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CHENGDU -- Qi Zhen and Qi Yuan, twin giant pandas at a giant panda research center in Chengdu, capital of southwest China's Sichuan Province, each gave birth to twin cubs on Sunday and Monday, sources with the center announced on Tuesday.

With the birth of the four panda cubs, the number of panda cubs born in captivity this year on the Chinese mainland rose to six, said Zhang Zhihe, director of the Chengdu Giant Panda Reproduction and Research Center.

A giant panda cub grovels on a mat at Wolong Giant Panda Protection Centre in Chengdu, Southwest China's Sichuan Province on August 7, 2006. Qi Zhen and Qi Yuan, twin giant pandas at the giant panda research center each gave birth to twin cubs on Sunday and Monday. [Xinhua]

The other two panda cubs were born on June 22 and August 7 at the China Wolong Giant Panda Protection and Research Center, also in Sichuan.

Both pandas gave birth to their twin cubs smoothly, said Zhang, and the two mothers and the newborn twins are all in good health. Zhang said that they had delayed announcing the births for the sake of the health of the newborn cubs.

Qi Yuan, the younger of the two mother pandas, gave birth to twin cubs on Sunday. The twin cubs, both males, weighed 169 grams and 119 grams.

Qi Yuan gave birth to her first cub at 5:40 p.m. Sunday and her second baby was born 17 minutes later.

Her elder sister Qi Zhen also gave birth to twin male cubs on Monday afternoon. The delivery lasted from 5:03 p.m. to 5:58 p.m.. The newborn twins weigh 145 grams and 51 grams.

It is the second time that Qi Yuan has given birth to cubs. Last year, she gave birth to a male cub. By contrast, her older sister Qi Zhen is a first-time mother.

Qi Zhen and Qi Yuan were born in the Chengdu center on September 4, 1999.

As giant pandas are used to nursing just one cub, scientists at the Chengdu center remove the cubs for their own safety and nurse them in special boxes, Zhang said, adding the cubs are taken to their mothers every four hours on a rotation basis.

Statistics from the China Giant Panda Breeding Technology Committee show that since the first panda cub was born in Beijing Zoo in 1963, most panda cubs born in captivity weigh between 83 and 190 grams. A 51-gram cub is very rare.

China began to artificially inseminate giant pandas in the 1960s but very few successful cases were reported.

Major breakthroughs began in the 1990s. Artificial insemination produced nine baby pandas in 2000, 12 in 2001, 10 in 2002 and 15 in 2003. The number rose to 25 last year, among which 21 survived.

Giant pandas show little instinctive behavior in captivity, especially sexual desire.

Forestry authority statistics show fewer than 10 percent of male giant pandas mate naturally and fewer than 30 percent of females conceive naturally.

Female pandas normally become sexually mature at age four or five and have only one chance of pregnancy a year. After a gestation period of about 160 days they deliver one or two cubs.

Studies from the State Forestry Administration show there are over 180 giant pandas living in captivity on the Chinese mainland.

Experts had previously estimated there were 1,590 giant pandas living in the wild in China, but Chinese and British scientists announced in June that there could be as many as 3,000 after a survey using a new method to profile DNA from giant panda feces.

The Chengdu center now has 48 adult giant pandas.