Published January 26, 2020
By Alice Klein
Shooting wild horses with contraceptive darts may help curb their numbers in parts of the US where their populations are ballooning out of control.
Horses were reintroduced to the US by the Spanish in the 16th century, after native North American horses went extinct about 10,000 years ago. Lacking natural predators, they have since spread across the rangelands in the western US.
More than 70,000 wild horses now occupy public rangelands, about 45,000 more than the limit set by the US government to ensure there is enough food and water to share with livestock and native wildlife.
The US Bureau of Land Management manages excess horses by adopting out a small number and housing the rest in long-term holding facilities. There are currently almost 50,000 horses being held, but these facilities are becoming overwhelmed and many scientists think they are cruel.
“Horses form these tight social bonds and then they’re torn apart and shoved in these really crowded conditions and spend the rest of their lives there,” says Tiana Pirtle at the University of Tasmania in Australia, where authorities are also struggling to manage a booming wild horse population.
Since the 1980s, the US has been experimenting with using contraception to reduce wild horse numbers. However, most contraceptives developed to date only last a short time or have to be injected by hand, making them unsuitable for areas where horses inhabit rough, hard-to-access terrain.
Now, Allen Rutberg at Tufts University in Massachusetts and his colleagues have shown that shooting wild horses with contraceptive darts is effective in reducing births. In the study, horses in rugged areas were targeted by luring them with baits and shooting them with darts containing a long-acting contraceptive called porcine zona pellucida-22 (PZP-22).
The researchers placed baits in areas of high horse traffic – as identified by the presence of tracks and faeces – in the mountainous Carson National Forest in New Mexico. When mares visited the bait sites, the researchers used rifles to shoot them with darts from about 10 to 15 metres away.
In the following year, only 2 of 17 treated females gave birth to foals. They were the last two to receive PZP-22, suggesting better results could be achieved by treating all mares earlier to ensure the contraceptive has enough time to work before the breeding season, says Rutberg. In comparison, 15 of 17 untreated females gave birth to foals.
In a separate study, the researchers found that shooting mares with “booster” darts of PZP-22 extended its contraceptive effect by an extra 4 years, suggesting it could provide at least 5 years of contraception, says Rutberg. No harmful side effects have been identified so far, he says.
Since the study only targeted a small number of mares, horse numbers in Carson National Forest didn’t decrease. “To actually reduce the population using contraception alone, you’d need to treat more than 90 per cent of the mares,” says Rutberg. His team is currently seeking permission to treat a larger proportion of horses in the area with PZP-22 to see if it successfully reduces their numbers.
In the short-term, fertility control will probably have to be used alongside housing wild horses to reduce their numbers, says Carolynne Joone at James Cook University in Australia. “The problem with contraception is that it doesn’t get rid of horses, it just stops them from breeding,” she says. “So even if a female isn’t contributing to the population, she’ll still be there for another 20 or 25 years.”
Once horse numbers have been brought down to a manageable level, fertility control could be used to keep them in check in the long-term, says Joone. “It’s a great strategy for maintaining populations,” she says.
Journal reference: Wildlife Research, DOI: 10.1071/WR18112
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