Environmental influences on the sexual dimorphism in body size of western bobcats

Abstract

Sexual size dimorphism might be influenced by environmental constraints on sexual selection or by intraspecific competition between males and females. We studied bobcats (Lynx rufus) in collections of museum specimens from western North America to examine these hypotheses. Structural body size was estimated from several measurements of the skull, ln-transformed and indexed through principal components analysis. Sexual dimorphism in body size was estimated from the difference in size index of males and females, and compared to geographic and climatic variables associated with biotic provinces (ecoregions). Of several climatic variables that were associated with bobcat body size, only seasonality of climate was associated with sexual dimorphism. Sexual size dimorphism, longitude, elevation, and seasonality were intercorrelated. As longitude decreased (moving inland from west-coastal ecoregions), sexual dimorphism decreased with the increased elevation and seasonality of continental climates of the Rocky Mountains. We suggest that increased seasonality and the need for fasting endurance by females may place constraints on the degree of sexual dimorphism in bobcats. Sexual dimorphism 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 dimorphism. Thus, both environmental constraints on sexual selection of body size and intersexual competition were supported as possible explanations of the degree of sexual size dimorphism that occurs in populations of bobcats.

When Rensch Meets Bergmann: Does Sexual Size Dimorphism Change Systematically With Latitude?

“…Identifying the mechanisms producing steeper male clines will require more studies quantifying latitudinal variation in sex-specific natural and sexual selection on body size. While the underlying mechanisms are often unclear, any latitudinal change in environmental factors (e.g., season length, food availability, or winter temperatures) that affects one sex more than the other can generate variation in male and female body size clines and thus in size dimorphism (e.g., Dobson and Wigginton 1996;Tamate and Maekawa 2006). In the house finch, for example, sexual selection favors large males but small females, whereas winter viability selection favors large females but small males, generating consistent spatial variation in sexual size dimorphism (Badyaev and Martin 2000;; for a similar example in house sparrows, see Fleischer and Johnston 1984).…”

The relationship between foot size of wild canids and regional snow conditions: evidence for selection against a high footload?

“…Other studies (e.g. Gittleman, 1985;Iriarte et al, 1990;Sikes & Kennedy, 1992;Dobson & Wiggington, 1996) have associated similar morphological variation among carnivore populations to selective forces imposed by local environmental conditions, thus, snow may be only one of a suite of environmentally mediated selective forces acting to shape carnivore morphology.…”

Potential environmental influences on variation in body size and sexual size dimorphism among Arizona populations of the western diamond-backed rattlesnake (Crotalus atrox)

“…The Lovich and Gibbons index is intuitive and has the most favorable statistical properties of all SSD indices. For example, sexinteractions in ANOVA, or mixed models, are biased by scale effects; thus, an index is the preferred method to investigate variation in SSD among populations (Dobson and Wigginton, 1996;Smith, 1999;Stillwell et al, 2007).…”

“…Environmental gradients are important determinants of resource variation, and in many species, body size varies along gradients of temperature (Ashton and Feldman, 2003;James, 1970), primary productivity (Beaupre, 1995a), and/or seasonality (Ashton, 2001;Boyce, 1979). Additionally, body size in males and females may respond differently to environmental gradients and thus result in geographic variation in sexual size dimorphism (SSD; Dobson and Wigginton, 1996;Stillwell et al, 2007).…”