Pregnancy is not something that’s easy to hide. From expanding pregnant bellies, to morning sickness and ultrasounds, whether someone is pregnant, eventually, is usually not that hard to figure out.
When it comes to giant pandas, however, scientists are still often unable to detect pregnancy — sometimes up until the actual moment a panda cub is delivered.
“Everything is complicated with giant pandas,” says Pierre Comizzoli, a research biologist with the Smithsonian National Zoo.
Comizolli has been monitoring Mei Xiang, a female giant panda at the National Zoo that gave birth to two cubs over the weekend. Mei Xiang was artificially inseminated with sperm from two other giant male pandas in April of this year. Zoo biologists noted that her progesterone levels were rising, but sustained progesterone levels are misleading because giant pandas can undergo pseudopregnancy, during which they show all the signs of a real pregnancy, even though they’re not with cub.
Sometimes they'll even build a nest for giving birth while undergoing pseudopregnancy.
“We have to wait until this period of time where the progesterone levels are decreasing. And when we are back to a level near to zero for the progesterone, we know that we are [at] the end either of pregnancy or pseudopregnancy,” Comizzoli says. “If we don't see a baby panda, unfortunately the female wasn't pregnant."
Ultrasounds are also often unhelpful in determining giant panda pregnancy.
“A female is about 100 kilograms and she's going to give birth to [a] baby that's on the [scale of] one hundred grams. So there is a huge difference between the size of the mother, the size of the abdomen of the mother and really the fetal development. So in terms of an ultrasound, in imaging [it] is really, really difficult to see something,” says Comizzoli.
Late last week, the zoo did finally pick up the panda baby on an ultrasound — but only days before giving birth.
Giant panda pregnancy terms are also difficult to determine. The breeding season for females happens only once a year, with a tiny 24-36 hour window for conception. Biologists are able to monitor female giant pandas in captivity in order to perfectly time artificial insemination. Once the insemination takes place, however, there is a long waiting period before scientists are able to tell whether or not a new cub will be born, and when it might be born.
“Pandas have a delayed implementation. That means that the length of the gestation is variable. It can be a little bit less than three months, but it can be also up to five months and a half,” Comizzoli says. “So the problem we have is that it's really difficult to know exactly what's the length of this pregnancy.”
Mei Xiang has now given birth to six cubs. Two died, two previous cubs survived, and now both cubs she birthed this weekend have survived as well.