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Effects of growth and tissue type on the kinetics of 13C and 15N incorporation in a rapidly growing ectotherm

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The use of stable isotopes to investigate animal diets, habitat use, and trophic level requires understanding the rate at which animals incorporate the 13C and 15N from their diets and the factors that determine the magnitude of the difference in isotopic composition between the animal’s diet and that of its tissues. We determined the contribution of growth and catabolic turnover to the rate of 13C and 15N incorporation into several tissues that can be sampled non-invasively (skin, scute, whole blood, red blood cells, and plasma solutes) in two age classes of a rapidly growing ectotherm (loggerhead turtles, Caretta caretta). We found significant differences in C and N incorporation rates and isotopic discrimination factors (Δ13C = δ13Ctissues − δ13Cdiet and Δ15N = δ15Ntissues − δ15Ndiet) among tissues and between age classes. Growth explained from 26 to 100% of the total rate of incorporation in hatchling turtles and from 15 to 52% of the total rate of incorporation in juvenile turtles. Because growth contributed significantly to the rate of isotopic incorporation, variation in rates among tissues was lower than reported in previous studies. The contribution of growth can homogenize the rate of isotopic incorporation and limit the application of stable isotopes to identify dietary changes at contrasting time scales and to determine the timing of diet shifts. The isotopic discrimination factor of nitrogen ranged from −0.64 to 1.77‰ in the turtles’ tissues. These values are lower than the commonly assumed average 3.4‰ discrimination factors reported for whole body and muscle isotopic analyses. The increasing reliance on non-invasive and non-destructive sampling in animal isotopic ecology requires that we recognize and understand why different tissues differ in isotopic discrimination factors.

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Without the insights and assistance of A. B. Bolten, this study would not have been possible. The authors thank L. Chapman, D. Hodell, and B. MacFadden for their assistance and support with this project and F. Davis, P. Eliazar, T. Garcia, H. Jacobson, S. Luciano, N. Osman, J. Pfaller, J. Sankar for their assistance with the logistics of turtle care and sample collection and preparation. We thank the University of Florida, Geological Sciences, Light Stable Isotope Lab and J. Curtis for assistance with stable isotope analyses. We also thank C. Layman for his helpful comments on this manuscript. This project was funded by Disney Wildlife Conservation Fund, National Marine Fisheries Service, US Fish and Wildlife Service, and Florida Fish and Wildlife Conservation Commission (FWC) Marine Turtle Grants Program and was conducted under permits issued by FWC (TP# 016) and the University of Florida Institutional Animal Care and Use Committee (permit Z094). C. Martínez del Rio was supported by NSF grant ISBN (0421738).

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Correspondence to Kimberly J. Reich.

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Communicated by Craig Osenberg.

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Reich, K.J., Bjorndal, K.A. & Martínez del Rio, C. Effects of growth and tissue type on the kinetics of 13C and 15N incorporation in a rapidly growing ectotherm. Oecologia 155, 651–663 (2008).

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  • Diet shift
  • Growth
  • Isotopic discrimination
  • Isotopic turnover
  • Caretta caretta