PILGRIM, M.A.; BOYD, A.A.; MARTIN, A.E.; RICKMYRE, J.L.; FARRELL, T.M.; Univ. of Arkansas, Fayetteville; Stetson Univ., DeLand, FL; Stetson Univ., DeLand, FL; Stetson Univ., DeLand, FL; Stetson Univ., DeLand, FL: Experimental validation of stable isotope techiques in determining reptile and amphibian diet.

Isotope ratios of common elements are often used in studies of energy flow and trophic structure without a complete understanding of isotope dynamics in specific organisms. We tested the hypothesis that stable isotopes can be used as dietary indicators with a controlled diet experiment using lab colonies of pigmy rattlesnakes (Sistrurus miliarius), corn snakes (Elaphe guttatta), and horned frogs (Ceratophrys cranwelli). We randomly assigned each animal to one of two isotopically distinct diets. At the start of the experiment, we collected a tissue sample from each animal and fed each animal its first experimental meal. We then fed each animal every 10 days and collected a tissue sample every 15 days for a period of 2.5 months. Next, we switched the animals� diets. For each colony, tissue delta 13C and delta 15N values diverged from their initial values, approached each set of diet isotope ratios, tracked the diet switch at day 60, and again approached diet isotope ratios. At equilibrium, the nitrogen enrichment values for each colony were less than 2.3 parts/mil. Unfortunately, most studies using stable isotopes as trophic indicators assume at least a 3.0 parts/mil nitrogen enrichment. We conducted a subsequent experiment using more complex diets (using combinations of three isotopically distinct prey). In this experiment, a mixing model could accurately predict diet composition of animals fed a low diversity of prey, but did not adequately predict the diet of animals fed a high diversity of prey. Furthermore, this experiment showed that incorrect assumptions about trophic enrichment yielded strongly biased estimates of diet composition. Our results have strong implications for field research.