Birds Perceive More Intraspecific Color Variation in Bird-Pollinated Than Bee-Pollinated Flowers.

Kenneth D Whitney,Charles F Williams,Thomas E White,Asher K Smith

doi:10.3389/fpls.2020.590347

Abstract

Pollinator-mediated selection is expected to constrain floral color variation within plant populations. Here, we test for patterns of constraint on floral color variation in 38 bee- and/or hummingbird-pollinated plant species from Colorado, United States. We collected reflectance spectra for at least 15 individuals in each of 1–3 populations of each species (total 78 populations) and modeled perceived color variation in both bee and bird visual spaces. We hypothesized that bees would perceive less intraspecific color variation in bee-pollinated species (vs. bird-pollinated species), and reciprocally, birds would perceive less color variation in bird-pollinated species (vs. bee-pollinated species). In keeping with the higher dimensionality of the bird visual system, birds typically perceived much more color variation than bees, regardless of plant pollination system. Contrary to our hypothesis, bees perceived equal color variation within plant species from the two pollination systems, and birds perceived more color variation in species that they pollinate than in bee-pollinated species. We propose hypotheses to account for the results, including reduced long-wavelength sensitivity in bees (vs. birds), and the ideas that potential categorical color vision in birds and larger cognitive capacities of birds (vs. bees) reduces their potential discrimination against floral color variants in species that they pollinate, resulting in less stabilizing selection on color within bird-pollinated vs. bee-pollinated species.

Highlights

Among other traits such as scent and size, flower color is a major signal used by pollinators to identify and choose their host plants (Fenster et al, 2004; Dyer et al, 2012; Schiestl and Johnson, 2013)
For >70% of populations, >95% of pairwise flower–flower comparisons were indistinguishable to bees, consistent with a history of stabilizing selection on flower color mediated by the bee visual system (Paine et al, 2019). These pairs of conspecific flowers were typically visually distinct to humans and birds. These findings suggest that human-perceived floral color variation within populations might persist because it is effectively invisible to pollinators
We examined 24 bee-pollinated species, seven birdpollinated species, and seven species with mixed pollination systems; for reflectance spectra, see Supplementary Appendix Figure S1

Summary

Introduction

Among other traits such as scent and size, flower color is a major signal used by pollinators to identify and choose their host plants (Fenster et al, 2004; Dyer et al, 2012; Schiestl and Johnson, 2013). When flower color variants arise via mutation within a plant population, they should be frequently selected against, as pollinators can exhibit positive frequency dependence in their floral color choices (Smithson, 2001; Eckhart et al, 2006). Central to testing hypotheses about constraints on flower color is the idea of discrimination thresholds within pollinator visual spaces. Greater distance between a pair of colors predicts greater discriminability, but all organisms have thresholds below which discrimination is not possible (e.g., Wyszecki and Stiles, 2000; Dyer and Chittka, 2004; Olsson et al, 2018). Bee and Bird Perception of Intrapopulation Floral Color Variation To address our focal question, we compared pairwise distances in color space to the relevant discrimination threshold, as delineated above (see Paine et al, 2019). The fraction of these intrapopulation comparisons that are discriminable to a given viewer we call the ‘fraction discriminable.’ We tabulated comparisons using a custom R script (R Core Team, 2020)

Methods

Results

Conclusion