Australian officials plan to kill more than 3,000 kangaroos on the outskirts of Canberra by July, because the population is growing too quickly and exhausting the grass supply. Future attempts to control the population may not rely on violent measures: Scientists are developing oral contraceptives to keep down the populations of nuisance animals like kangaroos. How do you get a wild animal on the pill?
You don't. The "oral contraception" under development for kangaroos is not a hormone pill like the one that women have been using for more than 40 years. Instead, scientists Down Under are working on an oral vaccine that will use a kangaroo's immune system to temporarily render her infertile. (These work by stimulating the animal to create antibodies that form an impermeable shell around her own eggs, making fertilization impossible.) This process is called immunocontraception and has been tested in a wide variety of animals, including white-tailed deer on Fire Island, N.Y., bears in New Jersey, wild horses out West, and even elephants in South Africa. It's also used in zoos to keep animal populations manageable. (It isn't easy to neuter a captive giraffe.)
If the oral contraception works as planned, government workers could trick kangaroos into eating food mixed with one of these vaccines. Some communities use a similar method of delivery for a pesticide product called OvoControl, which keeps Canada goose and pigeon populations from getting out of hand. OvoControl "looks like Corn Pops," according to one journalist, and is a bread-based bait left in birdfeeders. It works by preventing eggs from hatching.
But in most situations, wildlife officials deliver contraceptive vaccines by hand with syringes, or from a distance with darts. Getting shot with a dart surprises an animal but doesn't cause it too much distress. After a moment, it's ready to scamper through the wilderness again.
Though most attempts to limit the breeding capacity of wild animals use immunocontraception, there is at least one promising hormonal method. Researchers have tested implants placed between the shoulder blades of the koala bear, another animal Down Under whose population is booming rather inconveniently in some areas. Like Norplant, these secrete a hormone at relatively constant levels to keep the koalas infertile for two years.
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Explainer thanks Alan J. Conley of University of California-Davis, Kimberly Frank of the Science and Conservation Center, John Grandy of the Humane Society of the United States, John Rodger of the University of Newcastle, and Allen T. Rutberg of Tufts University.