Eggshells as hosts of bacterial communities: an experimental test of the antimicrobial egg coloration hypothesis

Oviparous animals have evolved multiple defenses to prevent microbes from penetrating their eggs and causing embryo mortality. In birds, egg constituents such as lysozyme and antibodies defend against microbial infestation, but eggshell pigments might also impact survival of bacteria. If so, microbes could exert an important selective pressure on the evolution of eggshell coloration. In a previous lab experiment, eggshell protoporphyrin caused drastic mortality in cultures of Gram positive, but not Gram negative, bacteria when exposed to light. Here, we test this ‘photodynamic antimicrobial hypothesis’ in a field experiment. In a paired experimental design we placed sanitized brown, protoporphyrin-rich chicken eggs alongside white eggs that lack protoporphyrin. We deployed eggs for 48 hrs without incubation, as can occur between laying and incubation, when microbial infection risk is highest. Eggs were placed on the open ground exposed to sunlight and in dark underground storm-petr...