippos being treated with a new method: darts laden with the contraceptive drug GonaCon.

Compared to surgery, GonaCon is "a much cheaper option," according to a Cornare statement. "However it is still complex since experts suggest giving three doses, based on studies and comparisons made in other large animals such as horses." The same drug has been tested on other wild animal populations, including wild horses in the United States, kangaroos in Australia and wild cattle in Hong Kong, it said.

Scientists must now track the efficacy of the drug by measuring hormone levels in hippo feces.

"It is the first time that we are implementing this procedure. We are going to follow up and monitor it to find out how successful it can be," said David Echeverri López, coordinator of the Cornare Forest and Biodiversity Group.

Cornare and USDA specialists involved in the hippo pilot project.

Hippos are seen at the Hacienda Napoles theme park, once the private zoo of drug kingpin Pablo Escobar at his Napoles ranch, in the Colombian town of Doradal on September 12, 2020.

Escobar's hippopotamus collection began with only one male and three females. Upon his death, other species of exotic animals were relocated, but the hippos were left because they were too difficult to capture and transport, according to the Biological Conservation study. They soon began to multiply, spreading around the Magdalena river basin from their original home some 100 miles east of the city of Medellin.

Research has shown negative effects of hippo waste on oxygen levels in bodies of water, which can affect fish and ultimately humans. Hippos also pose a threat to agriculture and to the security of people in affected areas, according to the Biological Conservation study. In May 2020, a hippo attack left a 45-year-old man seriously injured.