Smithsonian National Zoological Park l Friends of the National Zoo



Mating Mysteries

Cheetahs are notoriously difficult to breed in zoos. Smithsonian scientists are working to change that.

By Sara Bloom Leeds

There’s something pure and primal about the fastest land animals on Earth. It’s hard not to feel a sense of awe and appreciation for these magnificent cats. After all, cheetahs can be considered a pinnacle of evolution—vital and efficient predators within their ecosystems that also exude a distinct grace and beauty.

Yet these sleek, spotted cats also exude a sense of vulnerability. It may be the black marks that run like tear tracks down their faces. Or is it the haunted look a cat assumes as it pauses and scopes out its surroundings?

(Jessie Cohen/NZP)

Either way, that look is fitting, for the fleet felines are indeed vulnerable, as classified by the International Union for Conservation of Nature. Fewer than 10,000 survive in the wild. With such a small population, inbreeding is a growing concern. It can raise cubs’ mortality, lower animals’ resistance to disease, and cause other problems.

Inbreeding, of course, is also an issue for cheetahs in human care. There are 284 adult cheetahs in 53 zoos throughout North America. For that population to be sustainable—that is, capable of maintaining itself with reproductively viable individuals—at least 30 cubs would need to be born each year. Last year, 12 cubs were born in the United States.

“If we do not change the way we are managing the North American cheetah population,” explains Adrienne Crosier, a cheetah biologist at the Smithsonian Conservation Biology Institute (SCBI), “it will be extinct within the next 50 years.”

A Dearth of Births

Crosier is determined not to see that happen. For ten years, she’s been driven to untangle the mysteries of these complex, captivating cats. “Our tagline,” she says, “has always been that the more you know about a species’ biology, the better equipped you are to ensure its survival in the wild and in zoos.”

There are mysteries aplenty to occupy Crosier and her SCBI colleagues. “We’ve got at least a half-dozen pretty major projects going on,” she says, “all of which have never been looked at in cheetahs before.” One of the key mysteries scientists hope to solve is why it’s so unfathomably difficult to persuade cheetahs to breed in a zoo setting. North American cheetahs have excellent genetic variation as well as top-notch housing and veterinary care. Yet only 23 out of 111 females have had offspring.

One problem, scientists learned, was a lack of knowledge about wild cheetah behavior. Most institutions that possessed a pair of cheetahs figured that breeding—and cubs—would result from putting the male and female in the same enclosure. Sometimes that happened, but most times it didn’t. As Crosier puts it, “That’s just not the way cheetahs typically operate.”

Smithsonian Conservation Biology Institure: Cheetah Science Facility
(Meghan Murphy/NZP)

We now know that females in the wild are almost always solitary, seeking out other cheetahs only to mate. Males, in contrast, form coalitions, or groups, with their brothers and stay together for life. So zoos have learned to keep males and females apart. This knowledge underlay the design of both the Cheetah Conservation Station at Rock Creek (where cheetahs have bred successfully) and the Cheetah Science Facility in Front Royal (also a successful breeding site).

In the Mood?

Another challenge is that female cheetahs do not have regular estrous cycles, and it is extremely difficult to tell when they’re in heat. Many show no physical signs of estrus, at least not that humans can detect. They may not even engage in any mating behaviors, such as rolling, tail flagging, rubbing against things, or standing to be mounted—even in front of males. Therefore, it is also extremely difficult for researchers to know when a female becomes physically mature and ready to breed in the first place.

Scientists believe a wild female in estrus marks her territory with urine and feces, the smell of which alerts males to her receptivity to mating. It is virtually impossible to measure hormone levels within urine and fecal samples for indications of estrus. So humans often have to simply trust that a male cheetah might be a better judge.

The male’s highly attuned sense of smell helps him determine when a female is ready to mate. When he thinks she is, he will make specific vocalizations, called “stutter-barking.” When that happens, staff undertake a “fence-line” introduction, allowing the cats to see and smell each other through a fence. If the male becomes excited, and the female is receptive (or at least does not appear aggressive), staff attempt to let the animals in together. The staff closely monitor what happens—which may be nothing at all. After all, how this species chooses mates is still another aspect of cheetah reproduction that we do not yet quite fully understand.

Old Eggs in New Wombs

Last summer, SCBI scientists and collaborating researchers from North America and Namibia published results of a five-year study that solved a key cheetah mystery: the tendency of older females to have trouble reproducing.

By analyzing hormones, eggs, and the uteruses of cheetahs throughout the country, the team discovered that when a female cheetah turns eight or so, her uterus will likely begin to suffer from abnormal cell growth, cysts, and infections that prevent pregnancy. However, the researchers also discovered that, despite this, the animals can still produce normal hormone levels and eggs.

This is promising news because it means that eggs from an older female cheetah can potentially be collected and fertilized by sperm from a genetically ideal male in the lab and then implanted into a younger female surrogate who can carry the resulting embryos to term. Embryo transfer, successfully performed in other species, has never been attempted in cheetahs. If successful, it would be a huge step in not only producing more cubs, but keeping the gene pool as diverse as possible to prevent inbreeding among the younger generation.

Researchers are already hard at work examining how hormones control a cheetah’s ovarian function, as well as the hormonal preparation that would be required for surrogates to receive and carry early embryos.

(Meghan Murphy/NZP)

Crosier and her team are about halfway done with this research. “Once we have that figured out,” she says, “the next step would be to perform the actual procedure.” If all goes well, they may be ready to do so within the next year. Crosier and her team have also been trying to perfect artificial insemination techniques for cheetahs, a task with which scientists around the world have wrestled or more than 25 years. Because cheetah biology is so complex, this is still proving to be challenging.

“One of the biggest challenges to generating fast results is sample collection,” says Crosier. She explains that the researchers travel widely both within the country and internationally in a mobile research lab. This lab-on-wheels has all the equipment needed to do on-site semen collection and artificial insemination. Samples also often have to be shipped all over the country once collected. “There’s a lot to coordinate,” she says. Other reproductive research involves examining whether lower semen quality is associated with a cheetah’s social situation. For example, it is suspected that a dominant animal may often suppress the reproductive hormones of others, and this is believed to occur to some degree in both males and females. But how this occurs and to what extent is unknown.

Finally, researchers are also examining cheetah endocrinology, their behavior, and genetic diseases among family groups. They also seek to determine puberty onset in both males and females.

Built for Breeding

Scientists aren’t just amassing data. They’re putting what they learn into practice at eight special breeding centers in the United States, along with one in Toronto. The Zoo’s center, at SCBI headquarters in Front Royal, opened in 2007.

These facilities do their best to mimic conditions in the wild. Individual females are provided with roomy enclosures that are separated from males by distance and visual barriers. “An animal’s welfare often correlates directly to its reproductive success,” says Tony Barthel, curator of the Zoo’s Cheetah Conservation Station, “and cheetahs may just be the poster child for that statement.”

This new approach seems to be paying off. More than 90 percent of North America’s cheetah cubs are born at these breeding centers. So what went right? A key factor is that the centers have room for a variety of males and females. This allows researchers to mix and match genetically diverse males and females, allowing them to choose whomever they prefer, and thus increase the likelihood of a mating.

“If a female is in estrus and doesn’t like a certain male,” says Crosier, “you can try introducing her to another. You can also match young inexperienced animals with older more experienced ones. It is very difficult to breed young inexperienced males and females with each other because often neither cat knows what to do.”

Smithsonian Successes

So far, there have been five litters at the National Zoo—two at Rock Creek and three, more recently, at Front Royal. Twice, Zoo cheetahs have given birth to five cubs. This is striking because litters usually have two to four members.

Tragedy nearly struck the first litter in Front Royal. In December 2010, Amani, a new mother, gave birth to a single cub, dubbed Nick. That posed a problem, because cheetah mothers require the stimulation of nursing cubs to produce milk. One cub is not enough for this, and single cubs generally die of malnourishment. Luckily, Zoo staff knew that Zazi, an experienced mother, was also due to give birth. So they made the decision to hand-rear Nick until her litter was born. Then they hoped to introduce him into her litter.

Cheetah cub.
(Jessie Cohen/NZP)

That was risky. A mother might not accept another cub, ignoring or even killing it. She might even abandon the entire litter. Keepers knew, however, that Zazi was a very tolerant individual who’d been a good mother in the past. There seemed a good chance that she would accept a foster cub.

From the time Nick was born, keepers took careful measures to prepare him to be introduced to Zazi. “We were bottle-feeding him with fake fur around the nozzle to stimulate nursing and his kneading response,” says lead cheetah keeper Lacey Braun. “We also had him sleep in Zazi’s urine-soaked bedding so that he would smell familiar to her.”

Ten days later, Zazi ended up having a single cub as well. Soon afterward, staff members put Nick in with her and baby Maggie. They crossed their fingers while holding their breaths.

“I was very anxious about the foster,” Crosier says. Braun was nervous too when Nick was put into the nest box. But the story ended happily, and Zazi did indeed take care of both, proving to be the excellent mother that everyone thought she’d be. In addition, Nick still has a strong bond with the humans who initially raised him. He purrs when they come near and enjoys their company.

“It’s very rewarding,” says Braun. “My favorite part of the job is watching the cheetahs run and play with each other. The cubs keep me laughing all day long.”

-- SARA BLOOM LEEDS was an intern in the Zoo's Office of Communications.