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 lime disease, and so on. Even wildlife sanctuaries can suffer from overpopulation. This 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, 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 that 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.
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 .
Tests of the vaccine in zoos have shown that it also works on a wide range of exotic species such as African lions. It might even be the answer to the elephant problem in Africa's smaller wildlife reserves.
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.
Dept. of Anthropology, University of California, Davis, CA 95616.
e-mail: kjstewart@ucdavis.edu