Starvation effects on nitrogen and carbon stable isotopes of animals: an insight from meta-analysis of fasting experiments.

Nitrogen and carbon stable isotopic compositions (N and C) of consumers have been used for physiological and food web studies. Previous studies have shown N and C values are affected by several biological and environmental factors during starvation, but the generality of the effect of starvation on N and C values has not yet been tested. Here, we performed a meta-analysis to evaluate the effects of starvation on N and C values of consumers, and the underlying factors that may explain the observed variation. The δN and δC values were calculated as the differences between the final N and C values of consumers (post-starvation) and the pre-starvation values on each experiment. Our meta-analysis showed a large variation in the δN and δC values of consumers (δN range: -0.82 to 4.30‰; mean: 0.47‰ and δC range: -1.92 to 2.62‰; mean: 0.01‰). The δN values of most consumers increased along the length of the starvation period and were influenced by nitrogen excretion and thermoregulation types, probably because differences in nitrogen metabolism and thermoregulation affect nitrogen processing and excretion rates. None of our predictor variables accounted for the variation in δC values, which showed both increases and decreases due to fasting. Our findings suggest that starvation results in changes in consumer N values which are mainly explained by the length of the fasting period and by nitrogen and energy metabolism, but the underlying mechanisms of the starvation effects on C values seem to be more complex than previously thought.