Geographical variation in egg size dimorphism in rockhopper penguins

DEMONGIN, L.; POISBLEAU, M.; RAYA REY, A; SCHIAVINI, A.; QUILLFELDT, P.; EENS, M.; STRANGE, I.J.

POLAR BIOLOGY

SPRINGER

Lugar: Heidelberg; Año: 2010 vol. 33 p. 469 - 476

0722-4060

Abstract All crested penguins present a unique reversedhatching asynchrony: the larger second-laid egg (B-egg)hatches before the smaller Wrst-laid egg (A-egg). Althoughboth eggs often hatch, the A-chick generally dies of starvationwithin days after hatching. However, within rockhopperpenguins, the population at the Falkland Islands isunique in that some birds manage to raise both chicks.Although it has been suggested that the egg size dimorphismbetween A- and B-eggs may explain how long botheggs and chicks survive, this hypothesis has never beenexplicitly tested. We expect that both eggs are retainedlonger in the less dimorphic clutches than in the moredimorphic ones. In this paper, we have compiled egg measurementsfor three rockhopper penguin species (Eudypteschrysocome, E. Wlholi and E. moseleyi) in order to comparethe intra-clutch egg size dimorphism among these species.Furthermore, we have collected new data to compare eggsize dimorphism between two populations of E. chrysocome(Falkland Islands versus Staten Island). A-eggvolumes are more variable between species and populationsthan B-egg volumes. E. chrysocome and especially thepopulation from the Falkland Islands produces the largestA-eggs and the least dimorphic eggs. Nevertheless, asdiVerences in A-egg volumes between species and betweenthe populations of Falkland Islands and Staten Island arestronger and more signiWcant than diVerences in egg dimorphism,we suggest that A-egg volume, more than eggdimorphism, could be one of the factors inXuencing theprevalence of twins. A large A-egg and/or reduced eggdimorphism is probably necessary to enable rockhopperpenguins to raise two chicks, but other reasons may also beinvolved which enable them to keep both eggs and chicks.