Map turtles and sawbacks (Emydidae: Deirochelyinae: Graptemys) are a diverse group of turtles that are of ecological interest due to their diversity in trophic morphology, particularly in females, and their extreme sexual size dimorphism (with females larger). I used comparative analyses (independent contrasts in correlation analyses and GLM analyses) to examine hypotheses regarding the evolution of body size in the genus. Evolutionary changes in body size of both males and females were positively related to inferred shifts in latitude. Trophic morphology (relative head width, expressed either as a continuous or a discrete variable) was not an additive source of variation in male body size, but was for female body size, primarily due to the large body sizes exhibited by the megacephalic southern clade (G. pulchra, G. ernsti, G. gibbonsi, and G. barbouri). Status as allopatric or sympatric to other species of Graptemys was not an additive source of variation in female body size. These results argue against the hypothesis that character displacement of body size in females was important in the radiation of map turtle and sawback species and instead suggest a functional relationship whereby degree of molluscivory in females covaries with body size. Sexual size dimorphism was found to increase with body size of females, also due primarily to the large females of the megacephalic clade; this result means Graptemys is an exception to Rensch's Rule. In a latitude-corrected analysis of body size evolution in deirochelyine turtles, the exceptional degree of sexual size dimorphism in Graptemys appears to result more from reduction in male size than increase in female size. Decreased male size may indicate relaxed selection for large body size in Graptemys males as a consequence of the fact that they rarely leave the water for terrestrial excursions, because selection for larger male body size in other deirochelyines may be mediated by (a) predation pressure imposed by terrestrial predators, (b) enhancement of overland mobility for mate searching, and (c) the need for resistance to desiccation during overland excursions. Alternatively, male body size reduction in Graptemys may be explained by energetic requirements of searching for mates in the fluvial environment or unknown differences in social structure among deirochelyine turtles.