Four six-month-old giant panda cubs play at the Chongqing Zoo in southwest China's Chongqing on Dec. 22, 2019. The zoo held on Sunday a half-year-old birthday celebration for the four panda cubs named Shuangshuang, Chongchong and Xixi, Qingqing, which combine to symbolize "double joy and happiness" in Chinese. (Xinhua/Tang Yi)

Two pairs of panda twins marked their half-year anniversary Sunday in a zoo in southwest China's Chongqing Municipality, as they cuddled around and climbed a tree.

Born on the same day on Jun. 23, the four cubs are made up of a pair of male twins named Shuangshuang and Chongchong, and a female pair of twins named Xixi and Qingqing. The names combine to symbolize "double joy and happiness" in Chinese.

The Chongqing Zoo held a special ceremony for the cubs to mark the half-year anniversary as the age is about time that they learn to walk and start to increase outdoor activities, said Yin Yanqiang, technical supervisor of the bear in the zoo.

"At this age, they will have more time to see the tourists, and have stronger abilities to climb and run," Yin said.

The four cubs played around on the ground in the panda pavilion and cuddled around for a while before they went for a climb on a tree about 3 meters tall one by one.

They managed to stay in the tree for a while, but struggled a bit to get off. The audience broke into laughter after seeing their moves. The panda keepers finally helped them off the tree.

"Recently, the zoo has bought slides, platforms and racks for the cubs to practice their sports skills," Yin said.

The four cubs have been growing well, with Chongchong being the heaviest at 14.3 kg and Xixi the lightest at 9.8 kg, Yin said.

The giant panda cubs made their first public appearance at the Chongqing Zoo in September.