Geographic Variation in Sexual Size Dimorphism in Painted Turtles (Chrysemys picta)

Abstract

Geographic variation in body size may reflect adaptations to local environments, and sexual size dimorphism (SSD) arises from ultimate and proximate factors acting differently on males and females in those environments. The Painted Turtle (Chrysemys picta) is a wide-ranging North American freshwater turtle species with known female-biased SSD. We hypothesized that, in more seasonal environments, the disparity between adult female and male body size would be more pronounced (i.e., the sexual dimorphism index (SDI, female body size/male body size) would be higher) than in more moderate environments because selective pressures on females to maximize reproductive output would result in relatively larger body sizes (fecundity advantage hypothesis) in extreme environments. We predicted that the SDI would be higher in populations at northern latitudes and middle longitudes than in southern and coastal populations. We conducted linear and nonlinear regression analyses using data from the literature and museum records, extrapolated data, and unpublished data on adult male and female carapace and plastron lengths from 65 locations. In contrast to our prediction, SDI decreased with increasing latitude. With respect to longitude, the trend supported our prediction in that the SDI was slightly higher for interior populations and lower for coastal populations; however, the relationship was not significant. Future research should examine sex differences in carapace height and body volume which may more directly reflect selective pressures on female fecundity than straight-line shell lengths. Geographic variation in body size may reflect adaptations to local environments, and sexual size dimorphism (SSD) arises from ultimate and proximate factors acting differently on males and females in those environments. Sexual size dimorphism can result from sexual selection, selection for increased female fecundity, and ecological niche divergence (Hendrick and Temeles, 1989; Shine, 1989; Andersson, 1994). Sexual selection occurs through male-male competition or through female mate choice, both potentially resulting in large male body size relative to females (Weckerly, 1998; Cox et al., 2003). Relatively large female body size may evolve if maximization of body cavity volume maximizes the capacity for ovarian development (Darwin, 1871; Fairbairn and Shine, 1993; Honek, 1993). Ecological niche divergence refers to environmental pressures such as sex-specific food requirements and differences in trophic struc- tures that result in body size differences (Selander, 1966; Nudds and Kaminski, 1984; Shine, 1989; Temeles et al., 2000). In addition, proximate factors such as conspecific variation in growth rates (Shine, 1990; Rutherford, 2004), age and size at maturity (Gibbons and Lovich, 1990; Shine, 1990; Lovich et al., 1998; Zuffi et al., 2006), and sex-specific patterns of mortality (Stewart, 1985; Shine, 1990; Haenel and John-Alder, 2002) have been shown to affect SSD in reptile populations. Among freshwater turtle species, conspecific pat- terns of SSD have been attributed to habitat type combined with male mating strategy (Berry and Shine, 1980), sex-specific mobility (Bonnet et al., 2001; Kaddour et al., 2008), sex-specific feeding ecology (Tucker et al., 1995; Lindeman and Sharkey, 2001; Lindeman, 2006), and female-biased SSD has been linked to fecundity advantage (Berry and Shine, 1980; Gibbons and Lovich, 1990; Brophy, 2006; Munoz