Spotlight on Zoo Science
April 14, 2003
Predicting Giant Panda Pregnancy
Springtime in Washington means two things: cherry blossoms
burst into bloom across the city and suburbs, and the giant
pandas at the National Zoo experience their annual breeding
cycle. Around March 19, the giant pandas at the Smithsonian’s
National Zoo began to show signs that they might be entering
their breeding season. Tian Tian, the male, is approaching
his sixth birthday, and Mei Xiang, the female, her fifth.
Giant pandas have one breeding season a year, with mating occurring during a short, one-to-three-day period. Beginning about March 25, Zoo endocrinologists reported that Mei Xiang’s urinary estrogen levels had begun to rise above baseline. By April 1, her estrogen levels had risen substantially, and a sharp increase on April 3 indicated that ovulation was imminent.
Through April 3, Tian Tian and Mei Xiang were vocalizing more frequently, making chirps, whines, and bleats. Both pandas were scent marking more and more and the female was repeatedly raising her tail and walking backwards toward the male—all signs that the pair may be ready to mate.
On April 4, although Tian Tian attempted to mount Mei Xiang
several times, she did not exhibit the proper mating position.
Nevertheless, the female solicited the male’s attention
by direct approaches, chirps and whines. With scientists,
keepers, and volunteer observers maintaining a close watch
on the pair via direct observation and several video cameras
located in the pandas’ enclosures, Mei Xiang and Tian
Tian remained together indoors overnight.
A precipitous decline in urinary estrogens—a sure
sign of ovulation in giant pandas—occurred on April
4. And then, as if on cue, Mei Xiang and Tian Tian mated
very briefly—for about 15 seconds. In fact, the mating
was so brief that Zoo scientists had to review videotapes
of the pair’s behavior to confirm the mating.
Of course, the big question is whether Mei Xiang will produce a cub. Although amazingly little is known about the reproductive biology of giant pandas, Zoo scientists will be working hard to evaluate urinary hormones and behavioral patterns for clues regarding an impending birth.
Zoo endocrinologists conducted daily hormone evaluations and were able to precisely map Mei Xiang’s day-to-day hormone patterns around the time of ovulation, shown in the graph below. Scientists believe that ovulation occurs in giant pandas about 24 hours after urinary estrogen peaks and luteinizing hormone (a pituitary hormone, also known as LH) triggers ovulation.
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The clearly
defined peaks in estrogen and LH on April 3 preceded mating
by about 24 hours. The subsequent increase in urinary progestins
confirms that Mei Xiang did, in fact, ovulate. Although mating
occurred around the time of ovulation—the period of
peak fertility—it is still not certain that a cub was
conceived.
Only the National Zoo and the San Diego Zoo have this expertise
in wildlife endocrinology and are capable of providing
same-day hormonal analyses. This capability extends to
a host of other species as well.
Now, in the weeks to come, Zoo scientists will focus on assessing urinary progestin levels. But interpreting the hormone results will not be easy. Giant pandas that ovulate but don’t conceive, experience “obligate pseudopregnancy.” The ensuing hormone profile of pseudopregnancy, in which progestins rise, appears identical in every way to a true pregnancy.
This is related to another idiosyncratic characteristic of giant panda reproduction, delayed implantation. As in other bear species, the embryo floats unattached in the uterus after conception for 30 to 100 days before implanting in the uterine wall. Because a panda doesn't really "know" whether she might be pregnant, she protects the "potential pregnancy" by producing the appropriate hormonal signals, just in case. Once implantation occurs, gestation takes about five to six weeks.
This explains why all giant panda cubs are born at the same developmental age, despite the huge variation in the time between mating and birth, which ranges from 95 to 180 days. (It also explains why we can’t predict precisely when Mei might give birth if she is pregnant.)
Thus, using current methods of testing, Zoo scientists can’t
effectively diagnose whether Mei Xiang is pregnant. If, within
the next few months, Zoo reproductive physiologists notice
a secondary acceleration in progestin production, they will
be "on guard" for a pregnancy. However, if the
hormone levels drop precipitously after several additional
weeks, with no birth, scientists will conclude that Mei Xiang
experienced a pseudopregnancy.
For future giant panda breeding seasons, National Zoo scientists
are trying to develop a pregnancy test using a hormone
known as relaxin, but it will be some time before this
tool is available.
Funding for giant panda research comes from Friends of the National Zoo. Fujifilm helped to bring Tian Tian and Mei Xiang to the National Zoo and to create the Fujifilm Giant Panda Conservation Habitat and related conservation education programs.
