For animals living in an environment that is seasonal with respect to food availability and temperature/rainfall, timing of reproduction is one of the important components to maximize their fitness. Compared to the temperate regions generally exhibiting clear seasonality with predictable plant phenology, seasonality of tropical forests is often less predictable. In particular, lowland dipterocarp forests in Southeast Asia are characterized by masting events, which occur at unpredictable intervals every 2–6 years and involve widespread general flowering/fruiting. Animals including orangutans and bearded pigs are living in such “unpredictable forests”, but they also can be found in secondary forests, where those masting events are less prominent. In a previous study (Heldstab et al. 2021, Biol Rev), we reveal that various factors, e.g., gestation length, diets, temperature, etc., would impact on reproductive seasonality, especially in primates. It would, therefore, be interesting to investigate on the animals with dietary preference to fruits/seeds and different gestation length such as orangutans and bearded pigs both in predictable and unpredictable environments to understand intra- and inter-specific variation/flexibility in their reproductive strategy. We will overview our previous study about primate reproductive seasonality and show preliminary results of primate and bearded pig birth seasonality in Sabah, Borneo, Malaysia