Primates are one of the most important frugivorous taxa in tropical forests. In lowland dipterocarp forests in SE Asia, they experience unpredictable supra-annual fluctuations of fruit availability caused by mast fruiting. We summarize our studies in Danum Valley, Sabah, northern Borneo, on the functional and numerical responses by primates to seasonal fluctuations in fruit availability. Both orangutans and red leaf monkeys increased their feeding time on fruits or seeds when fruit availability increased. However, red leaf monkeys did not change their ranging pattern in response to seasonal fluctuations in fruit availability. According to route census conducted repeatedly, only orangutans increased their number in response to increased fruit availability. During the 15 years, we observed fruiting peak 7 times and orangutans always increased their number. Another study in Borneo revealed that energy intake by orangutans varied 3 times between fruiting and non-fruiting seasons and in non-fruiting seasons, they consume fat that has been accumulated during the fruiting season. For some species of primates, such as red leaf monkeys, increased fruit availability may be just a bonus, but the orangutans capitalize the mast fruiting by fat accumulation and also by migration to a fruit-rich area in that season.