Turnover of stable isotopes in Hokkaido brown bear (Ursus arctos yesoyensis)

Abstract

Stable isotope analysis has been used in many studies on the nutritional ecology of mammalian species. One of the merits of using stable isotope ratios as a dietary indicator is that stable isotope ratios of body tissue reflect those of foods assimilated, not just ingested foods (Hobson and Clark 1992; Hilderbrand et al. 1996; Kelly 2000). Another merit of using stable isotope ratios is that isotopic signatures indicate integrated dietary information over different time periods (e.g., weeks, months, years) because of the different turnover rates of dietderived stable isotopes in consumer tissues (Tieszen et al. 1983; Hobson and Clark 1992; Hilderbrand et al. 1996). Results of analyses of fecal and stomach contents, which many wildlife researchers have relied on, have problems such as overestimation of hardly-digestible diets or diets consumed largely at some particular moment (Hewitt and Robbins 1996). These shortcomings of traditional techniques have been overcome by the development of stable isotope analysis methods, and stable isotope analysis has provided deeper insights for nutritional ecology of wildlife (Hilderbrand et al. 1996; Hilderbrand et al. 1999a; Hilderbrand et al. 1999b; Jacoby et al. 1999; Felicetti et al. 2003b). Studies on mammals in which stable isotopes are used as dietary tracers require estimation of how long it takes for stable isotopes in tissues to be replaced by isotopes derived from the diet. When we estimate food habits using stable isotopes, accuracy of estimation depends on how appropriately sources in the mass balance models are selected (Minagawa 1992; Phillips and Gregg 2003), and knowledge of the isotopic turnover rates is necessary for appropriate selection of the sources. When we conduct experiments to estimate isotopic discrimination values, information on isotopic turnover rates is essential to decide periods of feeding trials (Hilderbrand et al. 1996; Felicetti et al. 2003b). Food habits of the Hokkaido brown bear have been investigated by fecal or stomach contents analysis (Aoi 1985; Sato 2002). However, shortcomings in the analysis method mentioned earlier would be more significant for omnivorous species like brown bears. Introduction of a method for stable isotope analysis for this species is needed. Moreover, recent research revealed the importance of nutritional ecology of ursid (bear) species for ecosystem conservation (Hilderbrand et al. 2004; Robbins et al. 2004). Nutritional ecological research on bears in Japan would also be necessary for better ecosystem conservation in Japan. When we treat hibernating ursid species, physiological changes during hibernation should be considered. In this paper, we report the results of experiments estimating isotopic turnover rates as well as determining possible seasonal fluctuation in turnover rates of captive Hokkaido brown bears.