ENVIRONMENTAL-INFLUENCES ON THE SEXUAL DIMORPHISM IN BODY-SIZE OF WESTERN BOBCATS

Sexual size dimorphism might be influenced by environmental constraint s on sexual selection or by intraspecific competition between males an d females. We studied bobcats (Lynx rufus) in collections of museum sp ecimens from western North America to examine these hypotheses. Struct ural body size was estimated from several measurements of the skull, I n-transformed and indexed through principal components analysis. Sexua l dimorphism in body size was estimated from the difference in size in dex of males and females, and compared to geographic and climatic vari ables associated with biotic provinces (ecoregions). Of several climat ic variables that were associated with bobcat body size, only seasonal ity of climate was associated with sexual dimorphism. Sexual size dimo rphism, longitude, elevation, and seasonality were intercorrelated. As longitude decreased (moving inland from west-coastal ecoregions), sex ual dimorphism decreased with the increased elevation and seasonality of continental climates of the Rocky Mountains. We suggest that increa sed seasonality and the need for fasting endurance by females may plac e constraints on the degree of sexual dimorphism in bobcats. Sexual di morphism of body size and sexual size dimorphism of trophic structures (teeth) exhibited a strong positive association over geography, thus indirectly supporting the hypothesis that intrasexual competition for prey could account for the geographic variation in sexual size dimorph ism. Thus, both environmental constraints on sexual selection of body size and intersexual competition were supported as possible explanatio ns of the degree of sexual size dimorphism that occurs in populations of bobcats.