Sexual difference in habitat use of Texas map turtles (Emydidae: Graptemys versa) and its relationship to size dimorphism and diet

Sexual differences in habitat use of map turtles (Graptemys spp.) have been attributed to differences in swimming ability as influenced by body size, because females are much larger than males, or to sexual differences in diet. Captures of young female Graptemys versa, which had body sizes similar to those of adult males but diet and trophic morphology more similar to those of larger females, allowed testing of these alternative hypotheses. A variety of single habitat variables measured at the sites of capture performed poorly in separating the three groups of turtles, but multivariate analysis and variables relating to position within the stream produced greater separation, indicating that complex combinations of factors probably influence habitat use. Young females were more similar in their habitat use to large females than to males, and their diet was also more similar to that of larger females, due primarily to the quantities of mollusks consumed. The data supported the hypothesis that habitat separation was the result of dietary rather than body size differences. While dietary differences are probably facilitated by sexual size dimorphism, they may not be the ultimate selective force that produced the size dimorphism.