Daily Energy Expenditure in Free-ranging Gopher Tortoises (Gopherus Polyphemus)
Studies of ecological energetics in chelonians are rare. Here, we report the first measurements of daily energy expenditure (DEE) and water influx rates (WIRs) in free-ranging adult Gopher Tortoises (Gopherus polyphemus). We used the doubly labeled water (DLW) method to measure DEE in six adult tortoises during the non-breeding season in south-central Mississippi, USA. Tortoise DEE ranged from 76.7–187.5 kJ/day and WIR ranged from 30.6–93.1 ml H2O/day. Daily energy expenditure did not differ between the sexes, but DEE was positively related to body mass. Water influx rates varied with the interaction of sex and body mass. We used a log/log regression model to assess the allometric relationship between DEE and body mass for Gopher Tortoises, Desert Tortoises (Gopherus agassizii), and Box Turtles (Terrapene carolina), the only chelonians for which DEE has been measured. The slope of this allometric model (0.626) was less than that previously calculated for herbivorous reptiles (0.813), suggesting that chelonians may expend energy at a slower rate per unit of body mass compared to other herbivorous reptiles. We used retrospective power analyses and data from the DLW isotope analyses to develop guidelines for sample sizes and duration of measurement intervals, respectively, for larger-scale energetic studies in this species.Abstract
Scaling relationships of (A) daily energy expenditure (kJ/d) and (B) water flux (mL/d) on body mass (g) for Gopher Tortoises, Desert Tortoises, and Box Turtles. Points represent mean values for the gender of each species for measurements made during summer months only. Energy expenditure, water flux, and body mass data for male and female Box Turtles from Penick et al. (2002). Energy expenditure, water flux, and body mass data for male Desert Tortoises interpolated from figures appearing in Peterson (1996). Energy expenditure and body mass data for female Desert Tortoises was taken from tables and text appearing in Henen (1997), while water flux data for female Desert Tortoises was interpolated from figures appearing in Henen et al. (1998)
Contributor Notes
(PGRJ) USGS-South Carolina Cooperative Fish and Wildlife Research Unit, G27 Lehotsky Hall, Clemson University, Clemson, South Carolina 29634; (DME) The Nature Conservancy, Camp Shelby Field Office, CSTS-ENV, Building 6678, Camp Shelby, Mississippi, 39407; and (GHV) Centre for Isotope Research, University of Groningen, Zoological Laboratory, The Netherlands. Present address: (DEM) Minerals Management Service, Environmental Sciences Section, MS 5432, 1201 Elmwood Park Blvd., New Orleans, Louisiana 70123-2394. (PGRJ) pjodice@clemson.edu Send reprint requests to PGRJ