Patterns of sexual size and shape dimorphism across the native range of the green anole lizard, Anolis carolinensis

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Ken S. Toyama, Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada and Rachel M. Goodman, Biology Department, Hampden-Sydney College, Hampden-Sydney, VA

Background/Question/Methods
Sexual dimorphism is a widespread phenomenon, commonly attributed to the relative influence of sexual selection, selection for fecundity and intersexual niche divergence. Variation in these selective forces can occur over the distribution range of a species and lead to geographic variation in sexual size and shape dimorphism (SSD and SSHD, respectively). In the current study, we quantified and compared the average levels of SSD and SSHD for 17 populations of green anoles (Anolis carolinensis) spanning the entire native range of this species and tested for their relationship with climatic variables. Our calculations of SSD were based on snout-vent length (SVL) and body mass, while to estimate SSHD we considered 10 ecologically relevant morphological characters. These measurements were obtained from preserved specimens and radiographs. We defined SSD as the ratio of male to female SVL or body mass, and SSHD as the distance between sexes in a morphological space built through a principal component analysis (PCA). We used climate data from 20 years prior to the collection date to compute 11 annual averages for variables related to temperature, humidity, and precipitation, and reduced the dimensionality of the data using a PCA. We considered latitude and longitude as independent variables in our analyses. Finally, we tested the relationship between dimorphism, body size, geographical coordinates and climate using multiple regression.
Results/Conclusions
In agreement with previous studies, we found that A. carolinensis follows the opposite of Bergmann’s rule, as populations having larger body sizes were found at lower latitudes. Moreover, A. carolinensis also follows Rensch’s rule, meaning that populations exhibiting larger body size also exhibit higher SSD. Our multiple regression analyses showed that populations experiencing higher average temperatures show higher SSD, even after accounting for body size. For body shape, most of the differences between sexes were related to head traits (proportionally larger in males, attributable to sexual selection) and pelvis traits (proportionally larger in females, attributable to fecundity selection). The magnitude of these differences was positively related to body size but not to latitude, longitude or any climatic variable. In conclusion, although we showed that body size was the main predictor of SSD and SSHD in A. carolinensis across its native range, only SSD was further affected by climate. Different degrees of SSHD are thus probably a consequence of allometry alone.