John L. Carr and Lori B. Woosley

In Macrochelys temminckii, the males attain a larger body size than females. Other than overall size, the major secondary sexual characteristic of males is a longer pre-vent tail length than that of females. Chelydra serpentina exhibits this same sexual dimorphism, and it has been hypothesized that the elongated male pre-vent tail length can be attributed to three factors in combination: (1) most importantly there is positive allometry of the pre-vent tail region in males; (2) the male plastron shifts anteriorly with respect to the carapace; and (3) the posterior portion of the male plastron grows relatively slower than the entire plastron. In this study, we used measurements of a total of 252 turtles (19 males, 18 females, 190 juveniles, and 25 unknown adults) to examine sexual dimorphism of the tail in Macrochelys temminckii. Our data show four growth trends in relation to carapace length (our measure of overall body size): (1) the plastron grows isometrically; (2) total tail length is negatively allometric; (3) in males, pre-vent tail length is positively allometric, while in females pre-vent tail length is isometric; (4) post-vent tail length is negatively allometric in both males and females, but it is more so in males. Our data suggest that the relative position of the plastron does not play a role in the elongation of the male pre-vent tail length, rather it can be accounted for by positive allometry of the pre-vent tail region in combination with negative allometry of the post-vent tail region.
Macrochelys Session Poster

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