Animal signals involved in sexual selection are often indicators of individual quality. The assumption that sexual characters such as breeding plumage may indicate immune state has rarely been tested in free-living migratory birds, particularly in relation to innate immunity. If sexual characters indeed reflect immune condition, then these could be used to evaluate individual quality. Melanin is a common pigment used in animal communication that mitigates the effects of oxidative stress and has positive effects on energy homeostasis, important functions during the strenuous activity of long-distance flights. However, melanin is also immunosuppressive, and the melanised patches of breeding plumage may to some extent compromise immune responsiveness. We studied melanin-based secondary sexual characters (SSC) in a long-distance migratory wader, the black-tailed godwit Limosa limosa, and found that breeding plumage features of male and female godwits are linked to components of innate immunity. Males with a larger colour extension had higher circulating levels of haptoglobin and hemolysis activity, while they also presented a lower body condition; whereas females presented a negative trend between colour and bar extension and hemolysis activity, and a positive trend for natural antibodies. The association between signal, immune state and physical condition in males suggests a cost for signal production and immune condition during prenuptial migration. Sex differences in how signals relate with immune capacity are a likely consequence of sex-specific signalling roles and energy demands. Our results indicate that male godwit breeding plumage reflects innate immunity state, and is therefore a likely signal for females to use during mate choice as an honest indicator of male's capacity to allocate energy/resources to both expensive traits during periods of energetic constraint.
Acknowledgements – We are thankful to the many people whose field efforts were invaluable towards bird catching. In the Netherlands and Iceland, we thank Y. Galama, G. Hoekstra, M. Verhoeven, Nathan Senner, Verónica Méndez and the local farmers for their co-operation and for allowing access to their private grounds. We acknowledge the cooperation of T. Piersma, J. Hooijmeijer and T. Gunnarsson, for their help with fieldwork logistics. We thank the MEEL lab (Dept of Biology, Lund Univ., Sweden) and Ana Domingos for providing the laboratory facilities. Funding – This work was supported by Fundação para a Ciência e a Tecnologia (FCT) to SP (SFRH/BD/84629/2012) and to JAA (SFRH/BPD/91527/2012) and benefited from the strategic program of MARE, financed by FCT (MARE – UID/MAR/04292/2013). Fieldwork in Iceland was supported by RANNIS – The Icelandic Research Fund (Grant of Excellence 130412-051) and H. Westerdahl financed molecular analysis through Swedish Research Council (621-2011-3674 and 2015-05149). Permits – In the Netherlands, blood sampling was conducted under permissions of Univ. of Groningen Animal Experimentation Committee (DEC-RUG) and under the license number 6350A and 6350E, following the Dutch Animal Welfare Act Articles 9 and 11. In Iceland, blood sampling was done under the permission of the Icelandic Natural History Inst. (license number 365).
© 2018 The Authors