How does a panda fake being pregnant?

Even though she had elevated progesterone levels, was sleeping a lot, and was building bamboo nests, it turns out that Mei Xiang, one of two giant pandas at the Smithsonian's National Zoo in Washington, D.C., was not pregnant. Nobody knows why some animals – including humans – experience so-called pseudo-pregnancies.

By , LiveScience Senior Writer

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    Mei Xiang, one of two giant pandas at the Smithsonian's National Zoo in Washington, D.C., appears in this 2001 photo. Zookeepers had thought that the panda was pregnant, but an ultrasound has revealed that she was experiencing a pseudo-pregnancy.
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After months of waiting and hoping, panda lovers' dreams were dashed again. There will be no baby panda at the National Zoo this year – mama bear was faking her pregnancy.

Zookeepers called off the pregnancy watch after noticing that the prospective panda mother's hormone levels were back to normal, and that there were no signs of a fetus during an ultrasound. Until now, the female had appeared for all the world as if she was carrying a baby panda: She had elevated levels of the hormone progesterone; she lost her appetite and slept a lot; and she even built bamboo nests and cradled objects as if they were cubs.

Yet no one could be sure she was with-panda, because it is often difficult to identify a panda fetus through an ultrasound. And female giant pandas regularly undergo pseudo-pregnancies, which mimic the real thing to a tee, sans the developing infant.

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This is the fifth time Mei Xiang (May Shee-ahng), one of two giant pandas who live at the Smithsonian's National Zoo in Washington, D.C., has undergone a false pregnancy. She gave birth to one panda cub, the male Tai Shan, in 2005. Zookeepers have been hoping for a repeat performance since then, and inseminated her again this year with sperm from her male companion at the zoo, Tian Tian (T-yen T-yen).

Pandas only have one shot to get pregnant every year – females can ovulate once a year, and are fertile for about two days. After that, the resulting pregnancy – or pseudo-pregnancy – generally lasts between three to six months.

Scientists don't know why pseudo-pregnancies happen, or if they have evolved for an evolutionary purpose.

"In a sense there's no answer, but there is speculation that perhaps pandas' bodies just rehearse pregnancy all the time," Lisa Stevens, curator of primates and pandas at the zoo, told LiveScience.

In the case of pandas, she said, some researchers think the phenomenon could be related to the fact that the bears have evolved to survive on a very low-energy diet. They eat almost nothing but bamboo, which contains barely more energy than it takes to consume it.

But giant pandas aren't the only ones that experience false pregnancies. Many animals, especially carnivores and other bears, can go through the same thing. Even humans can have symptoms associated with a pregnancy, without carrying an actual baby. It's just that in most species, especially humans, it's easier to tell for sure with an ultrasound.

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