Abstract
In crested penguins (Eudyptes spp.), second-laid eggs typically hatch before first eggs. Amongst a variety of factors that have been considered as mechanisms underlying this reversal, has been the idea that crested penguins can adjust the degree of hatching asynchrony by manipulating egg positions (i.e. placing the smaller first egg in the supposedly thermally disadvantaged anterior position) during incubation (termed Preferential Incubation Hypothesis). We tested this in the Snares crested penguin (Eudyptes robustus) and the closely related, but synchronously-hatching, yellow-eyed penguin (Megadyptes antipodes). Snares crested penguins were more likely to place their first eggs, which are smaller than second eggs, in the anterior incubation position than were yellow-eyed penguins, which have a clutch of two similar-sized eggs. But when yellow-eyed penguins, a non-brood reducing species, were provided with an artificial size-dimorphic clutch, they also placed smaller eggs more frequently in the anterior position, suggesting that a general preference exists among penguins to place smaller eggs in the anterior position. Egg temperatures of small first eggs of Snares crested penguins were higher in the anterior than in the posterior position. Large first eggs in lesser size-dimorphic clutches experienced high temperature differences in relation to position, while small first eggs in greater size-dimorphic clutches were incubated at similar temperatures. In yellow-eyed penguins, large eggs within clutches generally had higher egg temperatures than small eggs. Incubation periods of second eggs declined with increasing egg size. Egg-size variation, rather than egg positioning behaviour, influenced hatching patterns in Snares crested penguins. In lesser size-dimorphic clutches, second eggs were more likely to hatch first while in greater size-dimorphic clutches, small first eggs were more likely to hatch at the same time or before the second eggs. This was similar in yellow-eyed penguins, where second eggs hatched earlier in clutches with large first eggs. Our data contradicts the Preferential Incubation Hypothesis and we conclude that this hypothesis is unlikely to explain the reversed hatching asynchrony in crested penguins.
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Acknowledgments
We thank Alvin Setiawan, Mike Hazel, Sylviane Neumann, Danilo Hegg, Brad Robson, and Jasmine Lai for help in the field, and John Darby for providing helpful information about yellow-eyed penguins breeding at Boulder Beach. This study was approved by the Department of Conservation and the University of Otago Animal Ethics Committee and complies with the current laws of New Zealand. In particular, we thank Dean Nelson, Bruce McKinlay, Dave Houston, and Jeremy Karol from the Department of Conservation for support and permits. The study was supported by grants from the Yellow-eyed Penguin Trust, a University of Otago Postgraduate Scholarship to M.M. and an Otago Research Grant to L.S.D. Additionally, we thank Colleen Cassady St. Clair and Javier Viñuela, as reviewers, for the helpful comments they made on a previous draft.
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Massaro, M., Davis, L.S. Preferential incubation positions for different sized eggs and their influence on incubation period and hatching asynchrony in Snares crested (Eudyptes robustus) and yellow-eyed penguins (Megadyptes antipodes). Behav Ecol Sociobiol 56, 426–434 (2004). https://doi.org/10.1007/s00265-004-0803-8
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DOI: https://doi.org/10.1007/s00265-004-0803-8