Isis the lioness arrived at Chicago's Brookfield Zoo six years ago with a job: Give birth. Brookfield hadn't seen lion cubs in decades.
The zoo carefully planned her courtship with its new male, Zenda. Keepers took Isis off birth control. The community was excited.
But nothing happened.
And it wasn't just at Brookfield. The lioness Asali, at Ohio's Columbus Zoo, wasn't getting pregnant. Nor was Kiki, in Atlanta, or Neka, in Oregon.
At the same time, scientists at the St. Louis Zoo were coming to a startling conclusion: Dozens of lionesses, including Isis, had been on birth control — but the drug wasn't wearing off.
The implants, meant to last six months to a year, were still in effect three, four, even five years later.
Now researchers across the country are trying to figure out the extent of the problem, the impact on zoo breeding programs — and whether some of these animals will ever bear offspring.
The Wildlife Contraception Center, an arm of the Association of Zoos & Aquariums housed at the St. Louis Zoo, is the distributor of Suprelorin, the drug used on the lionesses, to accredited zoos in North America. The center has recorded more than 3,300 treatments for about 1,600 exotic animals, from polar bears to monkeys to meerkats. But it has documented only 88 animals that have "reversed" — gotten pregnant or produced sperm — in nearly a decade of treatments.
Of the 200-plus species treated, the center has seen reversals in just 50. Not one of the six or seven polar bears given Suprelorin, species experts said, has had cubs.
It's far too early to draw permanent conclusions, researchers say. Many of the zoos caring for those animals have not yet tried to breed them. Still, the zoo community is unsettled.
"I think we all should be worried," said Budhan Pukazhenthi, a reproductive physiologist at the Smithsonian Conservation Biology Institute at the National Zoo in Washington who works with lions. "I think we also should use a lot more caution when we make the decision to place an animal on contraception."
Spokesmen for Virbac Corp., the U.S. subsidiary of a French pharmaceutical group that makes Suprelorin, haven't returned multiple emails and phone calls on this subject.
The issue is key to the future of North American zoos. Still transitioning from the years of cages and pits, zoos now often say they exist for two primary reasons: To educate the public on wildlife conservation, and to provide a backup plan should species die out.
To accomplish the first goal, leaders prefer to exhibit animals in wild settings — herds of elephants, bands of gorillas, prides of lions. But to achieve the second, and for the ongoing health of their animals, they need to carefully watch those family groups, and only allow breeding between individuals of the right genetics, at the right time.
The alternative, zoo leaders note, is to kill unwanted animals, as do some European zoos. This month, the Copenhagen Zoo in Denmark shot a 2-year-old male giraffe with a bolt gun, dismembered it in public and fed the remains to its lions — despite an online protest petition with thousands of signatures.
Contraception, Copenhagen Zoo officials noted, has health repercussions.
At last fall's Association of Zoos & Aquariums conference in Kansas City, contraception center leaders called a special session on the topic to plead for data from the continent's zookeepers.
To fix the problem, they explained, they first need to understand how serious it is.
Lions were the poster children at the meeting. Keepers had already begun sharing data with the St. Louis contraception center on mating, health and reproduction.
And they had some numbers: 118 lionesses had been treated with Suprelorin, said Hollie Colahan, large mammal curator at the Denver Zoo and coordinator of the association's lion species survival plan. Only nine had documented reversals.
Keepers were perhaps the first to realize something was wrong.
In 2009, Zoo Atlanta had seven lions: four adults and three new cubs. "We had a lot of lions to manage," said Rebecca Snyder, the zoo's curator of mammals.
Atlanta put Kiki, its new mom, on one dose of Suprelorin. (It could have put Kiki's mate, Kamau, on the contraceptive. But Suprelorin makes male lions lose their manes, something no one likes.)
A year later, however, Kiki wasn't coming back into heat. Atlanta began to wonder if she would ever get pregnant again. "You start worrying, well, are there going to be enough lions to go around in zoos?" Snyder said.
In 2012, the zoo association's Population Management Center analyzed the status of zoo lions. In general, the study found, things were fine. Still, it warned, "If the females on contraceptive do not breed again, the population will experience a much more severe decline."
No matter how this ends, some zoo staffers say they'll be more cautious going forward.
"I think it was sort of recommended across the board without really knowing what the long-term consequences were going to be," said Zoo Atlanta's Snyder. "I think we all learned a lesson from that."