Egg temperature and embryonic metabolism of A- and B-eggs of macaroni and rockhopper penguins
Macaroni and rockhopper penguins lay two eggs but rear only one chick to independence. The eggs are markedly dimorphic in size and, although the smaller A-egg is laid several days before the B-egg, in nests where both eggs are incubated, the B-egg always hatches first. Incubation temperatures and embryonic oxygen consumption were measured to determine whether the observed hatching sequence could be accounted for by differences in egg temperatures or rate of embryonic development. Lowest egg temperatures were recorded from A-eggs incubated in the less favourable anterior position in the brood patch and highest temperatures from A-eggs incubated singly and B-eggs. Differences, however, were not significant. Levels of embryonic oxygen consumption of A-eggs of the same age showed a similar pattern to egg temperatures, but differences were slight. A-eggs incubated singly, and those incubated for several days after laying in a hot-room, still had incubation periods longer than B-eggs, suggesting that egg temperature alone does not account for differences in the hatching sequence. Consequently, it appears that there are inherent differences in embryonic metabolism of A- and B-eggs that result in the B-egg, which represents the greater parental investment, hatching first.