In this paper, we analyze predictions from the energetic bottleneck and opportunity models to explain the use of stones to crack open encased fruit by capuchins in dry environments. The energetic bottleneck model argues that tool use derives from the need to crack open hard-encased fruits which are key resources during periods of food scarcity. The opportunity model argues that tool use by capuchins derives from simultaneous access to stones and encased fruits. The study was conducted in the Caatinga biome, northeastern Brazil, at two areas where capuchin monkeys (Sapajus libidinosus and Sapajus spp.) regularly use stones to crack open encased fruit of Syagrus cearensis and Manihot dichotoma. Energetic gains were inferred based on the number of tool-use sites used and the mass of encased fruit consumed per month, and compared across seasons and areas occupied by the two groups. For the drier habitat, a significant increase in frequency of tool use (Ndry = 329 vs. Nwet = 59) and in the mean monthly mass of fruits consumed in the dry season (meandry = 193g vs. meanwet = 13.5 g) offered support for the energetic bottleneck model. However, our inference of low energetic payoffs for tool using individuals (in the drier caatinga habitat from 13 to 193 cal·ind−1·month−1 and in the wetter caatinga habitat from 805 to 1150 cal·ind−1·month−1) offer support for the opportunity model. Finally, our analyses indicate that consumption of six S. cearensis fruits would equal the daily requirements of capuchins for β-carotene, and the consumption of 1.22 g·day−1 of M. dichotoma encased fruit or 1.0 g·day−1 of S. cearensis can supply capuchin's daily requirement of vitamin C. So, specific nutritional requirements may play a role in explaining the continuous consumption of encased fruit and customary use of stones to crack open encased fruit. Am. J. Primatol. 74:332–343, 2012. © 2011 Wiley Periodicals, Inc.