The sight of green polar bears has prompted many questions from visitors concerned about whether the animals are sick or carrying mould, zoo officials said on Sunday.
According to zookeepers, high temperatures in July and August and less- frequent water changes because of the zoo's conservation efforts caused an algae growth in the bear pond and safety moat.
"We cannot change water in the pool and algae grows in there," said zookeeper Masaru Kimura.
"The polar bears go down to the pool to play and swim. Then, the algae sticks to the fur and their color turns green," he added.
Algae that enters hollow spaces in the bears' fur is hard to rinse off.
However, zoo officials confirmed the bears are expected to return to their natural color when the algae growth subsides in November.