I used novel quantitative measures to assess the impact of fruit seasonality on changes in diet and energy balance during 4918 hours of observation on >60 orangutans in Gunung Palung National Park, West Kalimantan, Indonesia, from September 1994 through September 1995. During this period a mast fruiting occurred resulting in large fluctuations in fruit availability measured by monitoring of 558 orangutan fruit trees. Orangutan diet varied tremendously in accordance with these fluctuations. During the month of highest fruit production, 100% of the orangutan diet was composed of fruit. In contrast, during the fruit-poor period as little as 21% of the diet was fruit, while 37% was bark. Nutritional analyses of 78 of the foods most commonly eaten during this study show that mast foods were significantly higher in caloric content than were nonmast foods, which translates into substantial changes in caloric intake. During the month of highest fruit consumption (January), males consumed an estimated 8422 kcal/day and females consumed 7404 kcal/day. During the month of lowest fruit consumption (May), males consumed 3824 kcal/day and females consumed 1793 kcal/day. These differences were significantly different between the 2 months for both males and females. In May, males ate significantly more kcal/day than females did. Time spent feeding did not differ between the two periods for either males or females. I assessed the impact of these changes in diet and caloric intake on orangutan physiological functioning by measuring ketones in 257 urine samples collected from adult males and females. Ketones-products of fat metabolism-were present in urine only during the fruit-poor period. These data suggest that orangutans are highly efficient at storing far during fruit-rich periods and utilizing the reserves during times of fruit shortfall. This response may have important implications for orangutan survivorship, reproduction, and behavior.
