Contrary to public perception, bananas are actually bad for monkeys. Nutritionists at Paignton Zoo in the U.K. have banned bananas from their monkeys' diets. Amy Plowman, the zoo's head of conservation and advocacy, told The Telegraph:
People usually try to improve their diet by eating more fruit -- but fruit cultivated for humans is much higher in sugar and much lower in protein and fiber than most wild fruit because we like our fruit to be so sweet and juicy. Giving this fruit to animals is equivalent to giving them cake and chocolate.
In the wild, monkeys like the Sulawesi crested black macaque (one of the species now going without bananas at the zoo) do mostly eat fruit, but their diet relies heavily on figs, not bananas. The crested black macaque does sometimes eat bananas, but not often; it supplements its fig-based diet with other fruits as well as leaves, eggs, insects, and small animals like lizards. And the bananas are unlikely to be the familiar Cavendish; in Indonesia, where the macaque lives, the pisang raja variety is much more common.