The Marginal Value Theorem in Caenorhabditis elegans

The Marginal Value Theorem (MVT) is an important part of Optimal Foraging Theory, predicting the optimal time to leave a food patch. It has been mostly studied in birds, insects and mammals, even though simpler organisms also need to forage efficiently in patchy environments. Here we test whether the nematode Caenorhabditis elegans implements the MVT. We recorded individual nematodes exploring patchy environments, across four inter-patch distances and three different food qualities, and found that C. elegans behavior matches MVT predictions: When food patches are further away, each food patch is exploited for a longer time. In previous studies animals achieved this by modulating the duration of visits to food patches. Similarly, we found that C. elegans also increases visit duration with inter-patch distance, but this only accounts for half of the increase in total exploitation time. The other half of the increase comes from C. elegans revisiting food patches multiple times, and the number of these revisits increasing with inter-patch distance. This increase in the number of revisits is not due to behavioral changes in response to distance, but rather to a passive interaction between trajectories and environment geometry. These results show that C. elegans can learn the statistics of an environment and use this information in a way consistent with the MVT, but also that part of the fitness-relevant outcomes can emerge passively.

Despite being key in understanding foraging in patchy resources, the Marginal Value Theorem (MVT) has been tested almost exclusively in relatively complex animals. We extensively tested the MVT in a simple, non-visual organism, showing that Caenorhabditis elegans increases patch exploitation time when inter-patch distance increases. This effect is partially driven by the same behavioral adaptation found in complex animals, but also by an increase in the number of patch revisits. This second driver, which had not been reported before and is probably key for non-visual organisms, requires no behavioral adaptation and produces around half of the fitness-relevant outcome. Our results highlight the need for adapting Optimal Foraging Theory to a wide range of taxa spanning from microbes to small invertebrates.