Sexual size dimorphism in bighorn sheep (Ovis canadensis): effects of population density

Sexual dimorphism is an important characteristic of many mammals, but littl e is known about how environmental variables may affect its phenotypic expr ession. The relationships between population size, body mass, seasonal mass changes, and sexual mass dimorphism were investigated using 22 years of da ta on individually marked bighorn sheep (Ovis canadensis) on Ram Mountain, Alberta. The number of adult ewes was artificially maintained low from 1972 to 1981 and then allowed to increase. The body mass of males from 0 to 7 y ears of age was negatively affected by population density. Female body mass was negatively affected by population density up to 2 years of age. As the number of ewes increased, sexual mass dimorphism of sheep aged 2-7 years d eclined. Population density had a negative effect on seasonal mass changes of young males and females. Density also had a weak but significant positiv e effect on yearly mass gain of 2-year-old females, suggesting compensatory growth. Females appear to compensate for resource shortages early in life, while males show a lifelong negative effect. We suggest that these sexual differences are due to the greater flexibility of resource allocation to gr owth or reproduction by females than by males.