Evidence for encounter-conditional, area-restricted search in a preliminary study of Colombian blowgun hunters.

Cody T Ross,Bruce Winterhalder

doi:10.1371/journal.pone.0207633

Abstract

Active search for prey is energetically costly, so understanding how foragers optimize search has been central to foraging theory. Some theoretical work has suggested that foragers of randomly distributed prey should search using Lévy flights, while work on area-restricted and intermittent search strategies has demonstrated that foragers can use the information provided by prey encounters to more effectively adapt search direction and velocity. Previous empirical comparisons of these search modes have tended to rely on distribution-level analyses, due to the difficulty of collecting event-level data on encounters linked to the GPS tracks of foragers. Here we use a preliminary event-level data-set (18.7 hours of encounter-annotated focal follows over 6 trips) to show that two Colombian blowgun hunters use adaptive encounter-conditional heuristics, not non-conditional Lévy flights, when searching for prey. Using a theoretically derived Bayesian model, we estimate changes in turning-angle and search velocity as a function of encounters with prey at lagged time-steps, and find that: 1) hunters increase average turning-angle in response to encounters, producing a more tortuous search of patches of higher prey density, but adopt more efficient uni-directional, inter-patch movement after failing to encounter prey over a sufficient period of time; and, 2) hunters reduce search velocity in response to encounters, causing them to spend more of their search time in patches with demonstrably higher prey density. These results illustrate the importance of using event-level data to contrast encounter-conditional, area-restricted search and Lévy flights in explaining the search behavior of humans and other organisms.

Highlights

Active search for prey is energetically costly [1], so understanding how foragers optimize random search has been central to foraging theory [2,3,4]
Previous theoretical work has shown that Levy flights can be efficient methods of search for random and not necessarily uniformly distributed prey items
This result is consistent with the idea that they use adaptive heuristics rather than non-conditional Levy flights to guide their movement

Summary

Introduction

Active search for prey is energetically costly [1], so understanding how foragers optimize random search has been central to foraging theory [2,3,4]. Evidence for encounter-conditional, area-restricted search in a preliminary study of Colombian blowgun hunters frequently more efficient than Brownian movement at encountering sparse, randomly distributed prey. Some studies have successfully tested this hypothesis in humans (e.g., [8, 9]) and nonhuman animals (e.g., [10, 11]), while others have either failed to find evidence of Levy movement [12] or documented different patterns of search in humans [13, 14] and other organisms [11, 15]. Issues with measurement have sometimes complicated tests of the underlying ideas [11, 16]

Methods

Results

Conclusion