SEXUAL SELECTION FOR INCREASED MALE COURTSHIP AND ACOUSTIC SIGNALS AND AGAINST LARGE MALE SIZE AT SHARP-TAILED GROUSE LEKS.

Abstract

Sexual dimorphism has been a long-standing problem for evolutionary biology (Darwin, 1871; Selander, 1966, 1972; Ralls, 1977; Alexander et al., 1979; Slatkin, 1984; Bradbury and Andersson, 1987). It is generally thought that sexual selection is the major force leading to the evolution of sexually dimorphic traits (Darwin, 1871; Fisher, 1958; Trivers, 1972; Lande, 1980). This may be particularly true of lek-breeding species, because the potential for intense sexual selection should be quite high (Darwin, 1871; Fisher, 1958; Bradbury, 1981). Typically, there is extreme variance in male mating success on leks. As a general explanation for sexual size dimorphism in lek-breeding species, the hypothesis has been proposed that male-male competition for mates has led to larger male size (Darwin, 1871; Wiley, 1974). This may occur because larger males are more dominant and by the time mating takes place they can subtly control access of smaller males such that females are essentially unavailable to subordinates (Beehler and Foster, 1988). In contrast, females may directly and disproportionately choose dominant males as mates (Bradbury and Gibson, 1983). Dominant males may be chosen, for example, because of the potential for greater offspring viability (Wiley, 1973, 1974; Alatalo et al., 1991). Trail (1990) recently proposed that the degree of sexual (plumage) dimorphism can be explained by differences among lek-breeding species in the amounts and kinds of intrasexual competition. He hypothesizes that in addition to intense male-male competition, intrasexual competition among females in some lek-breeding species has led to a greater elaboration of characters in females and, hence, sexual monomorphism. It is clear that a variety of intraand intersexual mechanisms may explain selection for increased elaboration of characters in both males and females of lekbreeding species. One hopes to explain not only the elaboration of characters, however, but also constraints on further elaboration. One hypothesis is that the further elaboration of sexually dimorphic traits is dampened only when traits become so costly that their bearers are unlikely to survive to mate (Fisher, 1958; Selander, 1965; Lande, 1980; Arnold, 1983). This explanation was originally applied to male secondary sexual characters, but may apply to body size as well. I studied how female choice acts on male body size