Stable isotopes (SIs) are emerging as a common tool in animal ecology. The rapid utilization of SIs raises concerns about the validity of blanket assumptions used to interpret data.  It is generally accepted that there is little trophic fractionation of ¹³C between consumers and resources, and ¹⁵N shows predictable fractionation between consumer and prey.  It has been emphasized by several authors that these fractionation values are mean estimates, and should not obscure the considerable variation among individuals.  Surprisingly, little is known about the sources of individual variation in SI fractionation, and no studies have explicitly examined whether plasticity in consumer traits affect SI patterns.  We used grey treefrog (Hyla chrysoscelis) tadpoles raised on different diets and at different densities to induce variation in growth and development rates and gut morphology.  We then examined the effects of diet and inducible changes in development rate and gut morphology on patterns of ¹³C and ¹⁵N fractionation.  We hypothesized that because diet induced changes in gut morphology increase gut transit time of food, they would reduce SI fractionation between tadpoles and their prey.
Results/Conclusions

The presence of competitors and reduced diet quality induced a larger gut. Reducing the protein content of food by 60% increased total gut volume by 76% - 125%, while quadrupling density [reducing food ration by 75%] only increased tadpole gut volume by 12 - 42%.  ¹³C and ¹⁵N fractionation were highly positively correlated, which contradicts observed field patterns of SI fractionation but is consistent with other laboratory studies.  Fractionation among tadpoles of both ¹³C and ¹⁵N ranged from -6 to 6 ‰ and -2 to 8 ‰ respectively.  Fractionation was significantly affected by tadpole diet, and varied as a function of development rate and induced gut length within and among tadpole diet treatments.  These results demonstrate that patterns of SI fractionation are not stable within tadpoles, but rather shift with changes in tadpole diet in part as a function of phenotypic change.  These results suggest it may be difficult to make inferences from SI data without accounting for the effects of diet on fractionation and phenotypic plasticity among individuals.