Managing wild horse populations a challenge worldwide
THE management of wild horse populations is a problem faced in many countries around the world.
The main countries with wild horses are the US, New Zealand and Australia, and growing numbers are causing concern, with methods to control them causing strong and at times angry debate among environmentalists and horse lovers.
Horses were reintroduced to the US by the Spanish in the 16th century, after native North American horses became extinct about 10,000 years ago.
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.
Now, another method is being trialed.
Allen Rutberg and his colleagues at Tufts University in Massachusetts 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 manure – in the mountainous Carson National Forest in New Mexico. When mares visited the bait sits, the researchers used rifles to shoot them with darts from about 10 to 15 metres away.
In the following year, only two of 17 treated females gave birth to foals. They were the last two to receive PZP-22, with Prof Rutberg suggesting better results could be achieved by treating all mares earlier to ensure the contraceptive has enough time to work before the breeding season. 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 four years, suggesting it could provide at least five years of contraception, says Rutberg. No harmful side effects have been identified so far, he said.
In New Zealand, horses were first reported in the Kaimanawa Range in 1876, although the first horses had been brought to New Zealand in 1814.
The herds grew as horses escaped or were released from sheep stations and cavalry bases. Some of the horses were recaptured by the locals to be riding mounts, as well as for their meat, hair and hides.
The number of horses declined as large-scale farms and forestry operations were built on the ranges, and there were only around 174 wild horses by 1979.
The New Zealand government started protecting the Kaimanawa horses in 1981 and numbers grew to 1576 by 1994.
Roundups have been carried out annually since 1993 to manage the size of the herd, removing around 2800 horses altogether. All suitable horses are available for rehoming after the roundups in April/May each year.
However, due to growing numbers New Zealand is also turning to contraception to as a control method.
This year’s muster saw the introduction of the immuno-contraceptive drug GonaCon Equine, which was injected into 60 mares.
The effects of the vaccine, which renders mares temporarily infertile, won't be seen for about three years.
In Australia, Brumbies are the descendants of escaped or lost horses, dating back in some cases to those belonging to the early European settlers.
Today they live in many areas, including some National Parks, notably Alpine National Parks and the Bahmah Forest in Victoria, Barrington Tops National Park in NSW and Carnarvon National Park in Queensland.
They are currently under threat and discussion is raging, particularly over management in the Barmah Forest area where environmentalists regard them as a pest and threat to native ecosystems but are they are greatly valued by others as part of Australia's heritage. Shooting the horses by registered hunters has been the common method of control but horse lovers very much fear this is inhumane, with the chances of horses being wounded and foals orphaned and left to starve.
Discussion of birth control in the Brumbies has been discussed as far back as 2008 and has been discarded as a control option on several occasions since then, but has been raised again.
The availability of non-lethal fertility control technologies to manage invasive and endemic species populations is changing the debate on wildlife management, with it being used on everything from Hippopotamus populations to pigeons.
In England, although there are no truly wild horses, there are herds of free-roaming ponies that live in wild conditions in various protected areas, such as The New Forest, Dartmoor and Exmoor.