Abstract
Blue is a favored color of many humans. While blue skies and oceans are a common visual experience, this color is less frequently observed in flowers. We first review how blue has been important in human culture, and thus how our perception of blue has likely influenced the way of scientifically evaluating signals produced in nature, including approaches as disparate as Goethe’s Farbenlehre, Linneaus’ plant taxonomy, and current studies of plant-pollinator networks. We discuss the fact that most animals, however, have different vision to humans; for example, bee pollinators have trichromatic vision based on UV-, Blue-, and Green-sensitive photoreceptors with innate preferences for predominantly short-wavelength reflecting colors, including what we perceive as blue. The subsequent evolution of blue flowers may be driven by increased competition for pollinators, both because of a harsher environment (as at high altitude) or from high diversity and density of flowering plants (as in nutrient-rich meadows). The adaptive value of blue flowers should also be reinforced by nutrient richness or other factors, abiotic and biotic, that may reduce extra costs of blue-pigments synthesis. We thus provide new perspectives emphasizing that, while humans view blue as a less frequently evolved color in nature, to understand signaling, it is essential to employ models of biologically relevant observers. By doing so, we conclude that short wavelength reflecting blue flowers are indeed frequent in nature when considering the color vision and preferences of bees.
Highlights
Why make so much of fragmentary blue In here and there a bird, or butterfly, Or flower, or wearing-stone, or open eye, When heaven presents in sheets the solid hue?Since earth is earth, perhaps, not heaven– Though some savants make earth include the sky; And blue so far above us comes so high, It only gives our wish for blue a whet. (Frost, 1920)Blue Flower ColorWhen we as human observers use our color vision to document the natural world, we need to be cognizant of the limitations and biases of our perception
Much work is required to understand what factors might lead to changes in flower color in different environments, and hopefully this review on fragmentary blue serves as a useful tool for bridging our understanding between how plants science and botany has classically thought of blue flower colors, and what the perception of such colors likely means for biologically relevant pollinators like bees
Through history humans have been attracted by the blue color which has been used to decorate items of economic or ritual value and inspired poets and artists
Summary
Why make so much of fragmentary blue In here and there a bird, or butterfly, Or flower, or wearing-stone, or open eye, When heaven presents in sheets the solid hue?Since earth is earth, perhaps, not heaven (as yet)– Though some savants make earth include the sky; And blue so far above us comes so high, It only gives our wish for blue a whet. (Frost, 1920)Blue Flower ColorWhen we as human observers use our color vision to document the natural world, we need to be cognizant of the limitations and biases of our perception. The difference in color visual systems among pollinators (see Figure 7) can influence which plant species may be successful in different environments, and such effects can be dynamic and depend upon the variety of flower visitors in an environment (Shrestha et al, 2013; Camargo et al, 2019), and how distributions may change due to factors like climate (Hegland et al, 2009; Shrestha et al, 2018).
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