Updated

Pandas are notoriously slow breeders, but this has been a particularly bad year for the increasingly rare species. Pandas are simply "too exhausted" to have more babies, according to research at a Chinese base.

In 2008, 18 baby pandas were born at the Chengdu Giant Panda Research Institute in Sichuan Province. This year the center's tired mothers could manage only four — and half the credit must go to Li Li, who gave birth to twin females Wen Li and Ya Li on July 19, The Times reported.

Researchers analyzing the sharp decline in births concluded that the mothers were too tired to conceive this year. The institute was set up 1987 with six pandas rescued from the wild. Just over two decades later it is home to 83 giant pandas, about a third of the total pandas kept in captivity in China.

They are a source of national pride in China and their dwindling numbers have become a countrywide concern. They are the rarest type of bears and, with fewer than 3,000 still in the wild, are considered to be among the world’s most endangered species.

For more on this topic, read the full story at the London Times.