Experimental evidence for mutual inter- and intrasexual selection favouring a crested auklet ornament

During the breeding season, female and male crested auklets, Aethia cristat ella (Alcidae), display similar conspicuous crest ornaments composed of elo ngated forward-curving feathers on their foreheads. Based on quantification s of brief agonistic interactions at a large breeding colony, we found that crest length was strongly correlated with dominance within both sexes. Acr oss the full range of crest length, individuals with longer crests were dom inant over shorter-crested individuals in agonistic interactions involving same-sex adults. Within subadults (2-year-olds of unknown sex), there was a similar trend towards longer-crested individuals being dominant. In agonis tic interactions involving individuals of different sex and age, adult male s were dominant over adult females and adults were dominant over subadults, regardless of crest length. In an experiment in which we manipulated crest length using Life-size realistic models, male auklets that responded were less aggressive to male models with longer crests than to models with norma l or shorter crests, confirming that crest length by itself signals dominan ce status. In a related experiment in which we controlled intrasexual compe tition, both males and females responded to opposite-sex models with more f requent sexual displays when the models had long crests compared with those having short crests, suggesting that crested auklets also have mating pref erences that favour long crest ornaments. Taken together, these results sup port the idea that the crest ornament is favoured by both intra- and inters exual selection. (C) 1999 The Association far the Study Animal Behaviour.