An affordable method to measure animal-background contrast using digital images

Abstract

Current color metrics commonly used to study the animal-substrate contrast, require researchers perform the analyses under controlled conditions of light. We argue that sometimes this approach can be suboptimal because it does not use the same light source as natural habitats, ignores the proportion of the different colors of the pattern, and ignores the behavioral repertoire displayed by animals when they are signaling. Here we present an affordable method to quantify animal-background contrast using photos taken under natural conditions. The method combines the use of free software to extract the color information and calculate the Color Overlapping Index (COI), which represent the degree of overlap between the color classes displayed by the animal and substrate. We showed that COI accurately reflects the degree of animal-substrate contrast seen in an image and that COI is a robust metric, yielding consistent results even for images with low resolution or with the un-calibrated color balance.