A delayed response in area-concentrated search can improve foraging success.

Background Area-concentrated search (ACS) is a simple movement rule implying that an animal searches for resources using a 'state-dependent correlated random walk'. Accordingly, a forager increases its searching intensity by reducing the directionality of movement ('intensive search mode' or ISM) when it detects a resource item, but if it searches unsuccessfully for a while, it returns to a more straight-line movement to search for new resource locations elsewhere ('extensive search mode' or ESM). In this study, we propose a modified ACS, called delayed-response ACS (dACS), which would be more efficient in resource collecting than the standard ACS. Methods Instead of immediately switching from ESM to ISM upon encountering a resource as done in the standard ACS, an individual foraging in the dACS mode delays this switch for x steps (the delay parameter) so that it continues moving in a straight line for a while before switching to ISM. We simulated the movement paths of an individual with ACS compared with dACS in a continuous landscape with a single resource cluster and varied two different movement parameters, that is x and the half-saturation constant h that affects how quickly the forager returns to straight line movement, and two landscape parameters, i.e., cluster size and resource density. Results Our results show that an individual with a suitable delay parameter x for the dACS achieves substantially higher foraging success than an individual with standard ACS (x=0). Optimal foraging success occurs when x is approximately similar to the patch radius r. This is because, with dACS, an individual can penetrate deeper into a cluster and stay longer within it, ultimately increasing the number of resources collected. Modifying the half-saturation constant h also affects foraging success but effects depend on resource density and cluster size. Generally, h modulates the optimal x value only slightly. Conclusions An individual following a dACS with an adequate delay x can collect more resources from a patch than one using standard ACS; this hold in particular if patches are small or resource density is low. dACS can be interpreted as a survey movement within a resource cluster before switching from ESM to ISM. The dACS rule does not rely on complex spatial memory but only on memorizing whether resources were found or not. It may thus occur also in a wide range of taxa, from organisms without a central nervous system to animals with complex brain systems.