We have been excited to discover that Bai Yun is pregnant with twins! Our veterinarians confirmed via ultrasound that there is a fetus in each of her uterine horns (the panda has a bifurcated uterus, shaped like a Y). When they were first discovered, vets could see that one fetus was well developed, with a robust heartbeat, and the other was smaller with a well-developed placenta but no heartbeat yet detected. A subsequent ultrasound revealed a fetal heartbeat on both sides. This morning, only the more developed cub could be seen. Does this mean that the other embryo is lost? Or was it just hidden from view? We can’t say for sure either way.
This is not the first time Bai Yun has been pregnant with twins. Veterinarians also saw two fetuses when looking in 2003, the year of Mei Sheng’s birth. Amazingly, they were also able to document the resorption of one fetus prior to birth. We were left to wonder if that fetus was somehow less viable than the other, and therefore died prematurely, or if Bai Yun’s body selectively pruned her litter of two down to one. Although Bai Yun herself was a twin, she has never given birth to twins. Will she prune her litter again this year?
Why would a female prune her litter? We know that raising one panda cub is an extremely difficult and labor-intensive task for a female. She invests a lot of energy in holding, suckling, cleaning, and protecting her young. We know from our comparative studies with other bear species that pandas are far more attentive mothers than American black bears or brown bears, and even the active sun bear isn’t quite as diligent as a panda. As demanding as raising a panda cub is, rearing twins is even more of a feat, so much so that it has been very rarely documented that a female might rear more than one cub in a natural setting. To save energy, it may be a good evolutionary strategy for a female panda to avoid putting herself in this position in the first place, and so she might have natural biological mechanisms in place that ensure a singleton birth.
Even in the event of a twin birth, panda mothers routinely make choices about how to rear their litter. In birth centers in China, females sometimes reject one of their cubs immediately upon their birth, deciding from the get-go that she will only raise one of the offspring. Others may choose to attempt to rear two for a few hours, but ultimately set one cub down to attend to the other. We don’t yet know what factors go into making the choice between cubs, but we are actively looking at this as a research project and hope to have the answers soon.
In the final analysis, a panda mother seems incapable of rearing twins on her own. There have been a few spotty reports of wild twins located in tree nests or wandering about in China. This suggests there might be a possibility of a panda caring for two cubs from birth. Certainly this is possible with most other bear species. However, to my knowledge there has been no confirmed case of a panda that gave birth to and successfully reared twins without support.
What would Bai Yun do if she did give birth to two? We don’t know. She’s never been put in that position before, and we aren’t sure how she might react. Bai Yun is an excellent, skilled, experienced mother; perhaps if she feels she’s up to the job of handling twins, she would try to raise both. She is also well-seasoned and knows just what’s involved in caring for the cubs, and splitting her efforts between two means neither one gets her full attention. For that reason she may reject the demands of rearing two in order to ensure the survival of one of her offspring. We’ll have to wait and see.
In a future update I will describe some of the conditions and considerations our staff will contend with in dealing with a twin birth. Stay tuned!
Suzanne Hall is a senior research technician for the San Diego Zoo’s Institute for Conservation Research.