Evaluating the spectral discrimination capabilities of different pollinators and their effect on the evolution of flower colors

Abstract

Important plant pollinators like bees and birds have very different color visual systems. Previous work has attempted to relate flower syndromes to the respective visual capabilities of the most important pollinators, but has often been limited by the lack of robust means to make between-species comparisons of how flower color signals are processed. In a recent study we solved this dilemma by comparing the raw spectral signals, quantifiable by major inflection points on a wavelength scale, from different flowers whose pollinators were known from direct observation. Here we elaborate on how this method allows robust cross species comparisons that are independent of the requirement to know the complex and often inaccessible physiological data about color processing in different animals. The use of this method should thus allow for the testing of pollinator syndrome hypotheses for different animal pollinators from different regions of the world.

Keywords: discrimination; evolution; flower color; vision.

Figure 1. Important hymenopteran pollinators are trichromatic and have a color visual system with UV-sensitive (maximum sensitivity about 350 nm), blue-sensitive (maximum sensitivity about 440 nm) and green-sensitive (maximum sensitivity about 540 nm) photoreceptors, while human trichromatic vision perceives longer-wavelength radiation (as indicated by the horizontal visible light spectral bar above the graph). The black line plots show sensitivity of the three photoreceptors in honeybees (dotted line) and bumblebees (solid line) normalized to a maximum of 1.0. Because of color-opponent processing mechanisms, the region of the spectrum where color discrimination is best is between overlapping photoreceptors, and the purple line shows an inverted Δλ/λ function that was empirically measured for the honeybee.^, This allows for key critical regions of the spectrum (e.g., about 400 and 500 nm for hymenopteran pollinators) to be identified for comparison with inflection points of floral reflectance curves.