The influence of sex, hormone status, and growth rate on dietary isotope fractionation: a case study with laboratory rats

Stable isotope analysis is increasingly used to discern ecological differences within and among species, especially those difficult to observe. In applied ecological studies, variations in stable isotope composition are often attributed to foraging behavior or trophic ecology rather than fractionation during metabolic processes. One physiological difference among individuals is gonadal hormone levels, which affects food intake, metabolism, and locomotor activity. However, it is unclear how these effects– isolated from ecology– affect metabolic dynamics and expression in stable isotope analysis. Here, we test the linkage between gonadal hormones and isotopic heterogeneity among individuals with captive rats. We found that sex and removal of gonadal hormones are factors either individually or interactive for δ15N and δ13C values in serum and blood as well as δ15N values of muscle and liver. Furthermore, the degree of fractionation in these tissues is related to growth rate. Gonadectomization also affected isotopic composition for liver δ13C values and kidney δ15N values, but fat δ13C values were affected only by sex. The pattern of differentiation between groups was most different for kidney δ15N and δ13C values, which also had the largest isotopic variability among groups. Overall, isotopic variation within one tissue for the four groups demonstrated up to 1‰ difference in δ13C and δ15N values suggesting that sex based within population variations should into consideration these potential physiological effects. What is already known There are isotopic differences between diet and various consumer tissues with variability among individuals, which is often characterized based on captive feeding studies. Effects of protein quality and quantity, variable growth rate based on ontogeny, and differences between sexes have been tested thus far in experiments. Furthermore, it is known that gonadal hormones affect energy regulation with difference in weight gain, which suggest differences in extent and rate of biochemical processes. What this study adds This study demonstrates effects of gonadal hormone status with metabolic biochemistry by using stable isotopes of carbon and nitrogen as dietary tracers. We find that gonadal hormone status accounts for some isotopic heterogeneity among individuals, which should be considered when interpreting stable isotope data from wild specimens. Rather than attributing isotopic differences to ecological factors, such as foraging preference, there may be underlying physiological processes that differentiate groups based on ontogeny and sex. ### Competing Interest Statement The authors have declared no competing interest.