Are bigger eggs better? Positive genetic and environmental correlations between egg size and hatching success in a precocial bird.

Egg size represents a primary form of parental investment, but whether it is causally related to offspring fitness is difficult to establish. We mated Japanese quail (Coturnix japonica) from replicated selection lines for high and low maternal investment in egg size to partners from the same and alternate selection line. We combined this with detailed pedigree information to partition variation in the size and hatching success of their eggs into their underlying genetic and non-genetic components, and estimated the correlation between these traits at all levels. Variation in egg size was primarily explained by additive genetic variation among mothers (h2 = 65%) and variation in hatching success also had an additive genetic component among mothers (h2 = 16%), with little evidence for additive effects of the father on both traits. Importantly, the correlations between egg size and hatching success were positive and similar at the genetic level among mothers and the non-genetic level within mothers. Furthermore, in one of the two replicates the eggs from females selected for high maternal investment in egg size had higher hatching success. We interpret these results as being most parsimoniously explained by a positive causal effect of egg size on hatching success.