Mart�nez del Rio, C.*; Sabat, P.; Schondube, J. E.: Stable Isotope Composition as a Covariate in Comparative Physiology

When animals ingest different diets, they must process contrasting arrays of nutrients, electrolytes, toxins and water. Hence, diet is a major determinant of an animal’s digestive and osmoregulatory traits and an important independent variable in comparative studies. Characterizing diets, however, can be difficult. We propose the use of the stable isotopic composition of an animal’s tissues as an indicator of its diet. We hypothesized that stable isotopic composition can be used as a covariate in comparative studies and used two systems to examine this conjecture: phyllostomid bats and passerine birds in the genus Cinclodes (Furnariidae). We used DELTA15N as a proxy variable for protein ingestion in phyllostomid bats. Nectarivorous and frugivorous species were characterized by low values of DELTA15N, whereas insectivorous, sanguinivorous, and carnivorous species were characterized by high values. The kidney’s concentrating capacity increased with DELTA15N. In contrast, the capacity of the gut to process sugars decreased with DELTA15N. The genus Cinclodes is unique among passerines in that it includes species that live in hyperarid environments and feed on maritime environments. It also includes species that live and feed on terrestrial and fresh-water habitats. DELTA13C characterized the marine and terrestrial/fresh water diet of these birds, and hence the salt loads that they ingested. Kidney mass and relative medulary thickness, as well as the osmotic concentration of urine and excreta, were positively correlated with DELTA13C within and among species. Our two examples highlight the potential value of stable isotope composition data for comparative analyses.