Abstract
Vivid plumage colors are some of the best examples of elaborate trait evolution resulting from sexual selection. Bird feathers often contain high concentrations of pigment or intricate microstructures for reflecting light at distal feather tips, in order to maximize color presentation to intended viewers (e.g., prospective mates, rivals). However, here we describe a rare form of “hidden” plumage coloration, whereby several species of heron (Pelecaniformes: Ardeidae) have powder down tracts liberally coated in yellow pigments that are hidden from view underneath contour feathers. We show using Raman spectroscopy and high-performance liquid chromatography that the yellow coloration is due to carotenoid pigmentation. We also show that fatty acids and carotenoids co-occur in samples and discuss the possible evidence and implications for the presence of esterified carotenoids in powder down. Our study of hidden feather coloration is in apparent contrast with the traditional view of carotenoid pigmentation in plumage, which holds that carotenoids are costly pigments that are displayed to advertise individual quality. We hypothesize that carotenoid pigments are transferred onto powder down as a consequence of the chemical affinity that these pigments have to lipoid secretions associated with subcutaneous fat. Further investigation may identify functions for carotenoid pigmentation in powder down (e.g., signaling, microbiological protection), but we cannot rule out the possibility that the powder down of herons may provide the first evidence for carotenoid coloration in feathers being a non-adaptive by-product of other processes.
Zusammenfassung
Versteckte Carotinoide in den Puderdunen von Reihern
Leuchtende Gefiederfarben gehören zu den besten Beispielen für die Evolution aufwendiger Merkmale durch sexuelle Selektion. Vogelfedern enthalten oft hohe Pigmentkonzentrationen oder besitzen komplexe lichtreflektierende Mikrostrukturen an den distalen Federspitzen, um die Wirkung der Farben auf die beabsichtigten Betrachter (beispielsweise mögliche Partner oder Rivalen) zu maximieren. Hier allerdings beschreiben wir eine seltene Form „versteckter Gefiederfärbung”, bei der verschiedene Reiherarten (Pelecaniformes: Ardeidae) Fluren reichlich mit gelben Pigmenten bedeckter Puderdunen aufweisen, welche unter dem Deckgefieder den Blicken verborgen bleiben. Mittels Raman-Spektroskopie und Hochleistungs-Flüssigkeitschromatografie zeigen wir, dass die Gelbfärbung auf Carotinoidpigmenten beruht. Außerdem zeigen wir, dass Fettsäuren und Karotinoide gemeinsam in den Proben vorkommen und erörtern die möglichen Belege für die Existenz veresterter Carotinoide in Puderdunen und deren eventuelle Konsequenzen. Unsere Untersuchung “versteckter” Gefiederfärbung steht in offenkundigem Gegensatz zur traditionellen Ansicht über carotinoidbasierte Gefiederpigmentierung, welche besagt, dass Carotinoide kostspielige Pigmente sind, die präsentiert werden, um die individuelle Qualität zu demonstrieren. Wir stellen die Hypothese auf, dass Carotinoidpigmente in Folge ihrer chemischen Affinität zu lipoiden Sekreten, welche im Zusammenhang mit subkutanem Fett stehen, auf die Puderdunen übertragen werden. Weitergehende Studien könnten Funktionen der carotinoidbasierten Pigmentierung der Puderdunen aufzeigen (beispielsweise Signalwirkung oder mikrobiologische Schutzfunktion), wir können allerdings die Möglichkeit nicht ausschließen, dass die Puderdunen der Reiher der erste Beleg dafür sind, dass Carotinoidfärbung bei Federn als ein nicht-adaptives Nebenprodukt anderer Prozesse entsteht.
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Acknowledgements
We thank H. F. James for specimen access and comments on a draft manuscript, C. Gebhard, National Museum of Natural History, Smithsonian Institution for collecting samples, and the Museum Conservation Institute, Smithsonian Institution for providing access to Raman instrumentation.
Funding
D. B. T. was funded by a Peter Buck Fellowship administered by the Smithsonian Institution.
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All analyses were on feathers from deceased birds that had not been specifically killed for this study. Analyses were performed in, and adhered to the laws of, the United States of America.
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Communicated by K. C. Klasing.
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Thomas, D.B., McGraw, K.J. Hidden carotenoids in the powder down of herons. J Ornithol 159, 785–792 (2018). https://doi.org/10.1007/s10336-018-1554-z
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DOI: https://doi.org/10.1007/s10336-018-1554-z