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.
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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."