Revisiting the positive correlation between female size and egg size

Positive correlations between maternal size and propagule (egg, seed, embryo) size could arise for several reasons. One of these is that larger mothers may improve the survival of their offspring during a stage when large propagules typically have lower survival than small propagules. We previously developed an optimality model that incorporated this effect and used it to explain the positive correlation between female size and egg size in some fishes. Our original analysis included the common assumption that large eggs have lower survival than small eggs when dissolved oxygen is low (because of surface-to-volume constraints). Recent empirical work, however, has suggested just the opposite: large eggs actually have higher survival than small eggs when dissolved oxygen is low. Here we re-analyse our original model in the light of this new evidence, showing that the original explanation for positive egg size–female size correlations no longer holds, but that new candidate explanations emerge. Specifically, larger females should produce larger eggs when they provide incubation environments of lower quality (i.e. lower dissolved oxygen). One way this might occur is that larger females produce larger clutches, which should have higher total oxygen demand. The re-analysis demonstrates that our theoretical approach can accommodate a variety of assumptions and may prove useful as a general framework for predicting variation in optimal egg size.