OSAKA (JAPAN): Losing weight has become monkey business at a Japanese zoo, which has put about 50 primates on a strict diet after some became so obese they couldn't move around.

The fat Rhesus monkeys are a popular attraction at the park, and gained fame in Japan after media dubbed them "metabo monkeys", a play on the Japanese slang for metabolic syndrome — a form of human obesity.

"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 Syoji Hasegawa. The keepers at Ohama Park in Western Japan have halved the animals' calorie intake, 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 — 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 saggy skin — evidence of weight loss. Although signs clearly tell visitors not to feed the monkeys, park officials say many people still throw them food.