It's wintertime and the king penguins at a zoo in northern Japan are putting on weight. But the keepers there have a solution: exercise.

Officials at Asahiyama Zoo on Thursday started taking the penguins on 500-yard walks on the snowy grounds twice a day, zoo spokesman Tetsuo Yamazaki said.

"Just like in humans ... the fat accumulates during the winter months, and the blood-sugar level rises," Yamazaki said.

The zoo's 15 king penguins aren't exactly obese. Penguin winter weight varies from 33 pounds to 40 pounds, said zoo official Kazunobu Maru. So far, only one of the flock is 40 pounds, he said.

The reason for weight gain is natural.

"In order to withstand the cold, the penguins have a habit of standing very still during winter months," Yamazaki said, while in the summer they can swim as much as they want.

To fend off obesity, the zoo instituted the winter exercise program in 2003.

The penguins have tender webbed feet and can walk significant distances only on snow, so the program can take place only during the colder months.

The zoo, on the northern island of Hokkaido about 550 miles northwest of Tokyo, takes the penguins on strolls from December until April.