Panda Pregnancy: Embryonic Diapause

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As we work our way into the month of August, many of you have been wondering about the status of the San Diego Zoo’s panda matriarch, Bai Yun. In recent years, she had already given birth by this date, and yet this time around we haven’t even been able to announce to you that she is indeed pregnant. I thought I would take this opportunity to discuss some of the reasons why pregnancy in pandas is a complicated affair, and what goes into the determination of a pregnancy. In this post, I’ll tackle one of the most interesting parts of the process with pandas: embryonic diapause.

Embryonic diapause, also known as delayed implantation, is an interesting biological event that comes into play after breeding occurs. In diapause, the female’s ova (eggs) experience a few rounds of cell division shortly after fertilization, and then development of the potential fetus arrests. This means the offspring virtually stops growing in the uterus and instead free-floats without growth until the time is right for it to implant in the uterine wall. Until implantation, the fertilized egg (called a blastocyst at this stage) cannot gain the important nutrition it needs to finish its transition into a fetus.

Pandas are not unique for experiencing diapause; in fact, it’s a phenomenon common to about 100 different types of mammals, from bears to weasels. We don’t yet understand what triggers the blastocyst to implant and begin growth in earnest, although scientists suspect photoperiod may play a big role. However, one of the big consequences of diapause in pandas is that this phenomenon requires us to wait until the period of delayed implantation is over to begin to see indicators of pregnancy behaviorally, physiologically, or via ultrasound.

The wait time can be variable from year to year, even with the same female. In Bai Yun, for example, we have observed total pregnancy lengths that have varied from 101 to 150 days. Since the length of time it takes to gestate a panda fetus from implantation to birth is thought to be fairly constant at about 50 days, the variation in pregnancy lengths most likely reflects the variation in diapause length for each of her pregnancies.

As the period of diapause comes to a close, hormone monitoring detects a rise in progestins that is probably associated with implantation. It is that rise in progestins that allows us to know that the female panda is undergoing a physiological preparation that could lead to birth, and it drives many of the behavioral and physiological indicators that are associated with pregnancy. However, once we transition out of diapause, we are subjected to another complicating factor in panda pregnancies: the possibility of pseudo-pregnancy.

In my next post, I will delve into this topic in more detail. See you next week!

Suzanne Hall is a senior research technician for the San Diego Zoo Institute for Conservation Research. Read her previous post, Farewell to Panda Dong Dong.

Note: Photos from Yun Zi’s 2nd birthday are now posted in our Panda Photo Gallery!