干旱区生态环境知识资源中心

Growth is a significant contributor to tissue stable isotope incorporation rates, particularly in ectotherms. However, few data exist describing how growth affects rates of carbon incorporation during discrete periods of growth and development within a species. Here we use a bi-directional diet switch experiment to estimate carbon incorporation rates and diet-to-tissue discrimination factors in two life-stages of Desert Box Turtles (Terrapene ornata luteola) growing at different rates. The younger more rapidly growing turtles had shorter carbon half lives in plasma (34 d) and red blood cells (RBCs; 69 d), relative to older, more slowly growing animals (61 d for plasma, while carbon isotope ratios in RBCs did not achieve equilibrium). Plasma carbon turnover was minimally influenced by growth in juvenile (25% +/- 7%) and sub-adult (20% +/- 7%) turtles. Growth was an important driver for RBC carbon incorporation rates, accounting for 50% +/- 22% in juvenile turtles. At equilibrium, diet-to-tissue discrimination factors varied in turtles feeding on a C-3 plant vs. a C-4 plant-based diet, a phenomenon probably related to differences in diet quality. We also found that the diet treatment affected the extent to which isotope ratios in the current diet influenced the stable isotope ratios in previously accrued growth rings (63-70% for turtles eating the C-3 diet vs. 32-37% for turtles eating the C-4 diet.). This study significantly adds to the comparative data available for reptiles, and increases our knowledge of how growth, developmental stage, and diet quality affect stable isotope dynamics in these organisms.