Achromatic Cues Are Important for Flower Visibility to Hawkmoths and Other Insects

Abstract

Studies on animal colour vision typically focus on the chromatic aspect of colour, which is related to the spectral distribution, and disregard the achromatic aspect, which is related to the intensity (“brightness”) of a stimulus. Although the chromatic component of vision is often most reliable for object recognition because it is fairly context independent, the achromatic component may provide a reliable signal under specific conditions, for example at night when light intensity is low. Here we make a case for the importance of achromatic cues in plant-pollinator signalling, based on experimental data on naïve Deilephila elpenor and Macroglossum stellatarum hawkmoths, optical modelling and synthesising published experiments on bees, flies, butterflies and moths. Our experiments show that in ecologically relevant light levels hawkmoths express a strong preference for brighter stimuli. Published experiments suggest that for flower-visiting bees, butterflies, moths and flies, achromatic cues may be more important for object detection than often considered. Our optical modelling enabled disentangling the contribution of pigments and scattering structures to the flower’s achromatic contrast, and illustrates how flower anatomy and background are important mediating factors. We discuss our findings in the context of the often-assumed dichotomy between detection and discrimination, chromatic versus achromatic vision, and the evolution of floral visual signals.

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

“…Achromatic contrast is another useful proxy for visibility of flowers by pollinators, particularly from long distances, and is straightforward to calculate (e.g. Spaethe et al , 2001 ; van der Kooi and Kelber, 2022 ).…”

“…The spectral differences between a flower and its surrounding are largely caused by differences in absorption by pigments. Achromatic contrast, also known as green contrast, is a (colour-blind) contrast between stimulus and background, which is perceived by bees and presumably other insects by the long-wavelength (green) photoreceptors ( Giurfa et al ., 1996 , 1997 ; Spaethe et al , 2001 ; Goyret and Kelber, 2012 ; for a review, see van der Kooi and Kelber, 2022 ). Achromatic contrast is caused by both differences in absorption by pigments as well as light-scattering structures ( Chittka et al , 1994 ; Kevan et al , 1996 ; van der Kooi and Kelber, 2022 ).…”

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

“…A second main research question considered in the current study was whether either species of bees tested might prefer stimulus intensity when colour hue and spectral purity was controlled. This question is important as several recent ecology studies on flower signalling have suggested that in some cases the brightness or intensity of a flower may be a factor in attracting pollinators (Hopkins and Rausher 2012 ; Renoult et al 2014 ; Sletvold et al 2016 ; van der Kooi and Kelber 2022 ). In honey bees we thus tested hypothesis (iii) and the results show that variations in stimulus intensity do significantly influence preferences when bees were conditioned towards the most intense colour (Fig.…”

“…In general, intensity is assumed to play a minor role in colour choice by bees (Daumer 1956 ; Backhaus 1991 ; Spaethe et al 2001 ; Reser et al 2012 ; Ng et al 2018 ; van der Kooi et al 2019 ). A recent study by van der Kooi and Kelber ( 2022 ) has shown that for nocturnal and diurnal hawkmoths the intensity of target stimuli plays a role for its visibility. Intensity has been considered as a potentially important factor for flower evolution (Hopkins and Rausher 2012 ; Renoult et al 2014 ; Sletvold et al 2016 ), and an experimental approach by Hempel de Ibarra et al ( 2000 ) demonstrated that high brightness contrast between a stimulus and its background can impact the choice behaviour of bees.…”

High diversity of arthropod colour vision: from genes to ecology

“…[9]), which is still an important field (e.g. [105–107]). In addition, many other object classes are now known to be detected and recognized by arthropods using colour.…”