In addition to factors related to body size and growth rate, isotopic turnover rates
vary among tissue types. Carleton and Martínez del Rio (2005) hypothesized that protein turnover is the primary determinant of isotopic turnover rate for the most commonly used tissues in isotopic ecology, especially since samples are typically lipid-extracted prior to analysis. While this prediction has not been tested by simultaneously measuring protein turnover and isotopic turnover in the same organism, there are data from the laboratory and field studies that suggest a close link between these processes. The first is the observation that splanchnic JNK inhibitor in vitro organs (e.g., liver) and plasma proteins, which have relatively Roscovitine order high rates of protein turnover, also have higher isotopic turnover rates than structural elements (e.g., collagen, striated muscle). Second, several studies have shown that protein intake, or the amount of dietary nitrogen is positively correlated with isotopic turnover rates. Because pinnipeds, cetaceans, and sea otters consume high quality, nitrogen-rich carnivorous diets, protein
intake rate is not likely to be an important source of variation in isotopic turnover. Diet quality could be an important factor for sirenians, which consume nitrogen-poor sea grass and algae. A relatively new contribution to the discussion of isotopic turnover is the concern that multiple isotope pools may exist within an organism and each of these pools may have different turnover rates. Ayliffe et al. (2004) were the first to discuss this issue when interpreting carbon isotope turnover in tail hair and breath CO2 from domestic horses. They
were able to isolate three carbon pools with distinct turnover rates ranging from MCE fast (t1/2 ∼ 0.2–0.5 d) to slow (t1/2 ∼ 50–140 d). Cerling et al. (2007) refined this approach further by presenting the “reaction-progress variable” as a method for determining whether isotopic turnover was best expressed using a single exponential function or by using multiple linear functions, an approach that has been effectively used in geochemical studies. Martínez del Rio and Anderson-Sprecher (2008) and Carleton et al. (2008) have evaluated the necessity of this approach by quantifying the uncertainty inherent in estimates of isotope retention by multicompartment models and by testing whether multicompartment models are more effective than single-compartment models. They argued that the appropriate model may depend upon the type of tissue. The significance of the these findings has yet to be determined for isotopic incorporation studies for marine mammals; turnover rates are determined by diet-switching experiments, which are difficult to perform on marine mammals, so few studies have produced data on isotopic turnover for metabolically active tissues (Table 1, Zhao et al. 2006, Newsome et al. 2006, Orr et al. 2009).