Visual constraints in foraging bumblebees: Flower size and color affect search time and flight behavior

In optimal foraging theory, search time is a key variable defining the valu e of a prey type. But the sensory-perceptual processes that constrain the s earch for food have rarely been considered. Here we evaluate the flight beh avior of bumblebees (Bombus terrestris) searching for artificial flowers of various sizes and colors. When flowers were large, search times correlated well with the color contrast of the targets with their green foliage-type background, as predicted by a model of color opponent coding using inputs f rom the bees' UV, blue, and green receptors. Targets that made poor color c ontrast with their backdrop, such as white, UV-reflecting ones, or red flow ers, took longest to detect, even though brightness contrast with the backg round was pronounced. When searching for small targets, bees changed their strategy in several ways. They flew significantly slower and closer to the ground, so increasing the minimum detectable area subtended by an object on the ground. In addition, they used a different neuronal channel for flower detection. Instead of color contrast, they used only the green receptor si gnal for detection. We relate these findings to temporal and spatial limita tions of different neuronal channels involved in stimulus detection and rec ognition, Thus, foraging speed may not be limited only by factors such as p rey density, flight energetics, and scramble competition. Our results show that understanding the behavioral ecology of foraging can substantially gai n from knowledge about mechanisms of visual information processing.