Not as similar as thought: sexual dichromatism in Chimango Caracaras is expressed in the exposed skin but not in the plumage

Abstract

Many of the mechanisms involved in visual signalling in birds are based on pigment-based plumage ornamentation and colouration rather than the expression of other, non-plumage-based displays. We have analysed the colouration of the tarsi and ceres of free-ranging Chimango Caracaras Milvago chimango, a controversial morphological trait for this species, and examined the proximal causes related to the expression of colour in these featherless body parts. The tarsi–ceres of 81 Chimango Caracaras captured in central Argentina between 2005 and 2008 were either yellow–yellow or bluish–pink coloured. Differences in colour were not linked with plasma carotenoid levels and did not vary seasonally. Rather, expression of the two colours was affected by the gender and age of individuals. Yellow-coloured tarsi and ceres were recorded only in adult males, while the remaining age and gender groups showed pinkish- and bluish-coloured bare parts, which is the suspected neutral developmental stage under low hormonal levels. These results support sexual rather than natural selection forces acting on the expression of yellow colouration in the bare parts of adult males. However, an exaggeration of yellowish colour during mating periods in accordance with an increase of circulating carotenoid levels may be expected due to the sexually-related nature of this trait. This documented partitioning of colour variation among sexes and ages that is unrelated to melanin-dependent plumages is unusual and contrary to the most widely observed types of sexual dichromatism in birds.

Zusammenfassung

Viele der Mechanismen für den Austausch visueller Signale bei Vögeln beruhen auf pigmentbasiertem Gefiederschmuck und -farbe statt auf der Zurschaustellung anderer, von den Federn unabhängiger Signale. In unserer Untersuchung analysierten wir die Farbgebung der Tarsi frei lebender Chimangokarakaras (Milvago chimango), einer für diese Art kontrovers beschriebenen morphologischen Eigenheit, und prüften die proximalen Ursachen für die Farbgebung dieser federlosen Körperteile. Die Tarsi-Ceres der 81 zwischen 2005 und 2008 in Argentinien gefangenen Chimangokarakaras waren entweder gelblich oder pink-blau. Die Farbunterschiede zeigten weder einen Zusammenhang mit den Carotinoid-Pegeln im Plasma, noch jahreszeitlich bedingte Unterschiede. Stattdessen gab es einen Zusammenhang mit dem Geschlecht und dem Alter der Tiere. Gelbe Tarsi und Ceres traten nur bei erwachsenen Männchen auf, während diese Körperteile bei den übrigen Geschlechts- und Altersgruppen pink und bläulich waren, dem vermuteten neutralen Entwicklungsstadium bei niedrigen Hormonpegeln. Diese Ergebnisse sprechen eher für einen Geschlechtsunterschied als für einen Selektionsdruck auf die Ausbildung gelber Färbung der federlosen Körperteile ausgewachsener Männchen. Es wäre dann allerdings zu erwarten, dass die gelbe Färbung während der Balzzeit besonders stark ausgeprägt sein würde, zusammen mit einem Anstieg des Karotinoid-Pegels. Im Gegensatz zu der in der Regel beobachteten geschlechtsbedingten Färbung bei Vögeln ist dieses Ergebnis insofern ungewöhnlich, als es eine unterschiedliche Farbgebung bei Geschlechts- und Altersgruppen aufzeigt, die nicht mit der Melanin-abhängigen Gefiederfärbung zusammenhängt.

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Acknowledgments

We are grateful to Miguel Santillán, Maximiliano Galmes, Claudina Solaro, María Soledad Liébana, Sonia Cabezas, Marcos Reyes, Laura Beinticinco, Hebe Álvarez, Andrea Costan, Juan Ignacio Zanón-Martínez, Elena Macías, and Camila and Lucía Sarasola for help in trapping Chimango Caracaras. This study received financial support from the Universidad Nacional de La Pampa (Argentina), Idea Wild (USA) and the Raptor Research Center, Boise State University. JHS was founded by the National Research Council of Argentina (CONICET) and the Spanish Ministry of Education and Science through the postdoctoral program “Juan de la Cierva”. This study was carried out with permission from the Dirección de Recursos Naturales de la Provincia de La Pampa.

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Correspondence to José Hernán Sarasola.

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Communicated by Jon Fjeldså.

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Sarasola, J.H., Negro, J.J., Bechard, M.J. et al. Not as similar as thought: sexual dichromatism in Chimango Caracaras is expressed in the exposed skin but not in the plumage. J Ornithol 152, 473–479 (2011). https://doi.org/10.1007/s10336-010-0606-9

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Keywords

  • Sexual dichromatism
  • Chimango Caracara
  • Milvago chimango
  • Skin colour
  • Sexual selection
  • Visual signalling