Geographic Variation in Body Size and Sexual Size Dimorphism of a Seed‐Feeding Beetle

Abstract

Body size of many animals varies with latitude: body size is either larger at higher latitudes (Bergmann's rule) or smaller at higher latitudes (converse Bergmann's rule). However, the causes underlying these patterns are poorly understood. Also, studies rarely explore how sexual size dimorphism varies with latitude. Here we investigate geographic variation in body size and sexual size dimorphism of the seed-feeding beetle Stator limbatus, collected from 95 locations along a 38 degrees range in latitude. We examine 14 variables to test whether clines in environmental factors are adequate to explain geographic patterns of body size. We found that body size and sexual size dimorphism of S. limbatus varied considerably with latitude; beetles were smaller but more dimorphic at lower latitudes. Body size was not correlated with a gradient in mean temperature, contrary to the commonly accepted hypothesis that clines are produced by latitudinal gradients in temperature. Instead, we found that three factors were adequate to explain the cline in body size: clinal variation in host plant seed size, moisture (humidity), and seasonality (variance in humidity, precipitation, and temperature). We also found that the cline in sexual size dimorphism was partially explainable by a gradient in moisture, though moisture alone was not sufficient to explain the cline. Other ecological or environmental variables must necessarily contribute to differences in selection on male versus female body size. The main implications of our study are that the sexes differ in the magnitude of clinal variation in body size, creating latitudinal variation in sexual size dimorphism, and that clines in body size of seed beetles are likely influenced by variation in host seed size, water availability, and seasonality.

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

“…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).…”

“…Temperature is often discussed as the primary driver behind large-scale patterns of body size clines (Stillwell et al, 2007). In general, body sizes of endotherms are negatively (i.e., Bergmann's Rule) and ectotherms positively correlated with temperature (Ashton and Feldman, 2003;James, 1970; but see: Olalla-Tárraga et al, 2006), but there is no general agreement on the proximate determinants responsible for these patterns.…”

Geographical variation in body size and sexual size dimorphism of introduced American bullfrogs in southwestern China

“…We estimated SSD of each age group (year) for each population using the SDI index (Lovich and Gibbons 1992), where SDI = [mean size of the larger sex/(mean size of the smaller sex-1)]. This index has the best statistical properties of all dimorphism indexes that have been proposed (Smith 1999;Stillwell et al 2007). We also calculated sex ratios (male:female) of each age group for each population.…”

Sex differences in phenotypic plasticity of a mechanism that controls body size: implications for sexual size dimorphism

“…The direction and magnitude of SSD varies considerably among the major taxa and among species due to variation in the sources of selection acting in concert to create SSD: variation among taxa/species in the magnitude of sexual selection favouring large size in males (owing to male -male competition or female choice), variation in fecundity selection favouring large size in females (larger females produce more eggs) and variation in a variety of sources of selection favouring small size in both sexes ). In addition, recent studies have shown that the magnitude of SSD, but often not the direction, changes considerably among populations within species (Blanckenhorn et al 2006(Blanckenhorn et al , 2007Stillwell et al 2007a). Although much of this intraspecific variation in SSD is partly genetically based and hence due to selection, some of this variation is also probably due to a sex difference in phenotypic plasticity in body size (Fairbairn 2005;.…”

“…Although many environmental and ecological variables induce plasticity in body size and other traits of ectothermic animals (Stillwell et al 2007a;Teder et al 2008;Blanckenhorn 2009), two are particularly important in inducing plasticity in growth and life-history traits: diet quantity/quality and temperature Stillwell et al 2007b). Insects typically mature at larger sizes when raised at lower temperatures and when raised on higher quality diets (Atkinson 1994;Berrigan & Charnov 1994;Stillwell & Fox 2005;Stillwell et al 2007b;Kingsolver & Huey 2008).…”