CSIRO PUBLISHING www.publish.csiro.au/journals/ajz Australian Journal of Zoology , 2004, 52, 635–647 © CSIRO 2004 10.1071/ZO04029 0004-959X/04/060635 Effects of diet change on carbon and nitrogen stable-isotope ratios in blood cells and plasma of the long-nosed bandicoot (Perameles nasuta) Marcel Klaassen A , Michele Thums B,C and Ian D. Hume B,D A Netherlands Institute of Ecology (NIOO-KNAW), Centre for Limnology, Department of Plant–Animal Interactions, PO Box 1299, 3600 BG Maarssen, The Netherlands. B School of Biological Sciences A08, University of Sydney, NSW 2006, Australia. C Present address: School of Zoology, University of Tasmania, Hobart, Tas. 7001, Australia. D Corresponding author. Email: ianhume@bio.usyd.edu.au Abstract Carbon (C) and nitrogen (N) stable isotopes offer a powerful tool for assessing the extent of tissue assimilation of dietary components. However, the method relies on knowledge of diet–tissue isotopic discrimination and how quickly diet shifts become apparent in various tissues. In the present study, blood plasma and blood cells, tissues that are easily obtained under field conditions, were used to validate the stable isotope method over a period of 4–5 weeks using captive long-nosed bandicoots (Perameles nasuta). Diet–tissue discrimination effects appeared to be small. For C, derived diet–tissue isotopic discriminations were 1.4‰ for blood plasma and –0.2‰ for blood cells. For N the values were 2.8‰ and 2.1‰, respectively, and were independent of the nitrogen content of the food. C and N turnover measurements in the blood plasma and cells of the bandicoots indicated that blood plasma provides dietary information integrated over a period of ~3 weeks, whereas blood cells give an impression of the assimilated diet over a period of as much as half a year. These turnover rates were low compared with the little information available for birds and eutherian mammals, and probably relate to the typically low metabolic rate of marsupials. ZO04029 Stable-isot ope rati osinbandicoot bl od M. Kl assen etal. Introduction Carbon and nitrogen stable-isotope ratios ( 13 C/ 12 C and 15 N/ 14 N) are increasingly used in studies of the diet and nutritional ecology of animals (e.g. Hobson and Clark 1992a, 1992b; Kelly 2000; Klaassen et al. 2001). All studies using C and N stable isotopes rely on the premise that these isotopes in the diets of animals are ultimately reflected in their tissues (i.e. you are what you eat: Gannes et al. 1997). However, during assimilation of dietary C and N into tissue, the preferential uptake of either the lighter or the heavier isotope of an element (isotopic discrimination) might occur. This will result in isotopic ratios in the tissues that are different from those in the diet. Moreover, different tissues might have different enrichments relative to the isotopic ratios in the diet. Studies comparing discrimination effects among and within species have identified various factors that affect the level of discrimination; these factors include body size, digestive physiology, diet quality (e.g. Tieszen and Boutton 1989), trophic level (e.g. Lee-Thorp et al. 1989) and even age (Roth and Hobson 2000). Sampling blood is a convenient means of obtaining animal tissues because it seldom invokes intolerable stress to the animal under study and can be done repeatedly, allowing the study of changes in stable-isotope ratios over time. Although many studies have estimated the C and N stable-isotope diet–tissue discrimination of whole blood, to date only four studies (on two birds and four eutherian mammals) have estimated the C and N stable-isotope diet–tissue discrimination of the two major blood components: plasma and cells (Hobson and Clark 1993; Roth and Hobson 2000; Lesage et al. 2002; Pearson et al. 2003).