Zookeepers around the world, facing limited capacity and pressure to maintain diverse and vibrant collections of endangered species, are often choosing between two controversial methods: birth control and euthanasia.
In the United States, the choice is contraception. Chimps take human birth control pills, giraffes are served hormones in their feed, and grizzly bears have slow-releasing hormones implanted in their forelegs. Even small rodents are included.
Cheryl Asa, who directs the Association of Zoos and Aquariums’ Wildlife Contraception Center at the St. Louis Zoo, said euthanasia was not a comfortable fit for zoos here. “On an emotional level, I can’t imagine doing it and I can’t imagine our culture accepting it,” she said.
Dr. Asa sees contraception as a better approach. “By preventing the birth of animals beyond carrying capacity,” she said, “more animals can be well cared for.”
But in Europe, some zookeepers would rather euthanize unneeded offspring after they mature than deny the animal parents the experience of procreating and nurturing their young.
“We’d rather they have as natural behavior as possible,” said director of conservation for the Copenhagen Zoo. “We have already taken away their predatory and antipredatory behaviors. If we take away their parenting behavior, they have not much left.”
So he and many of his European counterparts generally allow animals to raise their young until an age at which they would naturally separate from parents. It is then that zoo officials euthanize offspring that do not figure in breeding plans.
This spring, the Copenhagen Zoo put down, by lethal injection, two leopard cubs, about 2 years old, whose genes were already overrepresented in the collective zoo population. Leopards are considered near threatened by the IUCN. But as part of a breeding plan to maintain the genetic diversity of this species, the cubs’ fate was determined before they were born.
“We promised the species coordinator that the offspring would never leave the zoo,” the director said meaning they would not be bred with leopards from other zoos. The Copenhagen Zoo, he said, annually puts to death some 20 to 30 healthy exotic animals — gazelles, hippopotamuses, and on rare occasions even chimps.
The thinking is that this strategy mimics what would have occurred in the wild, where some 80 percent of feline offspring die from predation, starvation or injury, he said.
Terry Maple, the former director of Zoo Atlanta and co-editor of “Ethics on the Ark” said that while he knew of no studies assessing the importance of raising young to animals’ health or well-being, observation indicated that most zoo animals are motivated and protective parents that play frequently with offspring.
He acknowledged that American zoos once focused more on the intricacies of breeding endangered species than on their day-to-day well-being, but said this was changing. In meticulously planning their populations, Mr. Maple said, zoos will eventually avoid a surplus of animals and ensure that most breed and raise offspring. “I am not saying management euthanasia is wrong,” he said. “It is just not the best solution.”
International guidelines on the ethics of breeding zoo animals have been elusive, in part because philosophies vary, said Dave Morgan, chairman of the Population Management Committee at the World Association of Zoos and Aquariums. The African association of zoos lists euthanasia as a population management tool, whereas the precepts of Hinduism and Buddhism make the killing of even terminally ill animals difficult.
Both the US and Europe tolerate the euthanasia of feral cats and dogs. Euthanasia is permitted under the American zoo association’s regulations, but is mainly reserved for ill or elderly animals, said Steve Feldman, the association’s spokesman.
Although reliable data on the use of contraception is not kept by zoo associations, officials say that it is much more prevalent in North America but that it is starting to expand in Europe.
American zoos began developing contraception for highly fertile animals like lions in the 1970s, after breakthroughs in human birth control. Contraception use then expanded as it became quite difficult for zoos to sell or give away animals they could no longer accommodate.
This kind of family planning meant males and females no longer had to be kept apart to avoid unwanted pregnancies, which was ideal for the transition to more natural zoo environments. There were benefits, too, for zookeepers: hormones in contraceptives given males can take the edge off aggressive behaviors surrounding competition for a mate, which can result in mayhem and unsettle visitors.
There was a time when no one could have imagined that contraception would be needed for the Mexican wolf, a species hunted nearly to extinction in the 1970s. Zoos began with only seven survivors and bred a captive colony of nearly 300 wolves, saving the species. Ninety-two were reintroduced into the wild by the federal government starting in 1998, but then four years ago, the government used up the limited space that had been allotted for the program in New Mexico and Arizona.
So, for the last four years, birth control has helped the zoos breed selectively to maximize genetic diversity but not run out of holding space — or Mexican wolves — as a compromise is worked out with the states.
Genetic diversity is key to long-term species survival because it prevents inbreeding and preserves a broad array of traits that animals might one day need to survive in the wild.
Birth control is used for more than half of the female Western lowland gorillas held in 50 or so American zoos, said Kristen Lukas, who runs the species’ breeding plan for the zoo association. It enables a young female to stay in her family group without the danger of producing an inbred offspring.
Contraceptive science for most exotic animals, Dr. Asa notes, is not as advanced as for apes, which are more closely linked to humans. And side effects can and do occur. Large cats and canines on early hormonal implants were susceptible to uterine infections and tumors. Other animals like elephants have trouble restarting their reproductive cycle once they come off birth control.
European zoos that prefer euthanasia cite health risks of contraception as well as the enrichment of parenting — although putting down an animal can sometimes be controversial.
Denmark embraces the policy and is very open about educating its public. Germany, by contrast, allows euthanasia only in “reasonable” circumstances, which can be hard to define, said executive director of the European Association of Zoos and Aquaria. A few years ago at Zoo Magdeburg in northern Germany, it was discovered that a male tiger was a hybrid of two tiger subspecies, rendering the cubs it had sired genetically useless. When the three cubs were born, the zoo euthanized them immediately.
He characterized the decision as “courageous,” but animal rights advocates disagreed and had the zoo director and three employees prosecuted for violating the euthanasia law. They received suspended sentences.
Even when zoos wait to euthanize animals until their parents have had a chance to raise them, questions can come up. It might seem suspiciously convenient for zoos to destroy an animal just after it has completed its most adorable phase — given that baby animals are a top zoo attraction.
But Dr. Holst emphasized that the timing is dictated by nature. Zookeepers know it is time when the young leopards start picking fights with their mother.
“It may be painful for us,” he said, “but more natural to them.”