Extreme Intraclutch Egg-Size Dimorphism in Eudyptes Penguins, an Evolutionary Response to Clutch-Size Maladaptation

Eudyptes penguins (six species) are uniquely characterized by a two-egg clutch with extreme intraclutch egg-size dimorphism (ESD): the first-laid A-egg is 17.5%-56.9% smaller than the B-egg. Although A-eggs are viable, they almost never produce fledged chicks (genus average <1%). Using classical life-history theory and phylogenetic comparative methods, we demonstrate a marked slowdown in the life history of Eudyptes: age of first reproduction is 52% later and annual fecundity 48% lower compared with other two-egg clutch penguin species. All six Eudyptes species have retained a two-egg clutch, despite this pronounced life-history slowdown; this suggests evolutionary mismatch between clutch size and chicks fledged per clutch. Consistent with this, we show that Eudyptes fledge 43% fewer chicks per clutch than other two-egg clutch penguin species. Extreme intraclutch ESD in Eudyptes is associated primarily with a uniform (5%) increase in relative B-egg size, and B-egg size has evolved in accord with life history. We further show that intraclutch ESD is positively correlated with age of first reproduction in Eudyptes but not in other two-egg clutch penguin species. We argue that Eudyptes' persistent failure to evolve a one-egg clutch constitutes a unique genus-wide evolutionary maladaptation and that extreme intraclutch ESD evolved as a correlated response to selection favoring a slower life history imposed by their extreme pelagic overwintering and migration ecology.