Each year, hundreds of animals in European zoos are killed.
None of them are sick. Some are euthanized in order to maintain the number of animals a zoo is able to care for, others because zookeepers do not find them suitable for breeding. A giraffe named Marius was killed this year for the latter reason. The Copenhagen Zoo, which euthanizes approximately 25 healthy animals every year, shot the giraffe and then dismembered it in front of visitors.
You won’t hear anything as gory or heartless happening in the United States, though. That’s because American zoos prefer contraception over euthanasia to curb populations, prevent breeding, or preserve genetic qualities of a given species. Creatures as big as lions and hippos and as small as meerkats and mice take birth control — it’s crushed up in their feed, injected into their veins, and implanted under their skins.
The Association of Zoos and Aquariums, the governing body of U.S. zoos, allows euthanasia, but it’s usually reserved for ill or aging animals. Plus, our animal-loving society can’t quite stomach the idea of putting down a zoo animal for ostensibly no reason, especially chimpanzees and other primates, given their similarities to humans.
“On an emotional level, I can’t imagine doing it and I can’t imagine our culture accepting it,” Cheryl Asa, the director of the Wildlife Contraception Center, told Leslie Kaufman of The New York Times in 2012, suggesting that contraception is the better option. “By preventing the birth of animals beyond carrying capacity, more animals can be well cared for,” she said.
The Wildlife Contraception Center, an arm of the Association of Zoos and Aquariums, is the capital of exotic animal contraception in the U.S. Established in 1999, the St. Louis center provides information about and monitors the use of contraception for captive wildlife in 200 American zoos.
In the U.S., the administration of hormonal birth control in zoos began in the 1970s. Zoos were having trouble preventing unwanted pregnancies and were selling or giving away animals they no longer had room for. The practice coincided with a growing acceptance of birth control use by humans.
Animal birth control comes in many forms: hormonal treatments like progestins, estrogen, and progesterone; GnRH agonists, which suppress the reproductive endocrine system; vaccines, which create antibodies that block fertilization; and others.
Apes and monkeys are the easiest animals to treat with contraceptives, thanks to their biological resemblance to humans. They even experience some of the same side effects as women on hormonal birth control, like weight gain. Carnivores are the trickiest — the chemicals that work on primates actually stimulate carnivores’ uteruses, which can result in tumor growth. Birth control can also be administered in male animals, but zoos tend to avoid that practice, especially in lions. Some contraceptive drugs cause lions to lose their bushy manes — and, by some extension, what makes visitors attracted to them.
But contraception use in zoos has a dark side. Many drugs haven’t been commercially approved by the Food and Drug Administration, yet they remain in regular use across the country. The treatments are considered “experimental,” and their use depends on approval from the Institutional Animal Care and Use Committee, which reviews protocols for animal research. They must also be in compliance with the Animal Welfare Act, U.S. Department of Agriculture-enforced legislation that spells out animal welfare regulations.
One of these experimental drugs — and one of the first ever to be used in zoos, in 1975 — Melengestrol acetate (MGA), led to negative side effects in captive lions, including lesions, sterility, and even death. In the early 2000s, it was replaced by another experimental contraceptive called Suprelorin, a GnRH agonist about the size of a grain of rice that is implanted beneath the animal’s skin.
Suprelorin remains in use today. It has also worked almost too well, which has the zoo community worried. The slow-releasing hormones, meant to last six months to a year, were not wearing off several years after the implants were removed from lions. “I think it was sort of recommended across the board without really knowing what the long-term consequences were going to be,” Rebecca Snyder, the curator of mammals at Zoo Atlanta, told David Hunn of the St. Louis Post-Dispatch in February. There, zookeepers are still waiting for a lioness to get pregnant after she stopped receiving birth control. “I think we all learned a lesson from that.”
According to the Wildlife Contraception Center, out of more than 200 species treated with Suprelorin, only 50 have gotten pregnant or produced sperm again after ceasing treatment — 88 animals in total.
The use of contraceptive drugs is a relatively new venture in the zoo business (and the human business, too). Over the last few decades, experts have depended on trial and error to find the right dosage based on an animal’s age and behavioral and social factors. And they still can never be sure how long it will take for animals to regain their fertility once they’re off the medication.
Researchers say it’s far too early to draw conclusions. Many zoos have not yet tried to breed animals who were once administered birth control. But in the case of Suprelorin and other experimental drugs, the return of fertility seems not to be guaranteed.
“I think we all should be worried,” Budhan Pukazhenthi, a reproductive physiologist at the Smithsonian National Zoo’s Conservation Biology Institute, told Hunn. “I think we also should use a lot more caution when we make the decision to place an animal on contraception.”