In patch foraging problems, animals must choose to stick with a depleting resource or to search for a new, potentially more rewarding source. Many species of animals exhibit a tendency to stick with the depleting resource longer than is predicted by optimal foraging theory (Marginal Value Theorem; MVT). We attempted to determine the cognitive biases that underlie overharvesting in rats by characterizing rat foraging behavior in multiple foraging environments, and formally testing whether four separate cognitive biases – subjective costs, decreasing marginal utility for reward discounting of future reward, and ignoring post-reward delays – could explain rat foraging behavior. We found that, in some contexts, rats qualitatively follow predictions of MVT but overharvest, and all four biases could explain this behavior. In other contexts, rats deviated from predictions of MVT, and only one bias – in which rats ignore post-reward delays – could also explain these deviations of MVT.