Tool manufacture and use have been described for wild orangutans (Pongo pygmaeus), with appreciable interpopulational differences in tool complexes. The ecological factors that contribute to these differences require investigation. Significant interpopulational differences in diet suggest that ecological factors contribute to variation in tool-based insect foraging. Using 4 years of behavioral data from the Suaq Balimbing Research Station (Sumatra, Indonesia), we tested predictions of two ecological hypotheses for the invention of tool use for insect foraging. We found limited evidence for inter- and intrasexual differences, as well as temporal variation, in activity budget and diet. However, differences did not correspond to variation in either rate of tool use or specialization on tool-based insectivory. Compared to other populations, orangutans at Suaq Balimbing ate significantly more insects. Low temporal variation in insectivory and an abundance of social insects at Suaq Balimbing suggest that insects formed a staple in the diet rather than a fallback food. Our findings do not support the hypothesis that tool use is a response to the low availability of primary food sources. Rather, greater opportunities for invention likely contributed to insect-extraction tool use at Suaq Balimbing. Am J Phys Anthropol, 2004. © 2004 Wiley-Liss, Inc.