Behaviourally mediated sexual selection: characteristics of successful male black grouse

Recent studies of lekking animals suggest that a suite of characters may be favoured by sexual selection. Examples of such traits are high survival, increased androgen levels, territory features and morphological characters including exaggerated morphological ornaments. Here we argue that such selection is often mediated by behavioural differences and we use results from a field study of lekking black grouse, Tetrao tetrixas an example of our argument. In the absence of females, males display a range of stereotyped behaviours including vocalizations, visual displays and fighting. When females attend the lek, the behavioural repertoire of the males becomes more limited. Time budget analyses reveal that in the vast majority of cases, males in the presence of females perform only three types of behaviour: a vocal display called rookooing; a courtship behaviour called circling; and fighting. This suggests that female preference could be based on male courtship and fighting behaviour while females visit individual male territories and that displays not used in the presence of females could be ruled out as important for female preference. Sexual selection in black grouse, however, is a complex interplay between male-male competition and female choice and therefore female preference is not the only determinant of male success. Successful males were more often involved in fights in the absence of females on the lek and defended territories that were larger than expected. Therefore, we hypothesize that differences in male fighting ability result in some males occupying relatively large territories at the centre of the leks. Relatively large territories seem to be needed for successful courtship. Therefore behavioural differences mediate differences in male copulation success both through female preference and male-male competition.