In the quest of isotope equilibrium for trophic discrimination estimation: diet–tissue dynamics in Nile crocodiles ( <i>Crocodylus niloticus</i> )

Stable isotopes of carbon (<i>δ</i>¹³C) and nitrogen (<i>δ</i>¹⁵N) 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 (<i>Δ</i>¹³C and <i>Δ</i>¹⁵N) 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 (<i>Crocodylus niloticus</i>) 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 <i>δ</i>¹³C 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 <i>δ</i>¹⁵N 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 <i>Δ</i>¹⁵N 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.