The lek mating system in the black grouse : the role of sexual selection

Abstract

The aim of this thesis was to study the mechanisms behind the evolution of black grouse leks, and to examine the nature of sexual selection on leks. It was found that the mating preferences of females had an important role in the evolution of this particular mating system. Female preference for central males on a lek was suggested by both observational data from field and experimental data from aviary. Females favoured central males irrespective of male identity, and this preference was strong enough to yield a per capita benefit for males to be in the center. Therefore, males with the tendency to form tight clusters have been selected for. Sexual selection operated in slightly different ways depending on the habitat of lekking. On bogs, the male aggregations were spatially stable, while on ice-covered lakes the leks were mobile. This mobility was due to the lack of landmarks by which the males establish their territories, and it had consequences for female mate sampling and sexual selection. Females could choose from fewer males on lakes than on bogs, and the distribution of male mating success was more skewed on lakes. The dominant males prevented females from visiting all males on lakes. Still, females tried to approach other males than the first one visited, which induced interactions between males, and provided the females with information on the dominance status of the males. In a playback experiment, females were not attracted to leks with supplemented male vocal display. Instead, the number of males increased in the playback sites. These results indicate that the possible effect of playback on females was probably overrun by fidelity to breeding sites. In males, the response was most likely caused by young males with low site fidelity, who were attracted to the leks perceived as suitable-sized for them. In a comparative analysis of lekking, male mating skew and sexual dimorphism in body size in birds, no general patterns appeared between any of the variables examined. hylogenetic sources seemed to explain some of the variation in lek structure. It was concluded that with respect to these questions, intraspecific variation might offer more possibilities for studying adaptive processes.