2009 Pregnancy Watch
Zoo staff confirmed late yesterday that Mei Xiang is not pregnant but was experiencing a pseudo, or false, pregnancy during the past several months. On April 20, her level of urinary progesterone (a hormone associated with pregnancy) began to decline. Upon reaching normal baseline levels, this decline would end in either the birth of a cub or the end of a pseudopregnancy. Based on this information, and not having seen a fetus during the ultrasound exams, Zoo researchers have determined that Mei Xiang experienced a pseudopregnancy.
Female giant pandas almost always undergo a pseudopregnancy when they ovulate but fail to conceive. During a pseudopregnancy, hormonal changes and behaviors are identical to those of a true pregnancy, making it very difficult to determine if a giant panda is actually pregnant or not. This is the fifth time Mei Xiang has had a pseudopregnancy. Giant pandas ovulate once a year—Zoo scientists will determine whether Mei Xiang should be considered for breeding again in 2010.
Staff expect Mei Xiang to return to normal, hormonally and behaviorally (including an increase in appetite and activity level), in the coming days.
It has been about 50 days since Mei Xiang was inseminated. The duration of pregnancy in giant pandas is highly variable. Females have given birth as early as 80 days and as much as ten months after mating. Therefore, it is very difficult to determine when Mei Xiang will give birth if she is pregnant. Part of the variation in the length of pregnancy is attributed to a reproductive phenomenon known as delayed implantation.
Delayed implantation is common among some families in the order Carnivora, including mustelids (such as ferrets and sea otters) and ursids (such as polar bears and giant pandas). Following mating, the female’s egg becomes fertilized and undergoes cell division to become an embryo. However, at a certain stage, cell division slows or stops altogether—development is paused. We call this the "primary phase." Then for reasons we do not yet know, the "play" button is pushed and everything picks up where it left off. It is probably something environmental (extrinsic), but it could also be something that happens in the body (intrinsic). We can tell if the "secondary phase" of embryo development has started by a sudden increase in progestagens.
Mei's progestagen concentration has started to increase but we will need to analyze more samples to determine whether the secondary rise in progestagens has begun. One crucial thing to keep in mind is that we see both primary and secondary increases in progestagens in female pandas whether or not they are pregnant. Thus, the secondary rise will not tell us if Mei Xiang is pregnant. Because this phase generally lasts 40 to 50 days, the onset of the rise will give us a window of time when we may see the birth of a cub or the end of a pseudopregnancy. We will continue to closely monitor her over the coming weeks.
Mei Xiang's progestagens have started to increase more significantly in the past week. This rise is a bit early compared to previous cycles, but it is premature to consider it her secondary rise until the concentrations rise more sharply above 100 ng/mg Cr. The primarly progestagen rise in her past cycles averaged between 50 and 100 days; if this is the start of the secondary rise it would have been only 30 days. For this reason, we do not believe it is the secondary rise yet Once the progestagen concentrations increase significantly during the secondary phase, it will be about 40 to 50 days before we know if she is pregnant or if it is the end of the luteal phase.
The progestagen concentrations in Mei’s urine continue to rise, indicating she had a successful ovulation and formation of a functional corpus luteum (CL). We still do not know if she is pregnant, however. The formation of a CL is necessary for maintenance of the luteal phase, a phase that can be divided into two distinct sub-phases in the giant panda: the primary rise and the secondary rise. Progestagen dynamics during the primary rise are highly variable in duration (40-150 days) and magnitude (two to eight times the baseline), and it is also the period of embryonic diapause and before implantation if she is pregnant.
It is only after subsequent increase in progestagen production that the secondary rise begins. Following a more predictable pattern than the primary rise, the secondary rise typically lasts between 40 and 50 days, and is characterized by progestagen concentrations in excess of 30 times the baseline levels. Once the secondary rise begins in Mei Xiang, we should have some idea when the luteal phase will conclude (hopefully, with parturition). However, again, we see both a primary and a secondary rise even if the panda is not pregnant. So, we must be patient and wait.
We will continue to evaluate hormone patterns to monitor Mei Xiang’s reproductive physiology.
After the estrogen surge we look for concentrations of progestagens to increase, indicating ovulation has taken place. In the graph below, it is clear that Mei’s progestagen levels have begun to rise. The corpus luteum is structure on the ovary that forms following ovulation and that is what produces most of the progesterone that we see in the urine. Formation of a corpus luteum is necessary for maintenance of the luteal phase. We will continue to monitor the hormonal changes associated with Mei Xiang's luteal phase.
As we described last year, progesterone production during the luteal phase also is critical for maintaining a pregnancy. In most mammals, once the reproductive system has recognized that there is not a fertilized egg and that the female is not pregnant, the luteal phase ends. However, in bears, the luteal cycle lasts the same amount of time whether the female is pregnant or not. As a result, it is very difficult to use hormone analysis to determine whether a bear is pregnant. We can only know that she ovulated. The giant panda is a bear and, like other bears, it has a luteal cycle that is the same whether or not she is pregnant. Also, as in other bear species, the progesterone profile of the luteal cycle has two phases: a primary rise phase and a secondary rise phase. Following estrus the female begins the primary rise of her luteal cycle. We can see from the graph below that Mei Xiang has entered the primary phase of her luteal cycle.
The giant panda breeding season has come early this year, and Mei Xiang is in estrus. As the giant panda has only one estrus a year and a fertility window of only about 48 hours, we want to identify the time of ovulation as accurately as possible. We use several measures to do this; one of those measures is through monitoring hormonal dynamics associated with estrus. Hormones are the regulators of reproductive function. By assessing hormonal trends through urine analysis, we can track changes in physiology without disturbing Mei.
Typically, estrogen concentrations increase up to the time the female ovulates. Following ovulation, estrogen values drop off, so if we observe a drop in estrogens excreted in the urine, that will be an indicator that ovulation has occurred and that Mei is fertile. We look for a large decline in estrogens to baseline, and when that occurs we know it is time to schedule the artificial insemination. This year, her estrogen drop occurred on January 17, and that is the day she was inseminated. The scientists and veterinarians that conducted the AI said everything went very well. The timing looked to be perfect, and a very good quality semen sample from Tian Tian was used.