MAKIN, J/L*; HATCH, K/A; Brigham Young University; Brigham Young University: ¹⁵N enrichment of lizard uric acid and feces: can ¹⁵N be used as a measure of body condition in the wild?

Stable isotopes are used by animal physiological ecologists to determine such things as dietary sources, migration, and trophic levels. Attempts have been made to use ¹⁵N values as indicators of body condition as well. The assumption is that proteins are ¹⁵N enriched during anabolism whereas nitrogenous wastes are ¹⁵N depleted relative to diet. Catabolism then only further enriches the tissues as ¹⁵ N depleted urates are excreted. However, this model is based, in part, on only a single study of 8 cattle demonstrating ¹⁵N depletion of urea relative to diet. Because of their small size and ease with which uric acid can be collected, lizards are logical system to use to test the hypothesis that, across taxa, nitrogenous wastes are ¹⁵N depleted relative to diet. Based two studies, one of 15 side-blotched lizards (Uta stansburiana), and a second of 6 side-blotched lizards and 8 American anoles (Anoles carolinensis), we show that nitrogenous wastes are ¹⁵N depleted across taxa. We also discovered a difference in ¹⁵N between the side-blotched lizards and the American anoles. While the uric acid samples showed no difference, the feces samples of side-blotched lizards had a significantly higher ¹⁵N/¹⁴N ratio when compared to the feces of anoles.