It's Really Hard to Know When a Zoo Animal Is Pregnant

Poop mapping, dog pregnancy kits, bribing pandas for ultrasounds—zoos often have to get creative to figure out when their animals will give birth.

A panda sticking its tongue out
Guess (China Daily CDIC / Reuters )

Sex at the zoo is a highly managed affair.

When zookeepers do not want a species to reproduce, birth control is in order. “Chimps take human birth-control pills, giraffes are served hormones in their feed, and grizzly bears have slow-releasing hormones implanted in their forelegs,” writes The New York Times. When zookeepers do want a species to reproduce—especially an endangered or threatened one—the couplings must be carefully arranged. An animal might travel 1,500 miles to meet a partner.

But after all this meticulous planning, zookeepers can hit a wall of uncertainty: It’s sometimes quite hard to know whether a female is pregnant. In the case of pandas, their keepers might not be entirely certain until the baby pops out.

There are animals where it’s easier, sure. Great apes, for example, are related enough to humans that regular old pregnancy tests can work. The problem is getting individual apes to pee on a stick. To get around this, the St. Louis Zoo built special gutters where the great apes slept, which would route the urine outside. In the morning, someone would go out to collect the urine. “But the female has to be alone to do that,” says Cheryl Asa, a former director of reproductive research at the zoo. The system worked well for great-ape species like orangutans, which are more solitary, but not so well for chimps or gorillas, which prefer to sleep in groups and inevitably pee together.

Poop is easier to differentiate by individual. In some cases, says Asa, you can feed different animals food studded with beads of different colors. Or you can just watch as they go. Since mating animals are intensely surveilled anyway, staff members take note when females poop. “If they see a female defecate, they map it,” Asa says. A keeper later uses the map to retrieve the samples. The feces are tested for levels of the hormone progesterone, which rises with pregnancy.

Animals closely related to domestic pets also benefit from veterinary medicine. Wolf pregnancies, for example, can be tested using commercial dog pregnancy kits that detect a hormone called relaxin. The kits require a little bit of blood, which is easy enough to draw from dogs, and actually isn’t that difficult to get from wolves, either. “We all think of wolves as being really fierce, and they can be,” says Asa, “but most of the time when they’re cornered, they’ll just give up and you can hold them down and pull a leg free to get a quick blood sample.”

Some species, however, go through a strange phase called a pseudopregnancy, which looks just like a real pregnancy except there is no fetus. Pandas do this. After a mating, their progesterone levels may rise, and they may start acting like they’re about to give birth. “They build a nest. They get lethargic. They don’t really eat,” says Laurie Thompson, the assistant curator of giant pandas at Smithsonian’s National Zoo.

The only way to know for sure is to see a fetus on an ultrasound, which zookeepers can coax pandas into doing with honey water or fruit. But baby pandas are so tiny even at birth—only four ounces—that an unborn fetus is easy to miss. Worse, “sometimes the pandas don’t even want to participate and they’re just in their den,” says Thompson. The Smithsonian team has only successfully detected a panda pregnancy via an ultrasound once.


Then there are cheetahs, another species difficult to breed in captivity. Like pandas, they go through pseudopregnancies. They aren’t terribly cooperative with ultrasounds or blood draws, either. They do, however, poop.

Recently, researchers at the Smithsonian found a protein in cheetah feces called immunoglobulin J chain that rises during a true pregnancy. “The great thing about feces is that it’s noninvasive,” says Adrienne Crosier, a cheetah biologist at Smithsonian Conservation Biology Institute and an author on the study. “We can get feces any day of the week from a cat. It’s very easy. Any facility can collect it.”

The study looks at 26 female cheetahs living in seven different zoos. Crosier also relied on the Smithsonian’s own archive of freeze-dried cheetah feces, which goes back 12 years. In fact, the zoo stores frozen feces, urine, blood, sperm, oocytes, embryos, ovarian tissue, uterine tissue, and testicular tissue for dozens of species. “We keep absolutely everything so we can use it for research,” says Crosier. “We don’t know what we may be interested in looking at or need to look at in five to 10 years.”

At the San Diego Zoo Institute for Conservation Research, which famously has its own “frozen zoo,” there is talk of resurrecting species using tissue from long-dead animals. The zoo has sperm, for example, from northern white rhinos, of which only three remain on Earth. So even in death, reproduction can be a highly managed affair.