Eggshell morphology and gekkotan life-history evolution

Abstract

Eggshell structure is related to fundamental aspects of embryonic development (via water and gas exchange), adult ecology and behavior (via nest site selection), and demography (via effects on survival). We compared life-history characteristics between gekkotans that lay rigid- versus parchment- shelled eggs to determine if evolutionary shifts in eggshell structure are associated with life-history evolution. Ancestral gekkotans laid parchment-shelled eggs, with rigid-shelled eggs evolving later. Clutch size in oviparous gekkotans is fixed at one or two eggs, and this characteristic eliminates an egg size versus clutch size tradeoff as a life-history strategy. We found that species laying rigid-shelled eggs exhibit (1) smaller eggs relative to adult body size, (2) smaller hatchlings relative to the size of the egg, (3) earlier embryonic stage at oviposition, (4) longer incubation periods, and (5) smaller adult body sizes than species laying parchment-shelled eggs. These patterns hold when accounting for phylogenetic relatedness, and are not explained by geographic distributions of climate and habitat. In general, our data support the hypothesis that the spherical shapes of rigid-shelled eggs limit their size (volume), which in turn has restricted hatchling size and adult body size. In contrast, while parchment-shelled eggs are similarly constrained in width, elongate shapes allow egg sizes, and hence hatchling sizes, to increase relative to adult body sizes. Finally, the evolution of rigid-shelled eggs may have allowed gekkotans to become so successful; over 1,000 species lay rigid-shelled eggs, as compared to about 200 species that lay eggs exhibiting the ancestral parchment-shelled condition.