Abstract
ABSTRACTAvian eggshells are variedly coloured, yet only two pigments, biliverdin and protoporphyrin IX, are known to contribute to the dramatic diversity of their colours. By contrast, the contributions of structural or other chemical components of the eggshell are poorly understood. For example, unpigmented eggshells, which appear white to the human eye, vary in their ultraviolet (UV) reflectance, which may be detectable by birds. We investigated the proximate mechanisms for the variation in UV-reflectance of unpigmented bird eggshells using spectrophotometry, electron microscopy, chemical analyses, and experimental manipulations. We specifically tested how UV-reflectance is affected by the eggshell cuticle, the outermost layer of most avian eggshells. The chemical dissolution of the outer eggshell layers, including the cuticle, increased UV-reflectance for only eggshells that contained a cuticle. Our findings demonstrate that the outer eggshell layers, including the cuticle, absorb UV-light, probably because they contain higher levels of organic components and other chemicals, such as calcium phosphates, compared to the predominantly calcite-based eggshell matrix. These data highlight the need to examine factors other than the known pigments in studies of avian eggshell colour.
Highlights
Understanding the proximate causes of variation in morphological traits like colour is critical to understanding their functions and evolution (Hill and McGraw, 2006)
Ultra High Performance Liquid Chromatography (UHPLC) and Mass Spectrophotometry (MS) confirmed that none of the eggshells of the four species contained any detectable concentrations of protoporphyrin or biliverdin, whereas these pigments were detected in our positive controls (Fig. S1)
Despite the absence of known eggshell pigments, we found differences in the UV-reflectance of the four species’ eggshells
Summary
Understanding the proximate causes of variation in morphological traits like colour is critical to understanding their functions and evolution (Hill and McGraw, 2006). Eggshell coloration may serve several roles, including camouflage (Merilaita and Lind, 2005), sexual selection (Moreno and Osorno, 2003) or host-parasite egg mimicry and rejection (Yang et al, 2013). A recent study further suggested that colour produced by pigments modulates the amount of beneficial vs harmful UV-light reaching the embryo by acting as an absorbing barrier (Maurer et al, 2015). Many eggshells lack pigmentation (Hauber, 2014) and the mechanism by which they attenuate ultraviolet light is unknown (Kilner, 2006). Studying the proximate basis of egg coloration may help provide inspiration for applied systems, including the development of biomimetic materials by identifying important factors that contribute to light modulation (Yoo et al, 2009; Li et al, 2010).
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