Female Giant Panda's Hormones on the Rise at Smithsonian's National Zoo

National Zoo scientists have detected a secondary rise in urinary progesterone in its female giant panda, Mei Xiang. This means that she will either give birth to a cub or experience the end of a pseudopregnancy in 40 to 50 days. Mei Xiang has had five consecutive pseudopregnancies since 2007.

Mei Xiang returned to a more normal estrous cycle this year. She went into heat in April after three consecutive years of going into heat in January. Giant panda breeding season generally lasts from late winter to late spring, but scientists are unsure of what triggers the onset of estrus. Mei Xiang was artificially inseminated twice in late April with sperm collected and frozen in 2005 from the Zoo's male panda, Tian Tian. The second artificial insemination April 30 was live-tweeted for the first time.

As part of a cooperative agreement for giant panda breeding and behavioral research, Dr. Li Desheng from the China Conservation and Research Center for the Giant Panda in Wolong traveled to the National Zoo to assist scientists with the procedures. Both artificial inseminations were performed by Dr. Li alongside reproductive physiologists and veterinarians from the Zoo. The end of this breeding season will mark the conclusion of a two-year collaborative study on giant panda breeding between Smithsonian Conservation Biology Institute scientists and Chinese scientists.

Since the artificial inseminations National Zoo scientists have conducted weekly hormonal analyses on urine samples from Mei Xiang. Zoo veterinarians are also conducting ultrasounds frequently to monitor changes in her reproductive tract and evaluate for evidence of a fetus. Panda fetuses do not start developing until the final weeks of gestation. It may be too early to detect a fetus. Keepers are also monitoring Mei Xiang's behavior, which has been consistent with a rise in urinary progesterone. She has begun nest-building. Visitors to the Zoo's website can watch her progress on the panda cam.

Mei Xiang gave birth to her only cub, Tai Shan, July 9, 2005. Tai Shan was born as a result of artificial insemination and is now at the China Conservation and Research Center for the Giant Panda.

Fourteen year-old Mei Xiang and 15-year-old Tian Tian are at the National Zoo as part of the Giant Panda Cooperative Research and Breeding Agreement signed in 2011 by Chinese and Zoo officials. The agreement extends the Zoo's giant panda program through the end of 2015.