Sexual Dimorphism and Seasonal Variation of Reproductive Hormones in the Pascagoula Map Turtle,Graptemys gibbonsi

Abstract

The Pascagoula Map Turtle (Graptemys gibbonsi) is a narrowly endemic species found only in the Pascagoula River drainage in Mississippi. It is among the most poorly known turtle species because of research taxonomic biases and this species' relatively recent recognition as a unique taxon. A recent petition requested protective status for G. gibbonsi under the U.S. Endangered Species Act. We describe population parameters, quantitatively assess sexual dimorphism of G. gibbonsi, and document hormone secretion patterns from the Chickasawhay and Leaf rivers in Mississippi. We demonstrate a significant male-skewed sex ratio and a female-biased size dimorphism in both carapace length and height. Males showed a bimodal peak of plasma testosterone in fall and spring, consistent with the pattern shown by many other southeastern turtles with late summer-fall spermatogenesis and mating during spring and fall. Females did not show seasonal variation in estradiol secretion, an unexpected result that was possibly due to our small sample size of females, none of which were gravid when captured. Although this observation may be due to our limited capacity to sample females, given the reproductive issues reported for Graptemys flavimaculata from the same drainage (e.g., reproductive hormone abnormalities, low nesting frequency and success), this finding warrants concern and necessitates additional research. Finally, in order to put our hormone data in context, we briefly review hormone and reproductive patterns in southeastern U.S. turtles. Our review includes the timing of follicular enlargement, ovulation and nesting, clutch frequency, and estradiol cycles. The review for male turtles includes details on the spermatogenic cycle, spermiation, and the timing and frequency of testosterone peaks.