K. Zimmermann, Diplom. 2004. The evolutionary relationship between eggshell porosity and incubation period in Alcids. Fakultät fur Biologie, Ruprecht-Karls-Universität, Heidelberg, Germany

Abstract.- Eggshell porosity may be an important proximate determinant of incubation period, an evolutionarily conservative life-history trait in birds, through its role in regulating the rate of gaseous exchange between the developing embryo and the external environment. To examine the evolutionary relationship between eggshell porosity and incubation period, I measured egg mass and eggshell porosity in seven species of Alcidae that breed in British Columbia, Canada. This small family of marine birds exhibits an unusual degree of interspecific variation in incubation period. Multiple regression analysis showed that egg mass and eggshell porosity combined explained 87% of the variation in incubation period among our seven species, which include at least one member of each of the six major lineages of the Alcidae. As predicted, eggshell porosity and incubation period were negatively related, after controlling for egg mass. Although phylogenetic relationships remain poorly resolved in the Alcidae, our results suggest that evolutionary divergence in incubation period occurred in association with divergence in eggshell porosity at some point(s) during alcid evolution. Studies on other groups of birds could examine the same question, to assess whether the evolutionary relationship between eggshell porosity and incubation period is widespread.