Abstract
In the context of sexual selection, animals have developed a variety of cues conveying informationabout the sex of an individual to conspecifics. In many colonial seabird species,where females and males are monomorphic and do not show obvious differences in externalmorphology, acoustic cues are an important signal for individual and sex recognition. Here,we study the vocal and morphological sex dimorphism in the King Penguin Aptenodytespatagonicus, a colonial, monomorphic seabird for which our knowledge about the role ofvocalizations and morphology in mate choice is very limited. Data were collected at PossessionIsland, Crozet Archipelago, in a breeding colony consisting of about 16 000 breedingpairs. Using measurements of six morphological features and analysing acoustic parametersof call recordings of adult individuals, we show that King Penguins can be sexed based on asingle morphological measurement of the beak with an accuracy of 79%. We found a sexspecificsyntax in adult King Penguin calls that provided a 100% accurate method to distinguishbetween the sexes in our study population. To confirm the method at the specieslevel, we analysed calls recorded from King Penguin adults in Kerguelen Island, 1300 kmaway from our study population and found the same accuracy of the sex-specific syntax.This sex-specific syllable arrangement is rare in non-passerines and is a first step in understandingthe mate choice process in this species. Furthermore, it offers a cost-effective, noninvasivetechnique for researchers to sex King Penguins in the field.