It is easy to think that because wild gibbons are frugivores (“fruit eaters”), they should be fed large quantities of cultivated fruits. In reality, the fruits available for human consumption are nutritionally very different from those consumed in the wild. Wild forest fruits, such as wild figs and lipid- and sugar-rich flashy fruits, are higher in structural fiber, complex carbohydrates, proteins, and essential minerals like calcium. Conversely, agricultural fruits cultivated for human consumption, such as mangoes, papayas, and bananas, have been selectively bred for maximum taste, resulting in high levels of water and simple sugars (sucrose and fructose) and drastically reduced fiber. Furthermore, wild gibbons frequently consume fruits across various stages of ripeness, whereas market fruits are typically fed perfectly ripe.
Feeding a diet heavy in commercial fruits can cause unnatural sugar spikes in captive gibbons, leading directly to the metabolic and dental diseases. To combat this, cultivated fruit should be given as a small portion of the diet, or not at all, if possible. The nutritional composition of most wild fruits are closer to cultivated vegetables, than the fruits we know. This provides a good opportunity for us to shift captive gibbon diets from mostly fruit to mostly vegetable. When fruits are offered, caregivers should present the whole item whenever possible, leaving the peel, rind, and core intact to preserve whatever structural fiber is available. It is highly preferable to feed raw, unripe fruits, such as green bananas or green papayas, which contain higher levels of resistant starch and dietary fiber, rather than fully ripe or overripe fruit. Finally, carers must monitor individuals for specific dietary intolerances, as some acidic commercial fruits like citrus have been known to cause gastrointestinal problems in some gibbons.