Born Saturday night, the second cub had "developmental abnormalities" and was never alive, the Zoo said on its website. Stillbirth in giant pandas is "extremely rare," the Washington Post reported.
"Keepers watching Mei on the Panda Cam saw her groom it for 17 minutes," the Washington, D.C. zoo said on its website. "When she stopped grooming, it fell from Mei's body onto the floor … It lay motionless and made no sound."
Meanwhile, the frequently squealing firstborn cub had its first neonatal exam Sunday, and the Zoo reports it is "robust, fully formed, and is a bright, healthy shade of pink."
The youngster weighs about 4.8 ounces and seems to have a steady heart rate and well-functioning lungs. It's been feeding from Mei and digesting its food, "all signs that we have a very healthy cub," according to the zoo. (See more panda pictures.)
Twins are common among giant pandas, yet in the wild, one of them will eventually die, noted Marc Brody, senior advisor for conservation and sustainable development at China's Wolong Nature Reserve, which breeds pandas in captivity.
"The mother will choose which is the stronger of the twin, and will focus on that panda and abandon the other," Brody said.
"Immediately a mother panda knows it's a tough world out there and goes for the survival of the fittest."
However, thanks to "tremendous advances and sophistication" in veterinary care, many twins can survive when born in captivity, he noted.
For instance, in captivity, veterinarians never leave the panda mother with both newborns at a time, instead rotating the youngsters so that one's always in an incubator.