It has long been assumed that Sumatran forests are of higher quality for orangutans than Bornean forests, and that this is both the proximate and ultimate cause of many of the differences in socio-ecology between the two orangutan species. Yet this hypothesis has remained untested. This chapter presents data on the phenology and floristics of eight Bornean and three Sumatran forest sites where orangutans have been studied to examine the effects of floristic composition, habitat productivity, and seasonality on orangutan population density. The alternative hypotheses that higher orangutan densities in Sumatra are due to overall higher levels of plant productivity, the increased availability of preferred foods, the presence of more fallback foods, or differences in floristic composition between the two islands are tested empirically.