Nahua mushroom gatherers use area-restricted search strategies that conform to marginal value theorem predictions

Significance Using GPS trackers, we followed Nahua mushroom collectors to study how experienced foragers efficiently search for resources. Locating patchily distributed resources while reducing traveling costs requires a forager to balance time spent searching for new patches and searching within known patches. We present an analytical model of foraging behavior in which foragers can detect and change the way they search by adjusting direction and speed as a function of time since last resource encounter. This model can be used to test theoretical predictions concerning how long foragers should remain inside a given patch. Elucidating how organisms efficiently search is of widespread interest across academic disciplines: the potential applications of the method explained here range from conservation biology to robotic design.
We develop a method of analysis for testing the marginal value theorem (MVT) in natural settings that does not require an independent definition or mapping of patches. We draw on recent theoretical work on area-restricted search (ARS) that links turning-angle and step-size changes to geographically localized encounter-rates. These models allow us to estimate “giving-up times” using encounter-annotated GPS tracking data. Applied to a case study of Nahua mushroom foragers, these models identify distinct forms of intrapatch and interpatch search behavior, with intrapatch search transitioning to interpatch search after a predictable interval of time since the last encounter with a harvested mushroom. Our empirical estimate of giving-up time coincides with the theoretically optimal giving-up time derived under the MVT in the same environment. The MVT is currently underused in studies of human foraging and settlement patterns, due in large part to the difficulty of identifying discrete resource patches and quantifying their characteristics. Our methods mitigate the need to make such discrete maps of patches and thus have the potential to broaden the scope for empirical evaluations of the MVT and related theory in humans. Beyond studies of naturalistic foraging in humans and other animals, our approach has implications for optimization of search behavior in a range of applied fields where search dynamics must be adapted to shifting patterns of environmental heterogeneity affecting prey density and patchiness.