The protoporphyrin content of Kentish Plover Charadrius alexandrinus eggshells is better predicted by the fractal dimension of spottiness than by colour

Abstract

Quantifying the pigment content in avian eggshells is important for the validation of hypotheses on the functionality of eggshell coloration. The few studies that have analysed whether eggshell coloration and spottiness are related to pigment content have found contradictory results. In this study, we analysed whether the coloration and the degree of spottiness of Kentish Plover Charadrius alexandrinus eggshells are related to pigment concentrations (protoporphyrin and biliverdin). We also examined whether the concentrations of these pigments are related to the fractal dimension (FD) of spottiness. The FD of spottiness may be useful as a simple standardized method to quantify complex patterning. We found that protoporphyrin was more abundant than biliverdin in eggshells. Contrary to expectations, the content of protoporphyrin was not related to eggshell colour, probably due to a different allocation of pigments between the spots and the background of the eggshell, and/or to the different allocation of pigments among eggshell layers. However, we found a positive relationship between the FD of eggshell spottiness and the amount of protoporphyrin. It is likely that the FD of spottiness (indicative not only of spot size and the degree of spottiness, but also of how convoluted the outlines of spots are and how spots are distributed across an eggshell) may be related to the mechanical function of protoporphyrin (e.g. in strengthening eggshells, which may interfere with the fractal structure of potential fractures), and/or it may also improve egg camouflage. However, more studies in other avian species are needed to evaluate if the FD of spottiness is a good surrogate for protoporphyrin content in eggshells.