Losing weight has become monkey business at a Japanese zoo, which has put about 50 primates¹ on a strict diet after some became so obese² they couldn't move around.

The fat Rhesus（恒河猴） monkeys are a popular attraction at the park, and gained fame in Japan after media dubbed³ them "metabo monkeys", a play on the Japanese slang for metabolic⁴ syndrome（代谢综合症） -- a form of human obesity⁵.

"We don't really know the exact reason why the monkeys grew fat, but we reckon its because the monkeys are not only fed once a day by us, but also because this park is open 24 hours a day to visitors, who throw food into the enclosure," said Ohama park superintendent（监督人） Syoji Hasegawa.

The keepers at Ohama park in Western Japan have halved⁶ the animals' calorie intake⁷, replacing favorite foods such as sweet potatoes with wheat-based snacks.

The zookeepers imposed the diet late last year after finding that some of the monkeys weighed almost 30 kg (66 pounds) -- at least three times heavier than average.

Park officials have not weighed the animals recently but the diet appears to have worked as some monkeys now have saggy⁸ skin -- evidence of weight loss.

Although signs clearly tell visitors not to feed the monkeys, park officials say many people still throw them food.

That has prompted the park to consider building a new enclosure for the animals to cut them off from visitor titbits（量少味美的食品）.