Realistic Fasting Does Not Affect Stable Isotope Levels of a Metabolically Efficient Salamander
Stable isotopes are commonly used to examine various aspects of animal ecology. The use of stable isotopes generally proceeds under the implicit assumption that resource use is the only factor driving variation in stable isotope levels; however, a wealth of studies demonstrate that a range of common ecological factors can affect the behavior of stable isotopes in animal tissues and potentially confound inferences. For example, studies of some invertebrates and endothermic vertebrates show that animals fasted for ecologically realistic time periods have higher nitrogen (δ15N) or lower carbon (δ13C). We examined whether realistic fasting would influence the stable isotope composition of one of the most metabolically efficient ectothermic vertebrates, the Eastern Red-backed Salamander, Plethodon cinereus. We fasted salamanders for 7, 14, 21, 28, or 35-day intervals and examined whether δ15N or δ13C levels of tissues changed between fasted and fed animals. We investigated whether body condition (body mass to length and C:N [an index of lipid levels]) declined in fasted animals and whether there was a relationship between C:N and δ15N or δ13C. Body mass to length index and C:N, δ13C, and δ15N of tail and liver tissues did not differ between fasted and fed animals between 7 and 35 days. Because of their extreme metabolic efficiency, vertebrate ectotherms such as lungless salamanders (Plethodontidae) may not show the effects of fasting on stable isotopes observed in endothermic vertebrates and some invertebrates. This difference should lead to simpler interpretation of stable isotope results from field studies of these animals.Abstract
Relationship between snout–vent length (mm) and final dry body mass (mg) for fasted (r2 = 0.59; P < 0.001; dry mass = 161.1 + 7.4*SVL) and fed (control) animals (r2 = 0.42; P = 0.023; dry mass = −55.05 + 4.79*SVL). Filled squares (dotted line) represent fed (control) animals while open triangles (solid line) represent fasted animals. Mean final dry mass of fasted (123.2 ± 46.6 mg) and fed (control; 131.1 ± 19.4 mg) animals was not significantly different.
The relationship between C:N ratio and (A) δ13C and (B) δ15N for tail and liver tissue in fasted and fed (control) animals used to show differences in lipid content between the two tissues. Filled squares represent tail tissue from fed (control) animals, filled triangles represent tail tissue from fasted animals, open squares represent liver tissue in fed (control) animals, and open triangles represent liver tissue in fasted animals.
Results of general linear model examining the effects of the number of days fasted on the δ15N and δ13C of fasted and fed (control) animals for tail and liver tissue. Filled squares (dashed line) represent mean residual δ13C or raw δ15N values for animals fed every 7 days (fed [control]) while open circles (solid line) represent mean residual δ13C or raw δ15N values for animals fasted since day zero. Confidence bands represent SE bars (±1).
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