updated 12/7/2012 8:19:14 AM ET 2012-12-07T13:19:14

Xiao Liwu, the 4-month-old giant panda cub at the San Diego Zoo, is getting more svelte — though not less sleepy. The baby bear dozed off during his exam this week, caretakers said, but they were able to get a good look at him before his nap.

Tracy Clippinger, a veterinarian at the San Diego Zoo, said the cub's muscles are getting stronger and he is thinning out, improving his ability to crawl. Clippinger added that she counted eight teeth in his mouth and could feel more ready to break through the gums, before he fell asleep.

The cub weighed in at 12.5 pounds (5.7 kilograms) and measured 26.7 inches (68 centimeters) long during his 16th checkup Tuesday (Dec. 4). His paws are also growing, with his left back one measuring 4.7 inches (12 cm) long, according to a statement from the zoo.

Xiao Liwu, whose name means " Little Gift," was the sixth cub born to the zoo's panda mom Bai Yun. All of the San Diego Zoo giant pandas are on a research loan from China, the only place where the species still exists in the wild. Four of Xiao Liwu's siblings have already been moved out of California to join the Chinese panda conservation and breeding program.

Captive breeding is an important way to study and conserve the endangered species, as just 1,600 giant pandas are thought to be left in the wild. In addition to habitat loss from human activities and low reproductive rates, giant pandas' survival is also threatened by climate change. A study released last month in the journal Nature Climate Change found that global warming could wipe out much of the bears' chief food source, bamboo, over the next century.

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