Extra-pair paternity, testes size and testosterone level in relation to colour polymorphism in the barn owl Tyto alba


  • Alexandre Roulin,

  • Wendt Müller,

  • Lajos Sasvári,

  • Cor Dijkstra,

  • Anne-Lyse Ducrest,

  • Christian Riols,

  • Michael Wink,

  • Thomas Lubjuhn

A. Roulin (correspondence), Department of Zoology, University of Cambridge, Downing Street, CB2 3EJ Cambridge, UK. Present address of A. Roulin: Department of Ecology & Evolution, Biology Building, University of Lausanne, CH-1015 Lausanne, Switzerland. E-mail: Alexandre.Roulin@ie-zea.unil.ch. W. Müller and T. Lubjuhn, Institut für Evolutionsbiologie und Ökologie, Rheinische Friedrich-Wilhelms-Universität Bonn, An der Immenburg 1, D-53121 Bonn, Germany. L. Sasvári, Department of General Zoology, Eötvös University, Budapest, Hungary. C. Dijkstra, Department of Zoology, University of Groningen, NL-9750 Haren, The Netherlands. A.-L. Ducrest, Swiss Institute for Experimental Cancer Research (ISREC), chemin des Boveresses 155, CH-1066 Epalinges, Switzerland. C. Riols, Maison forestière des Genêts, F-11340 Espezel, France. M. Wink, Institut für Pharmazie und Molekulare Biotechnologie, Heidelberg University, Im Neuenheimer Feld 364, D-60120 Heidelberg, Germany.


In many bird populations, individuals display one of several genetically inherited colour morphs. Colour polymorphism can be maintained by several mechanisms one of which being frequency-dependent selection with colour morphs signalling alternative mating strategies. One morph may be dominant and territorial, and another one adopt a sneaky behaviour to gain access to fertile females. We tested this hypothesis in the barn owl Tyto alba in which coloration varies from reddish-brown to white. This trait is heritable and neither sensitive to the environment in which individuals live nor to body condition. In Switzerland, reddish-brown males were observed to feed their brood at a higher rate and to produce more offspring than white males. This observation lead us to hypothesize that white males may equalise fitness by investing more effort in extra-pair copulations. This hypothesis predicts that lighter coloured males produce more extra-pair young, have larger testes and higher levels of circulating testosterone. However, our results are not consistent with these three predictions. First, paternity analyses of 54 broods with a total of 211 offspring revealed that only one young was not sired by the male that was feeding it. Second, testes size was not correlated with male plumage coloration suggesting that white males are not sexually more active. Finally, in nestlings at the time of feather growth testosterone level was not related to plumage coloration suggesting that this androgen is not required for the expression of this plumage trait. Our study therefore indicates that in the barn owl colour polymorphism plays no role in the probability of producing extra-pair young.