Extreme Intra-Clutch Egg-Size Dimorphism in Eudyptes Penguins: A Unique Genus-Wide Evolutionary Maladaptation

The six species of Eudyptes penguins are characterized by a remarkable life history anomaly: their two-egg clutch consists of a first-laid A-egg that is 18 – 57% smaller than the second-laid B-egg, and the A-egg almost never (<1%) produces a fledged chick even though it is viable. Using phylogenetic comparative methods, we test a priori predictions derived from the novel “clutch-size constraint” hypothesis, which proposes that extreme intraclutch egg-size dimorphism in Eudyptes penguins evolved in association with their unique pelagic non-breeding ecology, which favoured a slower life history and resulted in constraints on clutch-size reduction. Consistent with a slower life history, we show that age of first breeding is 54% later and annual fecundity 54% lower in Eudyptes than in non-Eudyptes penguins with two-egg clutches. Consistent with constraint, we show that the fecundity resulting from a two-egg clutch is 51% lower in Eudyptes; this marked mismatch between clutch-size and realized fecundity provides a rare, unambiguous example of phylogenetic and evolutionary constraint. We further show that extreme egg-size dimorphism in Eudyptes penguins results primarily from a uniform increase in B-egg size, not a decrease in A-egg size, and propose that their extreme egg-size dimorphism is explained by an interaction between selection on B-egg size and evolutionary constraints on a transition to a one-egg clutch.