Growth versus metabolic tissue replacement in mouse tissues determined by stable carbon and nitrogen isotope analysis Stephen E. MacAvoy, Stephen A. Macko, and Lynne S. Arneson Abstract: Stable-isotope signatures in animal tissues presumably reflect the local food web. However, that assumption may be complicated by differential nutrient routing, fractionation, and the possibility that large organisms are not in isotopic equilibrium with seasonally available food sources. Additionally, the rate at which organisms incorporate the isotopic signature of a food is largely unknown. In this study we assessed the rate of carbon- and nitrogen-isotope turnover in liver, muscle, and blood in mice (Mus musculus L., 1758) following a diet change. We report the propor- tion of tissue turnover caused by growth versus that caused by metabolic tissue replacement. Growth accounted for approximately 10% of observed tissue turnover in adult mice. Blood carbon had the shortest half-life (16.9 days), fol- lowed by muscle carbon (23.9 days). Liver carbon turnover, which was slower than blood and muscle carbon turnovers, was not as well described by the exponential decay equations. All tissues primarily reflect the protein carbon signature rather than the carbohydrate carbon signature. The nitrogen signature in all tissues was enriched by 3‰–5‰ over their diets’ nitrogen signature, depending on tissue type, and the isotopic turnover rates of nitrogen in blood and muscle were comparable with those observed for carbon. Résumé : On présume que les signatures d’isotopes stables dans les tissus animaux reflètent le réseau alimentaire lo- cal. Cependant, cette supposition peut être compliquée par le cheminement différentiel des nutriments, par la fractiona- tion et par la possibilité que les grands organismes n’aient pas atteint un équilibre isotopique avec leurs sources de nourriture qui sont disponibles sur une base saisonnière. De plus, le taux auquel les organismes incorporent la signature isotopique d’un type de nourriture est en grande partie inconnu. Dans notre étude, nous avons mesuré le taux de rem- placement des isotopes de carbone et d’azote dans le foie, le muscle et le sang de souris (Mus musculus L., 1758) après un changement de régime alimentaire. Nous présentons la proportion du remplacement de tissus causée par la croissance, par rapport à celle due au renouvellement métabolique des tissus. La croissance est responsable d’environ 10 % du remplacement de tissus observé chez les souris. Le carbone sanguin a la demi-vie la plus courte (16,9 jours), suivi du carbone musculaire (23,9 jours). Le remplacement du carbone hépatique, plus lent que celui du sang ou du muscle, n’est pas aussi bien décrit par les équations exponentielles de dégradation. Tous les tissus exhibent surtout la signature du carbone des protéines, plutôt que celle du carbone des hydrates de carbone. La signature d’azote dans tous les tissus est enrichie de 3 ‰ – 5 ‰ par rapport au régime alimentaire, selon les tissus, et les taux de remplacement isotopique observés dans le sang et dans le muscle sont comparables à ceux observés pour le carbone. [Traduit par la Rédaction] MacAvoy et al. 641 Introduction Stable-isotope analysis is rapidly becoming a highly used and powerful tool in studying animal ecology. Analysis of stable-isotope ratios of elements such as carbon, nitrogen, sulfur, hydrogen, oxygen, and others has been used to trace migratory routes (reviewed in Hobson 1999), reconstruct di- etary sources (reviewed in Fry and Sherr 1984; Lajtha and Michener 1994), and determine physiological condition of individual animals (Hobson et al. 1993). These studies are based on the fact that stable-isotope ratios of consumers re- flect the isotopic signature of the local food web (Peterson et al. 1985; Peterson and Howarth 1987; Michener and Schell 1994). Different mechanisms of carbon fixation often result in different δ 13 C signatures among primary producers (Peter- son and Howarth 1987; Ehleringer and Monson 1993; Korontzi et al. 2000). Additionally, stable-isotope ratios of nitrogen ( 15 N/ 14 N) can be used to determine trophic relation- ships between organisms. An approximate 3‰–3.5‰ in- crease in δ 15 N per trophic level in a food chain has been observed in a wide range of terrestrial and aquatic systems (Minagawa and Wada 1984; Koch et al. 1994; Michener and Schell 1994). Stable-isotope signatures in an animal’s tissues are as- sumed to reflect the isotopic signature of the local food web. To satisfy this assumption, the animal must (i) be in isotopic equilibrium with its diet, (ii) reflect the bulk isotopic signa- ture of its food and not preferentially reflect any particular molecular component (protein, carbohydrate, etc.), and (iii) must show consistent diet–tissue discrimination (Hob- Can. J. Zool. 83: 631–641 (2005) doi: 10.1139/Z05-038 © 2005 NRC Canada 631 Received 23 August 2004. Accepted 15 March 2005. Published on the NRC Research Press Web site at http://cjz.nrc.ca on 23 June 2005. S.E. MacAvoy and L.S. Arneson. American University, Department of Biology, 4400 Massachusetts Avenue NW, Washington, DC 20016, USA. S.A. Macko. University of Virginia, Department of Environmental Sciences, Charlottesville, VA 22903, USA. 1 Corresponding author (e-mail: macavoy@american.edu).