Accept–reject decision-making revealed via a quantitative and ethological study of C. elegans foraging

Decision-making is a ubiquitous component of animal behavior that is often studied in the context of foraging. Foragers make a series of decisions while locating food (food search), choosing between food types (diet or patch choice), and allocating time spent within patches of food (patch-leaving). Here, we introduce a framework for investigating foraging decisions using detailed analysis of individual behavior and quantitative modeling in the nematode <i>Caenorhabditis elegans</i>. We demonstrate that <i>C. elegans</i> make <i>accept-reject</i> patch choice decisions upon encounter with food. Specifically, we show that when foraging among small, dispersed, and dilute patches of bacteria, <i>C. elegans</i> initially <i>reject</i> several bacterial patches, opting to prioritize exploration of the environment, before switching to a more exploitatory foraging strategy during subsequent encounters. Observed across a range of bacterial patch densities, sizes, and distributions, we use a quantitative model to show that this decision to <i>explore</i> or <i>exploit</i> is guided by available sensory information, internal satiety signals, and learned environmental statistics related to the bacterial density of recently encountered and <i>exploited</i> patches. We behaviorally validated model predictions on animals that had been food-deprived, animals foraging in environments with multiple patch densities, and null mutants with defective sensory modalities. Broadly, we present a framework to study ecologically relevant foraging decisions that could guide future investigations into the cellular and molecular mechanisms underlying decision-making.