Wild Horses: Population Control

Wild Horses

Potential Solutions



A more recent effort has been underway to help control the population of the wild horses in a more scientific manner. The following is an article written by Kelly Stewart of the Dept. of Anthropology, University of California, Davis, CA. Here Stewart describes in more detail the justifications, efforts, and future objectives of the wild animal birth control efforts at her university.

Population Control for Wild Horses

"It is a cruel irony that in this age of endangered species, conservationists sometimes have to cull the animals they want to protect. One reason is that wildlife can become a pest. White-tailed deer, for example, have reached unprecedented numbers in the USA, due to a decline in hunting. In urban areas, they eat peoples' gardens, collide with cars, spread lyme disease, and so on. Even wildlife sanctuaries can suffer from overpopulation, as is evident in some reserves in Kenya and South Africa, where elephants have been blamed for widespread destruction of trees and alteration of the landscape.

"In the past, we limited animal populations mainly through hunting or large-scale culls. But these methods are becoming less and less ethically acceptable to the public. Scientists from UC Davis are searching for alternative solutions, tackling the problem from the other end of life's procession. Rather than increase death rates, why not lower birth rates?

"Over the past decade [1990s], Professor Irwin Liu and his colleagues from the Department of Population, Health and Reproduction at UCD's Vet School have collaborated with scientists across the country to develop an effective means of birth control for wildlife. It sounds like the perfect solution: put animals on "the pill". But it's not that simple. For starters, how do you get wild animals to take the correct dosage?

"In the mid-eighties, reproductive biologist Jay Kirkpatrick, in association with UCD, began a wild horse contraception project on Assateague Island off the coast of Maryland and Virginia. Bands of horses have roamed free on the island for centuries, but their numbers had grown to a point where they were beginning to damage the habitat. Using a dart gun, Kirkpatrick delivered steroid hormones to stallions to lower their sperm counts and the equivalent of a "progesterone pill" to mares to prevent ovulation.

"The exercise was not a success. It proved too difficult to dart the animals often enough with sufficiently large doses to maintain contraception. Furthermore, scientists worried about the transfer of steroids up the food chain. Horse carcasses on Assateague are scavenged by vultures, foxes, and gulls, and it's possible that these animals could be adversely affected by eating hormone-laced horse meat.

"Meanwhile, back at UCD, Professor Liu was testing a new technique of birth control on captive horses — an inoculation against pregnancy. With this method, researchers inject a female with a vaccine made from pigs' ovaries. This stimulates her immune system to produce antibodies which then interfere with fertilization when she mates, possibly by preventing sperm from penetrating the egg.

"Kirkpatrick returned to Assateague with the vaccine and this time he hit the jackpot. Just two doses, given a few weeks apart, were sufficient to prevent conception in most mares for a year, after which an annual booster maintained the effect. Not only this, but the vaccine did not interfere with a current pregnancy, and its effect was reversible once the boosters were stopped. Happily for the mares and their stallions, inoculated females were still interested in sex and, in fact, all other normal wild horse behavior. Finally, no serious side effects appeared for four years of treatment, after which the vaccine began to inhibit ovulation.

. . .

"But after four years, a trio of researchers perfected a time release vaccine that seems to be 95% effective for at least one year with only one shot.

"The procedure is now being conducted on mares that are older than 9 years. Wild mares that are older than nine years are returned to the wild in the belief that they will not be adoptable. The new vaccine has already been tested on 200 animals in Nevada. The results will be clear after foaling season. The next efforts of the research will be to extend the effective length of time for the vaccine to two or three years.

"This solution is not without its critics. The Commission for the Preservation of Wild Horses objected to the fertility control project because results of the experiment had not been validated. But these objections will not deter future plans of implementing the new vaccine nor continued research in this area."
. . .

"Since the first trials on Assateague, "immunocontraception", as it is called, has successfully reduced birth rates in other species, including feral burros in the Virgin Islands and white-tailed deer on Fire Island National Seashore in New York state. This method of controlling deer numbers was far preferable to a previous trial with bow-hunting, when local residents, relaxing on their porches, were treated to the sight of wounded deer stumbling along the boardwalks. Current research at UCD includes a study of ways to regulate the elk population at Pt. Reyes using immunocontraception.

"We have entered an era in which wildlife must be managed in order to be saved. The days when we could conserve nature by leaving it alone are unfortunately gone. But we can try to be as "hands-off" as possible by searching for benign, non-invasive methods of management. In this quest, scientists at UC Davis are leading the way."

Reproduced with permission.
© Kelly Stewart, Ph.D.
Dept. of Anthropology, University of California, Davis, CA 95616



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