Protoporphyrin and Eggshell Strength: Preliminary Findings from a Passerine Bird

Abstract

Protoporphyrin pigmentation is a common feature of passerine eggshells worldwide, typically giving red to brown coloration. Although these may form a background colour to the egg, protoporphyrin more commonly forms speckles, blotches or patches of colour (maculation) against a white (unpigmented) or blue/green (biliverdin-pigmented) background. Conventional explanations for the presence of such pigmentation have focused on the eggs’ appearance, and include crypsis and strategies to deter brood parasites, such as cuckoos (Cuculidae). More recently, evidence has emerged that there may also be a sexually-selected visual function. However, the pattern of occurrence across taxa and upon and within the eggshell, suggest that non-visual functions may be at least as important, or even the principal function of some pigmentation. In particular, correlations between protoporphyrin maculation, eggshell thickness, rates of water-loss and local calcium availability in the Great Tit (Parus major), suggest that the pigment could serve a structural function. Despite strong circumstantial evidence, however, a direct relationship between eggshell strength and protoporphyrin pigmentation has not previously been available. Here, we present preliminary findings from a comparison of fracture toughness (resistance to fracture) and brittleness of pigmented and unpigmented shell from the same Great Tit eggshells. These suggest that while pigmented shell is as brittle as unpigmented shell, the fracture toughness is significantly greater and more than compensates for the reduced shell thickness observed at a pigment spot. The data also suggest that although the first two principal components of pigmentation influence fracture toughness, neither influences how the material responded to the propagation of a crack through its surface. We suggest that pigmentation provides structural overcompensation for shell thinning caused, for example, by a deficit of dietary calcium, and the reasons for this are discussed.