In the quest of isotope equilibrium for trophic discrimination estimation: diet-tissue dynamics in Nile crocodiles...

Stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) are increasingly employed to study the foraging ecology of ectothermic predators like crocodilians. However, accurate and precise estimations of trophic discrimination factors between diet and crocodile tissues (Δ¹³C and Δ¹⁵N) from captive experiments under controlled conditions are necessary to reliably quantify the contribution of different prey items make to their diet. The issue of an isotopically constant diet which leads to isotope equilibrium is an important factor influencing accurate estimations of diet-tissue discrimination factors. We raised Nile crocodiles (Crocodylus niloticus) under controlled experimental conditions feeding them with two isotopically distinct (but constant) diets until tissues reached isotopic equilibrium. We sampled blood (plasma and red blood cells, RBC), scute keratin and collagen, and nail tissues throughout the experiment to estimate diet-tissue discrimination factors. Overall, our estimations of average diet-tissue discrimination factors for δ¹³C were +0.2 ‰ for plasma, +0.1 ‰ for RBC, +0.2 ‰ for keratin, +1.9 ‰ for collagen, and +1.2 ‰ for nail tissue, while for δ¹⁵N values were -0.6 ‰ for plasma, +1.5 ‰ for RBC, +1.5 ‰ for keratin, +2.3 ‰ for collagen, and +1.8 ‰ for nail tissue. Body size did not have a significant effect on these tissue estimates, but plasma Δ¹⁵N was influenced slightly. Understanding these differences in ectotherm isotope ecology is crucial for interpreting trophic relationships within food webs that include animals such as reptiles.