title: Extreme intraclutch egg-size dimorphism in Eudyptes penguins, an evolutionary response to clutch-size maladaptation.
abstract: 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.
authors.affiliation: Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. rwstein@sfu.ca
authors.forename: R Will
authors.initials: RW
authors.lastname: Stein
authors.forename: Tony D
authors.initials: TD
authors.lastname: Williams
mesh_terms: D000818:Animals; D005075:Biological Evolution; D052284:Clutch Size; D005260:Female; D008297:Male; D010063:Ovum; D046429:Spheniscidae / Q000235:genetics*