Caution with colour calculations: spectral purity is a poor descriptor of flower colour visibility

Abstract

Background The colours of flowers are of key interest to plant and pollination biologists. An increasing number of studies investigates the importance of saturation of flower colours (often called “spectral purity” or “chroma”) for visibility to pollinators, but the conceptual, physiological and behavioural foundations for these metrics as well as used calculations rest on slender foundations. Methods We discuss the caveats of colour attributes that are derived from human perception, and in particular spectral purity and chroma, as variables in flower colour analysis. We reanalysed seven published datasets encompassing 774 measured reflectance spectra to test for correlations between colour contrast, spectral purity and chroma. Main findings and Conclusions We identify several concerns with common calculation procedures in animal colour spaces. Studies on animal colour vision provide no ground to assume that any pollinator perceives (or responds to) saturation, chroma or spectral purity in the way humans do. A reanalysis of published datasets revealed that values for colour contrast between flowers and their background is highly correlated with measures for spectral purity and chroma, which invalidates treating these factors as independent variables as is currently commonplace. Strikingly, spectral purity and chroma - which both are metrics for saturation and are often used synonymously - are not correlated at all. We conclude that alternative, behaviourally validated metrics for the visibility of flowers to pollinators, such as colour contrast and achromatic contrast, are better in understanding the role of flower colour in plant-pollinator signalling.

Comparative psychophysics of Western honey bee (Apis mellifera) and stingless bee (Tetragonula carbonaria) colour purity and intensity perception

“…Spectral purity is defined as the perceptual distance of a target colour from the uncoloured locus (Chittka and Kevan 2005 ), whereas colour contrast is defined as the perceptual distance between any two colours. Recently, van der Kooi and Spaethe ( 2022 ) pointed to this kind of correlation between values of spectral purity and colour contrast in several studies predominantly of flower colours. In our study, the spectral purity is calculated as the hexagon distance of a colour locus from the locus of the background colour in relation to the hexagon distance between the locus of the background colour and the corresponding spectral locus as proposed by Rhode et al (2013) and the colour contrast is calculated as the hexagon distance between a colour locus and the background locus.…”

Phenotypic Selection on Flower Traits in Food-Deceptive Plant Iris pumila L.: The Role of Pollinators

“…Although the pollinator community probably differs from that in the nature, the presence of potential I. pumila pollinators, such as Bombus, Apis, and small solitary bees, was confirmed [41], and it is likely that this pollinator's diversity may have led to this result. In addition to brightness, different aspects of the floral visual signals, such as green contrast, represent important cues for distant approach and are considered to provide good proxies for flower conspicuousness to pollinators [53,54]. Therefore, this would be an important direction for further investigation.…”

Bird‐flower colour on islands supports the bee‐avoidance hypothesis

“…This function also calculates the chromatic and achromatic contrasts, that is the parameters used to estimate the conspicuousness of different pollinators. The chromatic contrast represents the contrast of colour between the flower and the background; it was calculated in the vision models as the Euclidean distance between the colour loci of flowers and the achromatic centre of the colour space (van der Kooi & Spaethe, 2022). Larger chromatic contrast values indicate more conspicuousness for the specific pollinators to distinguish the flower from the background.…”

“…In the Canarian archipelago, at least six species of generalist passerine birds (Fringillidae, Paridae, and Sylviidae) and five species of lizards (Lacertidae and Gekkonidae) frequently visit flowers for nectar/pollen of at least 15 native plant species from seven families (Appendix S1: Table S1; Ojeda, 2013;Valido et al, 2004;Valido & Olesen, 2010). All these plant species are included in the so-called 'Macaronesian bird-flowered element' (Olesen, 1985) and can share similar floral characteristics such as red-orange flower colour, copious and hexose-rich nectar, and loss of conical cells on the epidermal surface (Dupont et al, 2004;Ojeda, 2013;Ojeda et al, 2016;Valido et al, 2004).…”