Geographic Variation in Body and Clutch Size of Gopher Tortoises

Abstract

We present body and clutch size information for two Gopher Tortoise (Gopherus polyphemus) populations, the southernmost populations for which such information is available. We combine these data with previous studies of 17 other populations to evaluate geographic variation in body and clutch size. Our study sites were Okeeheelee County Park, Palm Beach Co., Florida (OCP), and Archbold Biological Station, Highlands Co., Florida (ABS). Mean clutch sizes were 8.2 and 6.3 and mean adult female carapace lengths 308 mm and 274 mm at OCP and ABS, respectively. At OCP, clutch size showed a polynomial relationship with body size, with largest clutches at intermediate adult body sizes. This relationship may reflect senescence. Clutch sizes at ABS were not significantly associated with body size. Body size variation among populations showed a polynomial relationship to latitude, temperature, productivity, and seasonality; body sizes were smallest at intermediate values of all independent variables. Gopher Tortoises produce larger clutches in areas that are more southern, warmer, have greater productivity, and are less seasonal. Of environmental variables, clutch size variation is best explained by productivity. Future studies of other populations, particularly from the southern part of the range, are needed to evaluate the significance of the apparently unique clutch size--body size relationship observed at OCP, and to further clarify how and why body and clutch sizes vary throughout the range.