Mechanisms of overharvesting in patch foraging

Serial stay-or-search problems are ubiquitous across many domains, including employment, internet search, mate search, and animal foraging. For instance, in patch foraging problems, animals must decide whether to stick with a depleting reward vs search for a new source. The optimal strategy in patch foraging problems, described by the Marginal Value Theorem (MVT; Charnov, 1976), is to leave the depleting patch when the local reward rate within a patch matches the overall long-run reward rate. Many species of animals, ranging from birds to rodents, monkeys, and humans, adhere to this policy in important respects, but tend to overharvest, or stick with the depleting resource too long. Here we attempt to determine the cognitive biases that underlie overharvesting in one of these species (the rat). We characterized rat behavior in response to two basic manipulations in patch foraging tasks: to travel time between patches and depletion rate, and two novel manipulations to the foraging environment: the size of reward and length of delays, and placement of delays (prevs. post-reward). In response to the basic manipulations, rats qualitatively followed predictions of MVT, but stayed in patches for longer than is predicted. In the latter two manipulations, rats deviated from predictions of MVT, exhibiting changes in behavior not predicted by MVT. We formally tested whether four separate cognitive biases – subjective costs, decreasing marginal utility for reward discounting of future reward, and ignoring postreward delays – could explain overharvesting in the former two manipulations and deviations from MVT in the latter two. All of the biases tested explained overharvesting behavior in the former contexts, but only one bias – in which rats ignore post-reward delays – also explained deviations from MVT due to larger rewards with longer delays and due to introduction of a pre-reward delay. Our results show that multiple biases can explain certain aspects of overharvesting behavior, and, while foraging behavior may be the result of the use of multiple biases, inaccurate estimation of postreward delays likely contributes to overharvesting.