Keeping eggs warm: thermal and developmental advantages for parasitic cuckoos of laying unusually thick-shelled eggs
Obligate brood parasites have evolved unusually thick-shelled eggs, which are hypothesized to possess a variety of functions such as resistance to puncture ejection by their hosts. In this study, we tested the hypothesis that obligate brood parasites lay unusually thick-shelled eggs to retain more heat for the developing embryo and thus contribute to early hatching of parasite eggs. By doing so, we used an infrared thermal imaging system as a non-invasive method to quantify the temperature of eggshells of common cuckoos (Cuculus canorus) and their Oriental reed warbler (Acrocephalus orientalis) hosts in an experiment that artificially altered the duration of incubation. Our results showed that cuckoo eggshells had higher temperature than host eggs during incubation, but also less fluctuations in temperature during incubation disturbance. Therefore, there was a thermal and hence a developmental advantage for brood parasitic cuckoos of laying thick-shelled eggs, providing another possible explanation for the unusually thick-shelled eggs of obligate brood parasites and earlier hatching of cuckoo eggs compared to those of the host.
KeywordsAcrocephalus orientalis Cuculus canorus Eggshell thickness Incubation interval Infrared thermal imaging
We would like to thank Prof. Sven Thatje and two reviewers for their helpful comments. We thank Wenfeng Wang and Jianhua Ma from Zhalong National Nature Reserve, and Min Chen and Yungao Hu for their assistance with fieldwork, and the help and cooperation from Zhalong National Nature Reserve, Heilongjiang, China.
This work was supported by the National Natural Science Foundation of China (Nos. 31672303 to CY, 31660617 to LW, 31472013, and 31772453 to WL), the Program for New Century Excellent Talents in University (NCET-13-0761 to CY), and the Initial Fund Key Laboratories of Guizhou Province (grant No. 2011-4005) to LW.
Compliance with ethical standards
The experiments comply with the current laws of China, where they were performed. Fieldwork was carried out under the permission from Zhalong National Nature Reserves, China. Experimental procedures were in agreement with the Animal Research Ethics Committee of Hainan Provincial Education Centre for Ecology and Environment, Hainan Normal University (permit no. HNECEE-2012-003).
Conflict of interest
The authors declare that they have no competing interests.
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