Monitoring Reproductive Status in Giant Pandas and Other Carnivores

Giant panda reproduction is mysterious. Individuals live mostly solitary lives. Once a year, over a brief 2-day period coinciding the female’s estrus, males and females unite. About 135 days later, a cub is born.

But that number is only a rough estimate. Cubs have arrived anywhere from 85 to 185 days after conception. This is because following fertilization the embryo enters “diapause,” or a state of suspended animation. After some unknown signal occurs at some unknown time, the embryo implants into the uterine wall, the placenta begins to grow, and fetal development unfolds.

Only when the placenta and embryo become large enough to be detected with thermal imaging can we be certain of the presence of embryonic or placental tissues. This occurs between 65 and 80 days after breeding; the existence of a viable fetus can only be detected with ultrasound about 20 days prior to birth. To make things even more confusing, pandas exhibit “pseudopregnancy,” showing all the usual measures of pregnancy, such as hormone levels, but they are not pregnant!

What we know about panda reproduction has come from decades of research at our institution and others. Yet with so many reproductive mysteries remaining, our investigations are still very much active. With some successes and some failures, we are still adding to this unfinished puzzle, piece by piece.

A tried and true method of pinpointing panda estrus is observing changes in the size, shape and physiology of vaginal cells. These changes are caused by fluctuating hormone levels and different genes being turned on or off within the cells. By combining this established method with the measurement of gene changes throughout the reproductive cycle, we might be able pinpoint other events, like fertilization or implantation. In theory, this was a great idea. In practice, it has been problematic.

We discovered that genes do appear to be turned on and off throughout the reproductive cycle, but that we cannot consistently obtain high enough quality genetic material from vaginal cells to reliably measure them. Nevertheless, we are continuing to experiment and refine our protocols in hope of developing this approach further. Such is the nature of science.

We are also interested in developing novel, non-invasive methods that recreate the physiological events regulating pregnancy. One example is an in vitro system that will allow us to study the critical reproductive process of decidualization, which refers to a series of changes that transform the female’s uterus into a hospitable environment for the developing embryo. Obtaining decidualized uterine tissue is harmful to animal and certainly not something we would do to our pandas.

However, studies in other species show that treating isolated fibroblast cells growing is a dish with certain hormones and signaling molecules can promote decidualization. We are using this information to develop decidualization protocols for fibroblasts from endangered carnivore species stored the Frozen Zoo®.

Since decidualization occurs in the beginning of pregnancy, we hope to identify signals associated with early embryo development. If successful, we may use this information in the future to develop tests that will allow for pregnancy or pseudopregnancy diagnosis earlier than ever before.


Fibroblast cells from an American black bear are treated with hormones to mimic conditions that occur in the uterus during early pregnancy.

It should be noted that part or all of the reproductive phenomena of panda are shared with other members of the group Carnivora (dogs, cats, bears, and otters among others). As a result, many of our techniques, have potential application to many other endangered or threatened species. We are therefore expanding our efforts to include these species and hopefully unlock some of their reproductive secrets, much like we will continue to do for pandas.

Christopher Tubbs, Scientist, Reproductive Physiology Division, San Diego Zoo Institute for Conservation Research.