Staff at the Smithsonian’s National Zoo confirmed today that female giant panda Mei Xiang (may-SHONG) will not give birth to a cub this year. They believe that she experienced either a pseudopregnancy or the loss of a developing fetus. In a pseudopregnancy, an animal’s hormonal changes and behaviors are identical to a pregnancy, but no conception occurred. Fetal loss during early pregnancy is a common occurrence in mammals, but the reasons for this phenomenon are poorly understood.
National Zoo scientists, veterinarians and keepers were closely watching Mei Xiang, assessing her hormone levels and behavior and conducting weekly ultrasounds in an attempt to determine if she was pregnant. Veterinarians noted small changes in Mei Xiang’s uterus—but they were unable to confirm the presence of a fetus. Giant panda fetuses are very small—a new-born cub is only five inches long. At other zoos, fetuses have only been visible on ultrasound in the last weeks before birth.
In mid-July, Mei Xiang’s urinary progesterone levels (a hormone associated with pregnancy) began to decline. In pregnant pandas, declining hormones and increased maternal behaviors signal an impending birth. This year, Mei Xiang’s hormones declined as expected, but the decline lasted longer than normal and she continued to show maternal behavior even after her hormones reached baseline. The Zoo’s scientists and veterinarians speculate that Mei Xiang may have experienced the loss of an early-stage fetus that failed to develop normally, and it was reabsorbed into the lining of the uterus.
National Zoo staff are recognized leaders in the study of giant panda reproduction, but they still have much to learn. Combined with hormones and behavior, ultrasound technology can help them learn more about pseudopregnancy, as well as how common fetal loss may be for giant pandas.
National Zoo staff expect Mei Xiang to return to “normal,” both hormonally and behaviorally, in the coming days, which includes an increase in appetite and activity level. Zoo scientists artificially inseminated Mei Xiang with semen from the Zoo’s male giant panda Tian Tian (t-YEN t-YEN) March 19. Giant pandas ovulate once a year, so Zoo scientists will re-evaluate whether to consider Mei Xiang for breeding in 2009.
Mei Xiang is on exhibit at the National Zoo’s Fujifilm Giant Panda Habitat, along with the Zoo’s adult male giant panda, Tian Tian, and their 3-year-old cub, Tai Shan.
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