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 research base. In 2008, 18 baby pandas were born at the Chengdu Giant Panda Research Institute in Sichuan Province. This year the centre'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. Wen Li is considered the troublemaker of this year's litter, having made repeated attempts to escape from the wooden pen where she is kept. The more demure Ya Li is said to be content to look on. Researchers analysing 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. The pandas, whose scientific name, ailuropoda melanoleuca, literally translates as "cat-foot black-and-white", can live up to the age of 30. Resident pandas enjoy a fodder room and sleeping quarters. About 10,000 clumps of bamboo have been cultivated to keep the bears well fed.