Individual colour patches as multicomponent signals

Abstract

Colour patches are complex traits, the components of which may evolve independently through a variety of mechanisms. Although usually treated as simple, two-dimensional characters and classified as either structural or pigmentary, in reality colour patches are complicated, three-dimensional structures that often contain multiple pigment types and structural features. The basic dermal chromatophore unit of fishes, reptiles and amphibians consists of three contiguous cell layers. Xanthophores and erythrophores in the outermost layer contain carotenoid and pteridine pigments that absorb short-wave light ; iridophores in the middle layer contain crystalline platelets that reflect light back through the xanthophores ; and melanophores in the basal layer contain melanins that absorb light across the spectrum. Changes in any one component of a chromatophore unit can drastically alter the reflectance spectrum produced, and for any given adaptive outcome (e.g. an increase in visibility), there may be multiple biochemical or cellular routes that evolution could take, allowing for divergent responses by different populations or species to similar selection regimes. All of the mechanisms of signal evolution that previously have been applied to single ornaments (including whole colour patches) could potentially be applied to the individual components of colour patches. To reach a complete understanding of colour patch evolution, however, it may be necessary to take an explicitly multi-trait approach. Here, we review multiple trait evolution theory and the basic mechanisms of colour production in fishes, reptiles and amphibians, and use a combination of computer simulations and empirical examples to show how multiple trait evolution theory can be applied to the components of single colour patches. This integrative perspective on animal colouration opens up a host of new questions and hypotheses. We offer specific, testable functional hypotheses for the most common pigmentary (carotenoid, pteridine and melanin) and structural components of vertebrate colour patches.

Countergradient Variation in the Sexual Coloration of Guppies (Poecilia Reticulata): Drosopterin Synthesis Balances Carotenoid Availability

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Pterin pigments amplify iridescent ultraviolet signal in males of the orange sulphur butterfly,Colias eurytheme

“…Here we suggest that the combination of pigmentary and structural mechanisms on incident light reflections warrant close attention, because this interplay may be crucially important for the spectral composition of a signal and the information it may convey. Despite being mentioned recently in the literature (Grether et al 2004;Vukusic et al 2004) this idea has only just begun to be addressed empirically (Shawkey & Hill 2005).…”

A protean palette: colour materials and mixing in birds and butterflies

“…However, recent work has highlighted the importance of understanding both the scattering and absorptive properties of coloured traits (Grether et al 2004;Vukusic et al 2004;Rutowski et al 2005;Shawkey & Hill 2005;Morehouse et al 2007) because variations in either scattering efficiency and/or concentration of pigment molecules can result in distinct optical effects.…”

“…First, theoretical modelling of the optical properties of coloured animal tissues has approximated a number of systems with success (reviewed in Grether et al 2004;Prum 2006), suggesting that current theory adequately describes many basic optical mechanisms responsible for colour production. However, in many cases, explicit incorporation of the pigmentary components of coloured tissues (by inclusion of changes in absorbance and RI associated with pigmentation) should lead to better model fitting.…”

“…Some work has already looked at variation due to changes in the morphology of colour producing arrays (Shawkey et al 2003;Kemp et al 2006). However, variation in colour patterns due to changes in pigment deposition and associated shifts in RI and absorbance remain mostly unexplored (Grether et al 2004;Shawkey & Hill 2005;Morehouse et al 2007). Further work in this area promises to inform both empirical and theoretical understandings of the optical parameters important to variation in colour traits.…”