Color contrast and stability as key elements for effective warning signals

Abstract

Vivid warning signals (aposematism) have evolved repeatedly throughout the animal kingdom. However, relatively few studies consider what makes an effective signal, in terms of preventing attack and promoting avoidance learning by predators. Signal form varies substantially among and sometimes within species, but there has also been apparent convergence on relatively few main color types. We aimed to determine why warning signals often combine red, orange, yellow, and black colors, and specifically to determine whether these colors provide highly salient and reliable visual signals under a range of environmental conditions. Using digital image analysis, we modeled ladybird (ladybug) coloration to an avian visual system. We calculated the contrast of several different ladybird species against an average green background, based on predicted opponent color channel responses in bird vision. Our results suggest that longwave colors (i.e., red, orange) are more contrasting than colors such as blue, against green natural backgrounds. Moreover, these colors yield relatively unchanging (stable) signals throughout the day and under different weather conditions. These analyses show how aposematic signals have evolved under selection to be more effective by being more conspicuous and reliable to the visual system of their potential avian predators.

Image calibration and analysis toolbox – a free software suite for objectively measuring reflectance, colour and pattern

“…Illuminant colour can be measured from the relative ratios of the linear RGB values, or, if exposure time is controlled for the intensities between photographs can also be measured (see Lovell et al . and Arenas, Troscianko & Stevens ).…”

No evidence of quantitative signal honesty across species of aposematic burnet moths (Lepidoptera: Zygaenidae)

“…Field studies with model frogs and ladybirds have shown that chromatic contrast to the natural background is particularly important (Arenas et al., ; Hegna, Saporito, Gerow, & Donnelly, ), while experiments presenting different species of Lycaeidae seed bug larvae to domestic chicks suggest that prey with redder and more saturated signals are more strongly avoided (Gamberale‐Stille & Tullberg, ). Long‐wavelength colours are also thought to be more effective as warning signals, due to innate avoidance by some predators and their greater stability under different lighting conditions (Arenas, Troscianko, & Stevens, ). Finally, experiments with artificial stimuli and natural prey items such as Arctia plantaginis (wood tiger moth) larvae suggest that larger coloured markings generate greater avoidance (Forsman & Merilaita, ; Lindstedt, Lindström, & Mappes, ; Lindström, Alatalo, Mappes, Riipi, & Vertainen, ; Smith, Halpin, & Rowe, ).…”

Yellow and the Novel Aposematic Signal, Red, Protect Delias Butterflies from Predators

“…Higher signal stability may be due to the fact that both red and yellow are colours with longer wavelengths and thus, are less susceptible to scattering by atmospheric particles. As such, they are perceived as more uniform and stable colours across the time of day [51]. Atmospheric particles and cloud cover tend to scatter shorter wavelengths through the process of Rayleigh scattering, thus negatively affecting the stability of a signal [51].…”

“…As such, they are perceived as more uniform and stable colours across the time of day [51]. Atmospheric particles and cloud cover tend to scatter shorter wavelengths through the process of Rayleigh scattering, thus negatively affecting the stability of a signal [51]. In addition, signal contrast against the natural habitat of the prey, which tends to be green vegetation, is theorized to contribute to aposematic signal effectiveness [52].…”

“…For instance, when ladybird colouration was modelled to an avian visual system and the contrast of the ladybird’s colouration was measured against an average green background, red colours remained extremely stable throughout the day, unaffected by the varying illuminant spectra caused by differences in time of day and atmospheric conditions. Although yellow colours were found to not be as salient as red over varying light conditions, contrast values of yellow stimuli studied in the aforementioned study still revealed yellow to be a contrasting colour against natural backgrounds [51]. …”