Wildlife officials in Colombia are dealing with a rather large and unusual invasive species: hippopotamuses. Originally smuggled into the country by the notorious drug trafficker Pablo Escobar in the 1980s, the semiaquatic mammal from Africa has flourished in the tropical rivers of the South American country and now threaten local ecosystems.
To deal with the problem, biologists are injecting a number of hippos with a contraceptive to prevent the problem from procreating, report Stefano Pozzebon and Jack Guy of CNN. So far, 11 have been treated while another 24 are scheduled to receive the immunocastration drug GonaCon by dart rifles.
“It is a contraceptive that is effective in males and females,” Gina Paola Serna Trujillo, a veterinary doctor with the regional environmental agency Cornare, says in a statement. United States wildlife officials, who use the drug to control deer and wild horse populations, are assisting with the effort.
“The mission is to use our scientific experience to help in human conflicts with wildlife and at the same time, preserve the environment we share,” Douglas Eckery of the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service explains in the statement.
Currently, Cornare officials estimate there are at least 80 hippos in three locations in Magdalena Medio, a region in interior Colombia bordering the Magdalena River. Before he was gunned down in 1993, cocaine kingpin Escobar brought four hippos—one male and three females—along with other exotic animals to his Hacienda Nápoles estate, which is now a theme park about 155 miles northwest of the country’s capital of Bogotá.
After his death, authorities seized the property, sold the other species but left the hippos, Fernando Duarte reported for BBC News earlier this year. “It was logistically difficult to move them around, so the authorities just left them there, probably thinking the animals would die,” says Colombian biologist Nataly Castelblanco-Martínez, who currently conducts research at the University of Quintana Roo in Mexico, tells BBC News.
That turned out to be a ticking ecological timebomb as the invasive species—with ideal conditions for breeding and no predators to control the population—found themselves in a hippopotamus paradise.
Without natural controls, numbers soared in the jungles of Colombia, where ecologists have been warning about the impact to the environment. The mammals eat tons of vegetation and produce nutrients in their feces that promote algae blooms, which reduce oxygen levels in the water, reports Jonathan Edwards of the Washington Post.
In addition, hippos, which can grow to 6,000 pounds, pose a serious threat to humans. Last year, one attacked a local rancher, breaking his leg, hip and ribs, per the Washington Post. In Africa, hippos kill an average of 500 people each year, according to a 2016 BBC News report.
Environmental officials originally wanted to euthanize the hippos but public opinion was against that plan. In 2009, residents registered protests after three escaped from the theme park and one was killed by hunters, Hugh Bronstein of Reuters reported at the time. A judge later put a halt to the practice.
Cornare staff then started to castrate male hippos but that proved difficult, time-consuming and expensive—castration costs can easily exceed $50,000 per animal, reports the Washington Post.
After consulting with American officials, local biologists opted to use the sterilization drug. Developed by the USDA, GonaCon puts an animal into “a nonreproductive state” by controlling production of sex hormones, such as estrogen and testosterone, the Washington Post states.
In the statement, Cornare officials say they are hopeful they can “control the birth of this invasive species that is gradually dispersing throughout the Magdalena Medio.”