When giant pandas are born, they’re the opposite of giant. Researchers used to think bears had tiny babies was because they evolved to hibernate. In many bear species, pregnancy overlaps with long months of hibernation when the mother bear does not eat or drink; instead, she supplies her fetus with fat and protein from her own body. Even though pandas don’t hibernate, scientists assumed having tiny babies is just evolutionarily-baked into the bear family tree.
However, a new study published in Journal of Anatomy rules out hibernation as the cause—and the real reason why remains a mystery.
Placental animals, or mammals in the order Carnivora, give birth to relatively underdeveloped babies, reports Rafi Letzer at Live Science. Baby dogs, cats, seals, raccoons, and bears—to name only a few—all enter life small, blind, mostly hairless and pretty helpless. But panda babies are extreme; their mothers are about 400 times heavier than their babies. (Baby pandas weigh about 3.5 ounces and are roughly the size of a stick of butter. Their mama bears, on the other hand, weigh about 200 pounds and stand up to three feet tall and six feet long, according to Smithsonian’s National Zoo.)
Because such an extreme infant-to-mother ratio is rare in most mammals—only several others, including kangaroos, rival bear babies in size—scientists hypothesized that the size difference might be unique to bears. And since bears are some of the only mammals that are pregnant while they hibernate, scientists suspected having super-small offspring helped preserve the health of the mother bear, allowing extra growth to take place outside of the womb.
To test this idea, researchers from Duke University analyzed the skeletons of several baby pandas as well as other bear species and mammalian carnivores. Baby panda remains aren’t easy to find, but luckily, Smithsonian’s National Zoo has preserved the remains of five full term cubs born to the panda couple Ling-Ling and Hsing-Hsing in the 1980s, none of which survived long after birth.
The team created 3-D digital models of the skeletons of two of those cubs. They also scanned newborn grizzlies, sloth bears, polar bears, dogs, foxes and other related animals.
The team found that though they may be proportionally smaller than other mammals, the skeletons of most other baby bears—except pandas—are as mature as other species in terms of bone development.
The full-term panda cubs, on the other hand, had bones most similar to beagle puppies born several weeks prematurely. The cubs are similar to a human fetus at 28 weeks, the beginning of the third trimester.
The study does not support the idea that the panda’s small babies are a result of hibernation and confirms that panda cubs are oddballs, even for bears. “They’re basically undercooked,” says study co-author Peishu Li, a Duke vertebrate paleontologist, says in a statement.
So why are baby pandas so tiny? Another theory is that it is related to the bears’ all-bamboo diet, though Li says there’s no real research linking the two. Another possibility is the strangeness of bear pregnancy.
Michelle Starr at Science Alert reports that pregnant panda bears undergo a process called delayed implantation. After an egg is fertilized, it floats around in the womb for months before attaching to the uterine wall. In other bear species, the fetus grows for two months before delivery. Though the entire gestation period for pandas is 97 to 161 days, their fetuses only spend one month developing after implantation—half the time of other species.
“Development is just cut short,” says Duke biologist Kathleen Smith, whose lab conducted the study.
Why that developmental period is so short and why the size differential developed over time are questions the team can’t yet answer. Starr reports that the size mismatch in other bear species may indeed got back to ancestral bears. Over the past 20 million years, the size of adult bear species increased. It’s possible that the size of their offspring did not keep pace.
But that doesn’t explain the deal with pandas. “We really need more information about their ecology and reproduction in the wild,” Smith says.
Researchers are only now really beginning to understand the natural history of panda bears. A study earlier this year found that it’s likely the animals switched to their unusual all-bamboo diet just 5,000 to 7,000 years ago—possibly due to human encroachment. Another recent study shows that the animals migrate across the landscape following fresh new shoots of bamboo which has a high-protein nutritional profile more similar to meat than to plant matter. More research may reveal whether the animal's unique lifestyle has any impact on its unique offspring.