Isotopic discrimination and indications for turnover in hair and wing membranes of the temperate bat Nyctalus noctula

Abstract

Stable isotope ratios, especially of carbon (δ 13 C) and nitrogen (δ 15 N), are often used to make predictions of an animal's diet. Next to the isotope ratios of the studied an- imal and its diet, two factors are important for the interpreta- tion of stable isotope data: the discrimination factor and the turnover rate. Both parameters are species- and tissue-specific but sparsely reported, especially for insectivorous bats. We determined the diet-tissue discrimination factors (Δ 13 Ctissue and Δ 15 Ntissue) for the insectivorous common noctule bat (Nyctalus noctula) in hairs and wing membranes. No sex- related differences in discrimination of 13 Ca nd 15 Nc ould be detected, but wing membranes were significantly less enriched in 13 C( 4.0±0.6‰) than hairs (5.9±1.3‰). However, tissues were not significantly different in Δ 15 Ntissue (Δ 15 Nwing 3.7±0.6‰ and Δ 15 Nhair 3.4±0.6‰). Furthermore, we compared δ 15 Na ndδ 13 C of wing mem- branes and hairs from individuals feeding on a mealworm diet for 7 weeks (♀♀short and ♂♂short) and for an average of 124 weeks (range 27-298; ♀♀long and ♂♂long). As for ♀♀short and ♂♂short, no molting occurred after the dietary switch; we assumed that hairs still reflect the isotopic signa- ture of their natural diet. The metabolically more active wing membranes, however, should have incorporated, at least part- ly, the isotopic signature of the mealworms. Values of δ 15 N and δ 13 C indicate that a dietary switch after 7 weeks is reflected in wing membranes but not in hair. This provides further evidence that the turnover rate of wing membranes of insectivorous bats is only a few weeks.