A central issue in comparative biology is identifying the relative importance of historical (phylogenetic) versus present-day (ecological) factors in shaping phenotypic traits of organisms. Herein, we investigate effects of sex, ontogeny, and season on diet of Epipedobates flavopictus, a species restricted to open landscapes in central Brazil. Based on prey frequency, number, and volume, the most important prey categories were ants, termites, beetles, spiders, and orthopterans. Prey number and volume increased significantly with snout–vent length (SVL), and the consumption of termites also increased with SVL. There were few effects of sex and season upon diet composition, average prey number, or average prey volume, all independent of SVL. Reproductive females consumed larger prey, in great numbers, likely to increase energy uptake. As is true for most congeners, as well as species of closely related genera, ants are an important component in the diet of E. flavopictus. However, in contrast to its forest relatives, E. flavopictus consumes large amounts of termites, revealing the influence of prevailing ecological conditions. The small volume of ants in the diet of E. flavopictus and the high toxicity of its skin are not in agreement with the hypothesis of a causal relationship between skin toxicity and ant consumption.