Assessing the ecological significance of bee visual detection and colour discrimination on the evolution of flower colours

Bee pollinators interact with flowers in a complex signal-receiver system. Chromatic traits that allow reliable discrimination between rewarding and non-rewarding flowers have been proposed as an important feature of pollination syndromes: bee-pollinated flowers have spectral profiles that closely match the discrimination peaks of their pollinators across the visual spectrum. However, in the complexity of a natural environment, it may be hard for bees to even detect the presence of flowers. In particular, little is known about how discrimination and detection by bees may together contribute to pollinator-mediated selection on floral colour signals. We address here an unexplained feature of floral colour evolution: the extreme paucity of spectral patterns with pronounced changes in reflectance around 420–480 nm wavelength. We began by conducting experiments with honeybees in a Y-maze to determine their capacity to detect a stimulus rarely found in bee-pollinated flowers—one with a single sharp spectral reflectance change at 478 nm. We found bees to be poor at detecting this stimulus against a neutral background. We then conducted behaviourally-informed computer simulations that test how bee visual discrimination and detection interact, which yielded information about which flower colours most effectively facilitate cross-pollination. Finally, we identified from our previous work those bird-pollinated species whose floral colours had spectral characteristics similar to the stimulus used in the Y-maze experiment. These data demonstrate that plants can, and do, produce such spectra for pollinators other than bees. In combination, our results show that the interaction between colour discrimination and detection is important for understanding flower community assembly.