Using spectra and visual modeling to study animal coloration

Abstract

Animal coloration has many adaptive functions and plays an important role in signal communication both among intra- and interspecies. For example, it has been widely used in mate choice, intrasexual competition, and as aposematic or cryptic coloration in predator avoidance. Many colors and pigments also function in microbial resistance, structural support, photoprotection, and thermoregulation. Differing from human vision, based on RGB system, many other animals have tetrachromatic vision system, which includes the ultraviolet (UV) range that is undetectable by human eyes. Previous studies showed that ultraviolet is important in some species' social signaling and communication. Moreover, cone inner segments of most classes of vertebrate contain an oil droplet, which acts as a cut-off filter absorbing wavelengths below a critical value, and transmitting longer wavelengths. Animal and human vision is significantly different in that the classification of color by human standards may be a misleading for measuring animal coloration. Here, we illuminate how to use fiber spectrophotometer to quantify animal coloration, and analyze it by spectra analysis and visual modeling. As an example, we obtained plumage reflectance spectra from male and female scarlet minivets (Pericrocotus flammeus). This bird species is sexually dimorphic that the males have plumage color in black and red, while the females have grey and yellow accordingly. These plumage colors are typically generated from melanin and carotenoid pigments, which have an effect on antioxidant activity. Analysis of spectra segments provides hue, chroma, brightness and relative brightness of each wave range. Visual modeling maps color patches on tetrahedral color space and Robinson projection, meanwhile, calculates color span and color space volume which describe the color contrast and color diversity, respectively. In visual modeling, ambient light irradiance and spectral sensitivity of animal retinas are included, which provides an objective evaluation of coloration of animal vision.