Tissue carbon incorporation rates and diet-to-tissue discrimination in ectotherms: tortoises are really slow

Abstract Understanding carbon incorporation rates and diet-to-tissue discrimination (Δ(13)C(tissue-diet)) in animals is necessary to interpret stable isotope data collected from animals in the field. Our current understanding of the carbon dynamics in terrestrial ectotherms such as snakes, lizards, and turtles is poorly developed. Here we use a diet switch experiment to estimate carbon incorporation rates and diet-to-tissue discrimination factors in growing desert tortoises (Gopherus agassizii). Average carbon retention times for red blood cells (RBCs) and plasma were 126.7 ± 40.3 and 32.9 ± 14.5 days, respectively. Tissue carbon incorporation rates were affected by both growth and metabolism, with growth accounting for 50% of the carbon turnover in RBCs and 13% of carbon turnover in plasma. At equilibrium, scute keratin (0.8 ± 0.1) and plasma (1.0 ± 0.2) showed enriched discrimination values (Δ(13)C) compared to the test diet, but RBC Δ(13)C values were indistinguishable from diet (0.2 ± 0.3). We also found that new keratin continued to contribute significant material to previously grown keratin rings on the tortoise's shell. Changes in the δ(13)C of previously laid down growth rings indicated that the old rings closest to the region of new growth received about 73% of the carbon from the current diet; these data suggest that the interpretation of dietary history using growth rings must recognize that each ring may represent the weighted average of the diet over several seasons. These results continue to highlight the importance of laboratory experiments in interpreting isotopic data derived from field studies.