SEXUAL SIZE DIMORPHISM AND SELECTION IN THE WILD IN THE WATERSTRIDER AQUARIUS-REMIGIS - BODY-SIZE, COMPONENTS OF BODY-SIZE AND MALE MATING SUCCESS

Sexual size dimorphism is assumed to be adaptive and is expected to ev olve in response to a difference in the net selection pressures on the sexes. Although a demonstration of sexual selection is neither necess ary nor sufficient to explain the evolution of sexual size dimorphism, sexual selection is generally assumed to be a major evolutionary forc e. If contemporary sexual selection is important in the evolution and maintenance of sexual size dimorphism then we expect to see concordanc e between patterns of sexual selection and patterns of sexual dimorphi sm. We examined sexual selection in the wild, acting on male body size , and components of body size, in the waterstrider Aquarius remigis, a s part of a long term study examining net selection pressures on the t wo sexes in this species. Selection was estimated on both a daily and annual basis. Since our measure of fitness (mating success) was behavi oral, we estimated reliabilities to determine if males perform consist ently. Reliabilities were measured as kappa statistics and range from fair to perfect agreement with substantial agreement overall. We found significant univariate sexual selection favoring larger total length in the first year of our study but not in the second. Multivariate ana lysis of components of body size revealed that sexual selection for la rger males was not acting directly on total length but on genital leng th. Sexual selection for larger male body size was opposed by direct s election favoring smaller midfemoral lengths. While males of this spec ies are smaller than females, they have longer genital segments and wi der forefemora. Patterns of contemporary sexual selection and sexual s ize dimorphism agree only for genital length. For total length, and al l other components of body size examined, contemporary sexual selectio n was either nonsignificant or opposed the pattern of size dimporhism. Thus, while the net pressures of contemporary selection for the speci es may still act to maintain sexual size dimorphism, sexual selection alone does not.