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Until recently, biologists believed that panda populations are limited mostly by the supply of their staple food, bamboo. But Ron Swaisgood of the San Diego Zoo in California and Zejun Zhang and Fuwen Wei of the Key Laboratory of Animal Ecology and Conservation Biology in Beijing, China, suspected that past logging might have deprived pandas of suitable family accommodation.

Female pandas like to build dens in the cavities of ancient trees, where their cubs spend the first four months. But most of these old-growth trees have been cut down in many panda reserves. As a result, females in the Foping Nature Reserve in Shaanxi province in central China tend to resort to caves. These are in short supply and also liable to flood, Swaisgood told a meeting of the Society for Conservation Biology in Beijing, this week. "We think pandas are restricted by the quality and quantity of the dens available," he says.

Through painstaking video recording, the researchers have documented female pandas' struggles in this substandard housing - including one heart-rending incident from 2007 that has challenged conventional wisdom about panda breeding.

About half the time pandas in captivity give birth to twins but seem to focus on only one cub, leading biologists to assume that abandoning one twin is the norm. Swaisgood and his colleagues aren't so sure, after observing a female who successfully raised twin cubs in a rocky cave for 16 days, until heavy rains lashed the study site. When the team returned, the mother and one of the cubs had gone. The other cub was found dead, 22 metres from the den, suggesting the mother had tried to remove both cubs but could save only one.

The researchers believe that it may be possible to boost panda populations by building more suitable dens from wood or rocks on higher ground - and they are starting a pilot project to put that idea to the test.

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