Abstract
The study of animal colouration addresses fundamental and applied aspects relevant to a wide range of fields, including behavioural ecology, environmental adaptation and visual ecology. Although a variety of methods are available to measure animal colours, only few focus on chromatophores (specialized cells containing pigments) and pigment migration. Here, we illustrate a freely available and user-friendly method to quantify pigment cover (PiC) with high precision and low effort using digital images, where the foreground (i.e. pigments in chromatophores) can be detected and separated from the background. Images of the brown shrimp, Crangon crangon, were used to compare PiC with the traditional Chromatophore Index (CI). Results indicate that PiC outcompetes CI for pigment detection and transparency measures in terms of speed, accuracy and precision. The proposed methodology provides researchers with a useful tool to answer essential physiological, behavioural and evolutionary questions on animal colouration in a wide range of species.
Highlights
The study of animal colouration and colour patterns is essential to gather a better understanding on how animals visually communicate and how they can match different substrates
Animal colouration can be assessed by determining pigment dispersion in individual chromatophores or in multicellular chromatosomes (e.g. [19, 21])
Methods relying on the measurement of the diameter of the chromatosomes [19, 56] have the same problem, since they omit morphological variation [22]
Summary
The study of animal colouration and colour patterns is essential to gather a better understanding on how animals visually communicate and how they can match different substrates. Hogben and Slome [20] described changes in the pigment distribution in the frog Xenopus laevis by classifying chromatophores in five classes (Supplementary Fig. S1), applying a Melanophore Index (MI) for melanophores ( more generally called Chromatophore Index (CI) for chromatophores containing pigments other than melanin [20, 21]). This method has been extensively used (see Table 1 for some recent examples), concerns have been raised about its Received: 27 September 2016; Revised: 15 February 2017; Editorial decision: 17 February 2017; Accepted: 2 March 2017
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