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

Abstract

Sexual dimorphism is an important characteristic of many mammals, but little is known about how environmental variables may affect its phenotypic expression. The relationships between population size, body mass, seasonal mass changes, and sexual mass dimorphism were investigated using 22 years of data 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 years 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 declined. Population density had a negative effect on seasonal mass changes of young males and females. Density also had a weak but significant positive 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 growth or reproduction by females than by males. 1670Résumé : Le dimorphisme sexuel est une caractéristique importante chez plusieurs espèces de mammifères, mais peu est connu sur les facteurs qui peuvent influencer son expression phénotypique. La relation entre la taille de la population et le dimorphisme sexuel de masse a été étudiée grâce à une série de 22 années de données sur la masse corporelle individuelle des Mouflons d'Amérique (Ovis canadensis) de la population de Ram Mountain, Alberta. Le nombre de femelles adultes dans cette population a varié passablement entre 1972 et 1996. La masse des mâles âgés entre 0 et 7 ans a été négativement affectée par l'augmentation de la taille de la population. La masse des femelles a été affectée seulement jusqu'à l'âge de 2 ans. Le dimorphisme sexuel de masse des mouflons âgés entre 2 et 7 ans a diminué avec l'augmentation du nombre de femelles adultes. La densité a eu un effet négatif sur les changements saisonniers de masse des jeunes mâles et femelles. La densité a également eu un léger effet positif sur le gain de masse annuel des femelles de 2 ans. Les jeunes femelles sembleraient être capables de compenser pour une faible croissance survenue tôt dans la vie tandis que l'effet sur les mâles persisterait jusqu'à l'âge adulte. Nous suggérons que ces différences sexuelles sont attribuables à la plus grande flexibilité d'allocation des ressources de la reproduction vers la croissance de la part des femelles.LeBlanc et al.

Influence of Ecological and Social Factors on Body Mass of Wild Chimpanzees

“…Body mass was significantly lower during the period of small range and high density, which supports the suggestion that males can increase food availability by expanding the community range (Williams et al, 2004). Significant differences in body mass at different population densities have been noted in several nonprimate species (LeBlanc et al, 2001;Pettorelli et al, 2002), and Bulger and Hamilton (1987) provide some evidence that masses of female chacma baboons increased during a time of declining troop density.…”

“…Such invasive procedures are rarely used on wild primates, with the result that data on many species come from only a few measurements (Smith and Jungers, 1997). Repeated measures collected in the same population over time allow study of ecological effects such as season (Kurita et al, 2002), annual variation in rainfall or growth season (Langvatn et al, 1996;Bourgarel et al, 2002), or plant phenology (Ramsey et al, 2002), effects of human activity on food availability (Strum, 1991;Altmann et al, 1993), and the effects of social factors such as population density (LeBlanc et al, 2001;Pettorelli et al, 2002). However, they are rarely available for wild primates.…”

Sexual dimorphism and intercorhort variation in reindeer calf antler length is associated with density and weather

“…Female reindeer need larger antlers to aggress and repel young bulls during the rut, and to secure access to feeding craters during winter for themselves and their offspring (Reimers 1993). This suggest a sexual difference in the resource allocation strategy for growth of various life history traits and for reproduction (see LeBlanc et al 2001). Accordingly, Festa-Bianchet et al (2004) found that as resources become scarce, young bighorn rams allocate an increasing proportion of those resources to body growth rather than to horn growth, thereby increasing their probability of survival.…”

The role of foraging behaviour in the sexual segregation of the African elephant

“…Females, on the other hand, exhibit much higher levels of investment in reproduction and select mates on the basis of size and Wtness (Anderson 1994;McElligott et al 2001). They are limited by the number of oVspring they can produce (Clutton-Brock and Harvey 1978;Le Blanc et al 2001), and subsequently no signiWcant advantage is gained from larger body size.…”