BEIJING, Aug 9 (Reuters) - A panda once believed to be male and sent to Japan to breed with a female without success gave birth to twin cubs this week, state media reported on Thursday.

The panda, "Jinzhu", gave birth to two female cubs on Monday at the Wolong Nature Reserve in the mountainous southwestern province of Sichuan, 11 years after being declared male at birth in 1996, Xinhua news agency said.

"Jinzhu was believed to be male owing to her inconspicuous secondary sex characteristic and behaviour," the agency quoted Wei Rongping, assistant director of the reserve's research centre, as saying.

Jinzhu was sent to Japan in 2000 to mate with a female, the report said.

"When the pandas showed complete disinterest, experts decided to turn to artificial insemination, leading to the discovery that Jinzhu had no penis," it added.

Jinzhu was sent back to China in 2002, with experts arguing the panda was either a hermaphrodite or had "undeveloped" sexual organs.

"The penis of an adult panda is only about 3 cm (1.2 ins) long," Xinhua quoted Li Deshen, a panda expert, as saying, as a possible reason for the mix-up.

It was not until 2005 that scientists discovered nine-year-old Jinzhu's ovaries were positioned in the wrong place, and gave her a two-hour operation to make her a "normal girl", Xinhua said.

Jinzhu subsequently mated with a male in March 2007 and gave birth 142 days later, Xinhua said.

The giant panda is one of the world's most endangered species and is found only in China. An estimated 1,600 wild pandas live in nature reserves n China's Sichuan, Gansu and Shaanxi provinces.

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