Animal contraceptives are increasingly being used to manage wild populations.

A veternarian prepares to shoot a zebra with a contraceptive dart. Photo courtesy Cheryl Asa

Etosha has had two troubled pregnancies. Her first baby was breached and stillborn. The second was delivered by a dangerous cesarean section. A combination of skill and sheer luck helped her survive the delivery, but the procedure was harrowing enough that her doctor considered putting Etosha on hormonal birth control to prevent additional pregnancies.

It would have been a perfectly routine decision for millions of women. But Etosha is not a woman—she’s a South African lion at the San Diego Zoo.

These days, contraceptives are a staple of zoo breeding programs, with more than 12,000 zoo animals across 300 species on some form of hormonal birth control. But people are no longer tampering with the reproductive systems of zoo animals alone—animal contraceptives are increasingly being used in attempts to control wild populations.

Centuries ago, camel drivers put pebbles in female camels’ uteruses to prevent pregnancy during long voyages across the Sahara. But the recent history of animal contraception began in the mid-1970s, when zookeepers started to control the reproduction of big cats. Lions and tigers breed well in captivity, live for a long time, and eat pricey meats, so they put economic pressure on the zoos that house them. Giving these big cats contraceptives allowed zoos to manage their populations, and in the last few decades, the practice has spread to an ever-growing animal menagerie.

“It’s almost like every year another species is brought on board,” said Cheryl Asa, co-director of the

AZA Wildlife Contraception Center. “I don’t know a zoo that doesn’t use contraception.”

Still, contraception is not usually part of the public face a zoo puts forward. Baby pandas make splashy headlines and tiger cub webcams jam up servers, but a prevented conception is a public non-event. “I think at first people are surprised because they are only really hearing the other side: to make more and more animals,” Asa said.

But there are many reasons animal contraception is appealing to zoos. In addition to preventing the birth of animals a zoo doesn’t have room to house, contraceptives help zookeepers arrange suitable matings. Captive animals, unlike wild ones, tend to remain in their family groups for life—increasing the odds that their offspring will exhibit genetic abnormalities. So zookeepers use detailed family trees to match up unrelated pairs, and they employ contraceptives to keep animals from mating with relatives.
Finding the right birth control method for Noah’s entire ark, however, can be a challenge. “There isn’t a product that’s going to work perfectly well on all species,” Asa said. While human birth control pills can be used for primates—great apes, chimps, gorillas and orangutans— researchers have to be creative with other species.

For instance, scientists have long known that the steroid hormones in human birth control pills can over-stimulate the uteruses of carnivores, increasing the risk of infection or cancer. So the AZA is researching a new non-steroid form of animal birth control, called deslorelin. The drug acts as a chemical castration agent, shutting down the endocrine reproductive system for up to a year in dogs and two years in lions. Currently, it’s approved for use only in Australia and New Zealand in male dogs, though Asa’s research group administers it experimentally to a variety of species.

One of the most successful animal contraceptives is the porcine zona pellucida (PZP) vaccine, which is made from pig proteins and targets the zona pellucida, the membrane that surrounds all mammal eggs. “If you inject a female deer with this foreign pig protein she’ll make antibodies to it,” said Allen Rutberg from the Center for Animals and Public Policy at Tufts Cummings School of Veterinary Medicine. “The antibodies bind to the egg of the deer and they block sperm from coming in.”

It’s a delicate balancing act: They must introduce a protein foreign enough that the deer’s body will produce antibodies to it, but similar enough to its own egg proteins that the antibodies actually attach. The vaccine doesn’t work in all animals and the success rate is too variable to make it useful in humans, who have vastly different immune responses.

“With that kind of variability it’s a little bit too unpredictable for the human market,” Rutberg said, but “individual deer are not going to sue us because they have fawns before they expect to.”

Still, the vaccine is successful enough to make it useful for controlling the overall size of certain wild animal populations. Rather than having to be culled by hunters, the animals can be shot with a dart filled with the PZP vaccine. This will prevent enough females from having offspring, thereby controlling the total population size. The approach has been successfully used with wild mustang populations in Wyoming and Montana since 1971, and has been experimentally introduced in populations of other wild animals, including Indian elephants, brown bears, kangaroos, and koalas.

But the idea of using contraceptives to control wild animal populations is accompanied by heated politics. Hunters have been particularly vocal critics because they fear that the practice will leave them without animals to hunt. OvoControl, a drug approved by the EPA and FDA to control populations of Canada geese, has been denounced by the hunting lobby, said Erick Wolf, CEO of Innolytics, the company that makes OvoControl. EPA approval of the use of OvoControl on pigeons is pending, and Wolf hopes that plan will be less controversial. “Pigeons,” he said, “have very few friends.”

Wildlife groups, however, are still divided on this issue. While the Humane Society endorses the PZP vaccine and OvoControl as non-lethal population management tools, the environmental group Friends of Wetlands and Wildlife recently denounced plans to put West Bengal elephants on birth control as another “killing exercise.”

“It makes people uncomfortable because they like to have a wall between nature and people. This really crosses the boundary between human cultural activity and ‘the wild,’” Rutberg said. “That is kind of the joke: The human influence on so-called wild populations is so pervasive that this is just another way in which people are attempting to influence wildlife.”

Originally published November 30, 2006

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