The diet of Sumatran orangutans at Perth Zoo was reviewed and altered to simulate more closely foods available in the wild. Diversity of offered plants was increased to over eighty species. Cereal-based concentrates were replaced by foods of animal origin, including fish and cheese. Dietary supplements were eliminated. The proportion of different foods was adjusted to meet Dietary Reference Intakes for humans with respect to minerals and vitamins, as recommended by the U.S. Institute of Medicine.
Target body mass for non-pregnant non-lactating (NPNL) females was set at 45 kg, taking into account skeletal development in captivity is greater than in the wild. To achieve this target, dietary energy offered to each NPNL female was reduced from 9.2 MJ to 5.8 MJ per day. Males, with double the body mass, were allocated approximately twice the ration of NPNL females. Energy reductions took place in five stages, over 7 mo.
Acceptance of the diet has been high. Changes resulted in loss of body mass, in all six specimens considered to be overweight, out of a total of eleven adults. Fully mature NPNL females were able to maintain body mass on less than 0.4 MJ kgW0.75/day with regular menstrual cycles. A boost in dietary energy, equivalent to that of a mast fruiting season in the wild, is planned for the coming spring, to take advantage of a possible benefit of increasing day length on conception.