The Internet exploded over the weekend over the birth of tiny twin pandas at the National Zoo. And it's fair — baby pandas are quite possibly the cutest things on the planet. Of course, this wasn't the first time news of twin pandas hit the Internet. A popular pair resides in Atlanta, while another pair was born in China last month. In fact, "in about 50 percent of births there are twins in pandas," Barbara Durrant, Director of Reproductive Physiology at the San Diego Zoo tells Refinery29. "The reason most people think that they usually have single births is because most of the time, only one survives." Which is, sadly, what happened to the pair at the National Zoo today as the smaller cub passed away. Twin animals aren't really that rare, Durrant explained. Identical twins, for sure, but fraternal? "Technically any two, or three, from a litter would be twins," she says. Puppies in a litter are twins — so are triplets, quadruplets, quintuplets and so on. These sets of fraternal twins are quite common in medium-sized animals like sheep, small antelope, and bears.
Identical twins, however, are much more rare. To be considered identical twins, animals would need to have the exact same DNA and share the same placenta, Durrant explained. So those twin pandas born over the weekend were probably fraternal twins — especially as the births were a few hours apart. "Very rarely are there identical twins," Durrant says, who has witnessed hundreds of births at the San Diego Zoo. "And the thing with giant pandas is no one has ever been able to recover a giant panda placenta, because they're either not passed or the female consumes them right away." So no one knows, without a DNA test. Only one animal consistently gives birth to identical twins, Durrant says, and that's the nine-banded armadillo. "They ovulate only one egg," Durrant says, "and then the embryo splits four ways, and four identical quadruplets are born." According to American Scientist magazine, this is because the nine-banded armadillo females only have a small implantation site for the egg; egg-splitting allows larger litters for the female. It's probably safe to say, however, that baby armadillos aren't as cute as those pandas. Or these twin tigers.