The Analysis of 13C/12C Ratios in Exhaled CO2: Its Advantages and Potential Application to Field Research to Infer Diet, Changes in Diet Over Time, and Substrate Metabolism in Birds

Stable isotopes are becoming an increasingly powerful tool forstudying the physiological ecology of animals. The ¹³C/¹²C ratiosof animal tissues are frequently used to reconstruct the dietof animals. This usually requires killing the subjects. Whilethere is an extensive medical literature on measuring the ¹³C/¹²Cratio of exhaled CO₂ to determine substrate digestion and oxidation,we found little evidence that animal physiologists or physiologicalecologists have applied ¹³C/¹²C breath analysis in their studies.The analysis breath ¹³C/¹²C ratios has the advantage of beingnon-invasive and non-destructive and can be repeatedly usedon the same individual. Herein we briefly discuss the medicalliterature. We then discuss research which shows that, not onlycan the breath¹³C/¹²C ratio indicate what an animal is currentlyeating, but also the animal's diet in the past, and any changesin diet have occurred over time. We show that naturally occurring¹³C/¹²C ratios in exhaled CO₂ provides quantitative measureof the relative contribution of carbohydrates and lipids toflight metabolism. This technique is ripe for application tofield research, and we encourage physiological ecologists toadd this technique to their toolbox.