After significant strides were made in the field of human contraception in the 1960s, zoos across the United States jumped on the birth control bandwagon. Sterilization seemed to be a perfect solution for keeping zoo populations at a manageable level, both in terms of space and financial resources.

The New York Times reported that "Chimps take human birth control pills, giraffes are served hormones in their feed, and grizzly bears have slow-releasing hormones implanted in their forelegs.

Even small rodents are included. The ethical argument is simple: prevent the birth of more animals than the zoo would like, and spend your resources caring for the animals you have."

Prior to contraception, zoos either had to keep animals of the opposite sex separated, or they were forced to accommodate growing numbers of residents if they couldn't sell or give the offspring to other zoos.

"By preventing the birth of animals beyond carrying capacity, more animals can be well cared for," explains Cheryl Asa, director of the Association of Zoos and Aquariums' Wildlife Contraception Center (WCC) at the St. Louis Zoo.

Another benefit of birth control is that males and females can be kept together--a more natural arrangement in many cases--without worrying about pregnancy.

Birth control is regularly administered to many female Western lowland gorillas in American zoos, according to the WCC. As a result, a young female is able to stay with her family group upon reaching maturity without the unwanted possibility of inbreeding, which is critically counterproductive to zoos' laborious efforts to perform reproductive management and maintain genetic diversity among the wildlife in their care.

Another species that has benefitted from birth control is the Mexican wolf, a species that was nearly hunted to extinction in the 1970s. Zoo breeding programs put the wolf on the path to recovery, starting with just seven surviving wolves. As the captive population grew, biologists began reintroducing captive-bred wolves into the American southwest in 1998. Since then, the federal government has reduced funding for captive breeding facilities, so, birth control has helped zoos perform selective breeding to maximize genetic diversity of Mexican wolves without running out of room altogether.

Because contraceptives can be given to both sexes, zookeepers have also found it helpful to medicate mature males of some species in order to subdue their instinctively aggressive tendencies toward other animals in their exhibits. Competitive behavior involving disputes over mating and territory not only endanger other zoo inhabitants but also pose potential threats to keepers and disturb visitors.

The WCC contends that contraception is an essential tool in worldwide wildlife conservation. "Contraceptive research carried out in zoos and aquariums is directly applicable to the management of wild populations in parks and reserves around the world."

The Science and Conservation Center at Zoo Montana has applied contraceptive technology to the management of specific species and populations of animals that are at risk of expanding to the point of crisis, including the wild horse population of Assateague Island National Seashore, white-tailed deer in seven states, African elephants (to eliminate the need to legally kill them in African National Parks), water buffalo in Guam, feral burros in Virgin Islands National Park, and elk on Point Reyes National Seashore.

Although hormonal birth control is generally accepted by the zoo community as being a humane tool for animal management, it is not a blanket science that can be applied uniformly to all species in zoos. A contraception method that is safe and effective for one species may not be appropriate for another. Apes, for example, are closely linked to humans, and caution must be exercised to reduce similar side effects from the use of birth control pills. Also like some humans, elephants can have trouble restarting their reproductive cycle after being treated with chemical contraception. Disturbingly, hormonal implants were found to cause uterine infections and tumors in big cats and canine species, so the type of birth control had to be modified.

The science of zoo contraception is constantly evolving, and the experts at the Wildlife Contraception Center work diligently to make sure our nation's zoos are safe and hospitable places where animals can coexist as naturally as possible.