Behavioral Ecology Vol. 15 No. 3: 380-389
Behavioral Ecology vol. 15 no. 3 © International Society for Behavioral Ecology 2004; all rights reserved
Mutual ornamentation, sexual selection, and social dominance in the black swan
a Department of Zoology, University of Melbourne, Melbourne, VIC 3010, Australia, b Ballarat Field Naturalists, PO Box 328W, Ballarat West 3350, Australia, and c Centre for Ecological and Evolutionary Studies, University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands
Address correspondence to K. Kraaijeveld, who is now at the Department of Biology, Galton Laboratory, Wolfson House, University College London, 4 Stephenson Way, London NW1 2HE, UK. E-mail: k.kraaijeveld{at}ucl.ac.uk.
We investigated the adaptive significance of a sexually monomorphic ornament in the black swan Cygnus atratus. Both sexes grow curled feathers on their wings (range 7–22 curled feathers per wing), which are displayed prominently in a range of social interactions. The number of curled feathers increased until the birds reached sexual maturity (at 2 years of age) but did not vary with age thereafter. We found evidence for both sexual and social functions of the ornament. Paired, mature individuals of both sexes had higher numbers of curled feathers than unpaired, mature birds, and individuals paired assortatively with respect to curled feather number, suggesting the feathers may be involved in mutual sexual selection. More ornamented individuals were dominant in agonistic interactions with birds of the same sex and pairing status. Highly ornamented pairs were also more likely to maintain extended tenancy of preferred cygnet feeding areas, which resulted in improved offspring survival. The curled feathers thus appear to function as a signal of social dominance, which is highly correlated with reproductive success and is therefore a reliable signal of parental quality in mate choice.
Key words: black swans, Cygnus atratus, dominance, mutual sexual selection, ornamentation, signaling, social selection.
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