The 10 pandas for the 2010 World Expo at the Shanghai Zoo have grown by 6 to 7 kilograms over the past 10 days.

At a weigh-in yesterday, the heaviest panda, Yun Yun, was measured at 62 kilograms. A Ling was the lightest at 50 kilograms, a weight that is within the normal range.

Keepers said Yun Yun had a strong constitution because she was born as a single and her mother had enough milk for her when she was a cub.

A Ling, born in September 2008, is the youngest among the 10, thus it's expected that she'd be the lightest. Besides, some pandas, such as Han Yuan, always grabbed A Ling's food because she was timid and shy.

"A Ling eats very slowly, while Han Yuan eats much faster," said Li Guo, a keeper from Sichuan Province's Wolong Panda Research Center. "So after Han Yuan finished her food, she would jump to grab A Ling's."

Li said the keepers now will part Han Yuan and A Ling at dinner time, and give Han Yuan extra food after she finishes to ensure A Ling gets enough to eat.

Cajoling pandas onto the electric scale was tough work. Although keepers weigh them once every 10 days, some of the pandas still resisted.

"We attracted them with carrot first, and for those who were not willing to go onto the scale anyway, we would hold them up from the back, and stand onto the scale," Li explained. "It's exhausting because they are too heavy."

Usually pandas grow to 70 kilograms when they are two years old. Keepers said they are on pace to easily reach the weight.

The pandas came to the city from the Wolong center's Ya'an branch in early January. They will move to Shanghai Wildlife Park in July.