Isotopic characterization of bat populations and evidence of altitudinal migration in Central Peru
Altitudinal migration by birds is known to occur in tropical forests and it is likely associated with tracking food resources such as fruits and nectar. It has been suggested that Neotropical bats that depend on similar resources could exhibit a similar migration pattern. However, evidence based on the use of traditional techniques to study migratory movements, is circumstantial and limited to changes in population sizes and recaptures of few banded individuals. In our study, we used a fairly recent technique, the analysis of stable isotopes, to first describe the triple-isotopic composition (ranges, gender effect, and trophic effect in values of hydrogen, carbon, and nitrogen) of 7 species (6 frugivores and 1 sanguinivore) collected at 3 altitudes (1500, 1800, and 2300 m) in the Central Andes of Peru. Then, we evaluated potential altitudinal movements by comparing expected hydrogen isotope values from precipitation data with observed isotopic values in bat hair.
Greater intraspecific variation in isotopes (hydrogen, 85‰; nitrogen, 4.29‰; carbon, 15.85‰) was found at low altitudes (1500 m), which likely corresponds with greater resource variety as compared to high altitudes which contains fewer plants and/or fruits. Differences in nitrogen isotopes among groups with different diets are known to occur and this pattern also extends to hydrogen profiles. Our results show that trophic differences in hydrogen between frugivores and sanguinivores ranged between 35.88‰ and 45.49‰. These are the first values reported for these species and agree with recent observations in other Neotropical bats. No significant differences were found in any isotopes between genders of the two species tested (Sturnira erythromos and Carollia brevicauda). Hair keratin of S. erythromos captured at both 1800 and 2300 m was more enriched (positive) in hydrogen than expected. This may indicate that keratin was synthesized at higher elevations, potentially between 70 to 680 m away from the capture sites. This is a small distance for a bat; in addition, intraspecific variation in hydrogen was found to be too large to make further conclusions about migratory movements.