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The protoporphyrin content of Kentish Plover Charadrius alexandrinus eggshells is better predicted by the fractal dimension of spottiness than by colour

  • Jesús Gómez
  • Gustavo Liñán-Cembrano
  • Macarena Castro
  • Alejandro Pérez-Hurtado
  • Cristina Ramo
  • Juan A. Amat
Original Article

Abstract

Quantifying the pigment content in avian eggshells is important for the validation of hypotheses on the functionality of eggshell coloration. The few studies that have analysed whether eggshell coloration and spottiness are related to pigment content have found contradictory results. In this study, we analysed whether the coloration and the degree of spottiness of Kentish Plover Charadrius alexandrinus eggshells are related to pigment concentrations (protoporphyrin and biliverdin). We also examined whether the concentrations of these pigments are related to the fractal dimension (FD) of spottiness. The FD of spottiness may be useful as a simple standardized method to quantify complex patterning. We found that protoporphyrin was more abundant than biliverdin in eggshells. Contrary to expectations, the content of protoporphyrin was not related to eggshell colour, probably due to a different allocation of pigments between the spots and the background of the eggshell, and/or to the different allocation of pigments among eggshell layers. However, we found a positive relationship between the FD of eggshell spottiness and the amount of protoporphyrin. It is likely that the FD of spottiness (indicative not only of spot size and the degree of spottiness, but also of how convoluted the outlines of spots are and how spots are distributed across an eggshell) may be related to the mechanical function of protoporphyrin (e.g. in strengthening eggshells, which may interfere with the fractal structure of potential fractures), and/or it may also improve egg camouflage. However, more studies in other avian species are needed to evaluate if the FD of spottiness is a good surrogate for protoporphyrin content in eggshells.

Keywords

Biliverdin Coloration Digital photography Eggs Fractal analysis Pigmentation 

Zusammenfassung

Die fraktale Dimension der Fleckung prognostiziert den Gehalt an Protoporphyrin in Eierschalen des Seeregenpfeifers Charadrius alexandrines besser als die Färbung.

Die Quantifizierung des Pigmentgehaltes in Eierschalen von Vögeln ist wichtig, um Hypothesen über die Funktionalität der Eierschalenfärbung zu überprüfen. Wenige Studien haben analysiert, ob die Färbung und Fleckung der Eierschalen mit dessen Pigmentgehalt zusammenhängt, wobei widersprüchliche Ergebnisse gefunden wurden. In dieser Studie haben wir untersucht, ob die Färbung und das Fleckungsausmaß der Eierschalen des Seeregenpfeifers Charadrius alexandrinus mit der Pigmentkonzentration (Protoporphyrin und Biliverdin) wie auch mit der fraktalen Dimension der Fleckung zusammenhängt. Die Verwendung der fraktalen Dimension der Fleckung sollte dabei als eine einfache standardisierte Methode zur Quantifizierung der komplexen Musterung dienen. Wir fanden heraus, dass in den Eierschalen Protoporphyrin reichlicher vorhanden war als Biliverdin. Entgegen den Erwartungen stand der Gehalt an Protoporphyrin nicht in Zusammenhang mit der Eierschalenfärbung, möglicherweise aufgrund der unterschiedlichen Zuordnung der Pigmente zwischen den Flecken und dem Eierschalenhintergrund und/oder der unterschiedlichen Verteilung der Pigmente innerhalb der Eierschalenschichten. Wir fanden einen positiven Zusammenhang zwischen der fraktalen Dimension der Eierschalenfleckung und der Menge an Protoporphyrin. Anzunehmen ist, dass die fraktale Dimension der Fleckung (nicht nur bezeichnend für die Fleckengröße und das Fleckungsausmaß, sondern auch dafür, wie gekrümmt die Fleckenumrisse und wie die Flecken über der Eierschale verteilt sind) mit den mechanischen Funktionen des Protoporphyrins (z.B. durch die Stärkung der Eierschalen zur Vermeidung einer möglichen fraktalen Struktur bei Brüchen) zusammenhängt und/oder die Eiertarnung verbessern könnte. Allerdings sind noch weitere Studien bei anderen Vogelarten nötig, um abzuschätzen, inwieweit die fraktale Dimension der Fleckung als guten Ersatzparameter für den Gehalt an Protoporphyrin in Eierschalen geeignet ist.

Notes

Acknowledgements

Thanks to Lorenzo Pérez-Rodríguez and Roger Jovani for their comments on previous versions of the manuscript. J. G. was supported by a FPU pre-doctoral fellowship (FPU-12/01616) from the Ministerio de Educación, Cultura y Deporte, Spain. Our project was funded by grant CGL2011-24230 from the Ministerio de Ciencia e Innovación, Spain, with EU-ERDF financial support, and was approved by the Comité Ético de Bienestar Animal from EBD-CSIC (reference CEBA-EBD_2011_01). During the preparation of the paper we were funded by grant CGL2017-83518-P from the Ministerio de Economía, Industria y Competitividad, Spain, with EU-ERDF financial support. The analyses of pigments were performed at the laboratory of chemical ecology of the Estación Biológica de Doñana (LEQ-EBD) by Isabel García and Juan Antonio Canales. Thanks also to Antonio Gómez Ferrer for providing facilities at the study site and to the Consejería de Medio Ambiente of the Junta de Andalucía, which authorised our study.

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Copyright information

© Deutsche Ornithologen-Gesellschaft e.V. 2018

Authors and Affiliations

  • Jesús Gómez
    • 1
  • Gustavo Liñán-Cembrano
    • 2
  • Macarena Castro
    • 3
  • Alejandro Pérez-Hurtado
    • 3
  • Cristina Ramo
    • 1
  • Juan A. Amat
    • 1
  1. 1.Departamento de Ecología de HumedalesEstación Biológica de Doñana (EBD-CSIC)SevilleSpain
  2. 2.Instituto de Microelectrónica de Sevilla (IMSE-CNM)Consejo Superior de Investigaciones Científicas (CSIC) and Universidad de SevillaSevilleSpain
  3. 3.Departamento de Biología, Facultad de Ciencias del Mar y AmbientalesUniversidad de CádizPuerto RealSpain

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