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

Abstract

In optimal foraging theory, search time is a key variable defining the value of a prey type. But the sensory-perceptual processes that constrain the search for food have rarely been considered. Here we evaluate the flight behavior 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 from the bees' UV, blue, and green receptors. Targets that made poor color contrast with their backdrop, such as white, UV-reflecting ones, or red flowers, took longest to detect, even though brightness contrast with the background 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 signal for detection. We relate these findings to temporal and spatial limitations of different neuronal channels involved in stimulus detection and recognition. Thus, foraging speed may not be limited only by factors such as prey density, flight energetics, and scramble competition. Our results show that understanding the behavioral ecology of foraging can substantially gain from knowledge about mechanisms of visual information processing.

Figure 1

Color stimuli used in our study. (a) Spectral reflection curves of the artificial flowers and background. (b) Color loci of the stimuli in the color hexagon. The color space inside the central circle (< 0.1 hexagon units) appears achromatic for the bees. Colors for humans: 1, yellow; 2, UV-absorbing white; 3, blue; 4, turquoise; 5, red; 6, UV-reflecting white; 7, lemon.

Figure 2

Search time for detecting blue flowers of various sizes. Same letters indicate no significant differences (Wilcoxon-matched-pairs test); mean ± SE; n = 7; P < 0.05.

Figure 3

Relation between search time and color distance (a) and green contrast (b) for the three different flower sizes. Circles indicate 28-mm flower diameter, triangles indicate 15-mm flower diameter, and diamonds indicate 8-mm flower diameter. Significant correlation is indicated by filled symbols and regression lines (for details see text).

Figure 4

Flight height (a) and flight velocity in the vertical x-y plane (b) while searching for flowers of different flower sizes (Wilcoxon-matched-pairs test; numbers indicate P level). Mean ± SD; n = 7. ns, not significant.

Figure 5

This sketch illustrates the geometry necessary to calculate the radius of the detection area, r, within which a bee flying at a given height, h, will be able to detect a flower with diameter d, given a resolution of α ≥ 5° (α ≥ 15° for the color recognition area; r = radius of the circular detection area, sin α = visual angle subtended by the flower when the bee is situated at point e).