Bambi on Birth Control

The biggest challenge to managing animal populations with contraceptives may be getting people to agree about it

By Jennifer Weeks

Except in cartoons, animals aren’t known to accessorize. But in the town of Estes Park, Colorado, elk have been spotted sporting Christmas lights, laundry, and even bicycles in their antlers. While it might seem amusing, the occasional sock-adorned creature is indicative of a serious problem: There are too many elk in the surrounding area, which drives them into communities they wouldn’t otherwise enter. To keep the herd size from growing even larger, wildlife managers at nearby Rocky Mountain National Park are considering putting the animals on birth control.

From California to New Jersey, communities are using contraception to control deer, squirrels, and other critters that are multiplying and damaging habitats or spreading diseases. Not everyone agrees this approach is the best solution to animal overpopulation. But for the past 15 years, wildlife contraceptives have been used in a variety of settings in which people and animals overlap, including barrier islands, office campuses, and public lands.    

Encounters between humans and animals are on the rise for several reasons. Suburban development is pushing into many formerly wild areas, especially in western states. In the Northeast, forests have been growing for a century on farmland abandoned in the 1800s, creating more habitat for beavers, moose, black bears, and other large creatures. And some species, such as wild turkeys and white-tailed deer, are thriving in suburbs where there are fewer predators and hunting is banned or severely limited.

Many communities that need a targeted and safe way to address overpopulation are turning to birth control, which is generally seen as a humane alternative to hunting or culling herds. “The demand [for contraceptives] is overwhelming,” says wildlife fertility specialist Jay Kirkpatrick, director of the Science and Conservation Center in Billings, Montana.

That’s because birth control is often quite effective. Enforced infertility has been used to control creatures as small as parakeets and as large as elephants. Currently, Santa Monica, California is using a contraceptive to reduce persistent squirrel overpopulation in Palisades Park. In the Northeast, several communities have used the approach to stem the ballooning number of deer within their boundaries. And Australian officials are researching contraceptives for koala bears and kangaroos.

Early versions of animal birth control relied on steroids or sex hormones. However, these substances persist in the animals’ bodies, potentially posing threats to people who eat them—an important factor for game species like deer—and they increase risks of cancer and infections in animals. Increasingly, the tool of choice for curbing animal overpopulation is immunocontraception, or vaccines that stimulate an animal’s immune systems to block reproductive functions.

One experimental vaccine
, PZP, uses a protein found in pig ovaries. In order to fertilize an egg, sperm must attach to and then penetrate the zona pellucida, a membrane that surrounds mammalian eggs. When a female animal is injected with PZP, her body produces antibodies to it; those antibodies then bind to the zona pellucida, preventing sperm from attaching and fertilizing the egg. Another vaccine, GonaCon, blocks the gonadotropin-releasing hormone, which signals the body to produce sex hormones like estrogen and testosterone. By tying up the hormone, GonaCon short-circuits the reproductive cycle, rendering both males and females infertile for up to four years.

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When I lived in Berkeley, the city decided to trap and sterilize squirrels in order to control the exploding population. I worked at a newspaper at the time, and we had loads of fun coming up with off-color (and, of course, unused) headlines for that one.

TNR is probably a better option than hormonal birth control, at least with populations of wild animals that aren't endangered.

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