The researchers say they only looked at skeletons in this study, and it could be that other organs like the brain tell a different story. “They’re basically undercooked,” said Li, now a Ph.D. The researchers say the panda bear’s embryonic appearance likely has to do with a quirk of panda pregnancy.Īll bears experience what’s called “delayed implantation.” After the egg is fertilized, the future fetus enters a state of suspended animation, floating in the womb for several months before implanting in the uterine wall to resume its development and get ready for birth.īut while other bears gestate for two months after implantation, giant pandas are done in a month. Other factors might have pushed panda babies toward smaller sizes over time - some researchers blame their bamboo-only diet - but data are scarce, Li said. “That would be like a 28-week human fetus” at the beginning of the third trimester, Smith said. Even in a full-term baby panda, the bones look a lot like those of a beagle puppy delivered several weeks premature. The panda bear is the one exception to this rule, results show. In fact, despite being small, the researchers found that most bear skeletons are just as mature at birth as their close animal cousins. The researchers didn’t find any significant differences in bone growth between hibernating bears and their counterparts that stay active year-round and don’t fast during pregnancy. “It’s certainly an appealing hypothesis,” Smith said.īut the Duke team’s research shows this scenario is unlikely. But the idea is that small birthweight is ‘locked in’ to the bear family tree, preventing non-hibernating relatives from evolving bigger babies too. Proponents of the theory concede that not all bears - including pandas - hibernate during the winter. By cutting pregnancy short and giving birth to small, immature babies, bears would shift more of their growth to outside the womb, where babies can live off their mother’s fat-rich milk instead of depleting her muscles. The thinking is that, energetically, females can only afford to nourish their babies this way for so long before this tissue breakdown threatens their health. Pregnant females don’t eat or drink during this time, relying mostly on their fat reserves to survive, but also breaking down muscle to supply protein to the fetus. One decades-old idea links low birthweights in bears to the fact that, for some species, pregnancy overlaps with winter hibernation.

For the vast majority of baby mammals, including humans, the average is closer to 1:26. A newborn polar bear’s birthweight as a fraction of mom’s is less than 1:400, or less than one-half of one percent of her body mass. The panda may be an extreme example, but all bears have disproportionately small babies, Li said. They looked at whether the teeth had started to calcify or erupt, and the degree of fusion between the bony plates that make up the skull. The researchers examined the degree of ossification, or how much the skeleton has formed by the time of birth. They used the scans to create 3-D digital models of each baby’s bony interior at birth.Īs a baby animal grows and develops inside the womb, its bones and teeth do, too.

The researchers took micro-CT scans of two of those cubs, along with newborn grizzlies, sloth bears, polar bears, dogs, a fox, and other closely related animals from the Smithsonian National Museum of Natural History and the North Carolina State College of Veterinary Medicine. The National Zoo’s first panda couple, Ling-Ling and Hsing-Hsing, had five full-term cubs in the 1980s, but none of them survived long after birth. Li and Smith published their findings this month in the Journal of Anatomy.īaby panda skeletons are hard to come by, but the researchers were able to study the preserved remains of baby pandas born at the Smithsonian’s National Zoo in Washington, D.C. No one knows why, but a new study of bones across 10 species of bears and other animals finds that some of the current theories don’t hold up. With a few exceptions among animals such as echidnas and kangaroos, no other mammal newborns are so tiny relative to their mothers. This unusual size difference has left researchers puzzled for years. Their mothers are 900 times more massive than that.

“It just seemed like a jelly bean popping out of a vending machine,” Li said.īorn pink, blind, and helpless, giant pandas typically weigh about 100 grams at birth - the equivalent of a stick of butter. Duke student Peishu Li ’19 remembers the first time he saw a baby panda being born, in a video for a class taught by biology professor Kathleen Smith.

Baby panda