Trophic position and dietary breadth of bats revealed by nitrogen isotopic composition of amino acids

Caitlin J Campbell,David M Nelson,Nanako O Ogawa,Naohiko Ohkouchi,Yoshito Chikaraishi

doi:10.1038/s41598-017-15440-3

Abstract

Bats perform important ecosystem services, but it remains difficult to quantify their dietary strategies and trophic position (TP) in situ. We conducted measurements of nitrogen isotopes of individual amino acids (δ15NAA) and bulk-tissue carbon (δ13Cbulk) and nitrogen (δ15Nbulk) isotopes for nine bat species from different feeding guilds (nectarivory, frugivory, sanguivory, piscivory, carnivory, and insectivory). Our objective was to assess the precision of δ15NAA-based estimates of TP relative to other approaches. TPs calculated from δ15N values of glutamic acid and phenylalanine, which range from 8.3–33.1‰ and 0.7–15.4‰ respectively, varied between 1.8 and 3.8 for individuals of each species and were generally within the ranges of those anticipated based on qualitative dietary information. The δ15NAA approach reveals variation in TP within and among species that is not apparent from δ15Nbulk data, and δ15NAA data suggest that two insectivorous species (Lasiurus noctivagans and Lasiurus cinereus) are more omnivorous than previously thought. These results indicate that bats exhibit a trophic discrimination factor (TDF) similar to other terrestrial organisms and that δ15NAA provides a reliable approach for addressing questions about variation in the TP of bats that have heretofore proven elusive.

Highlights

Bats exhibit a diversity of feeding strategies, including nectarivory, frugivory, sanguivory, piscivory, carnivory, and insectivory
C-N bonds are not created or broken during the metabolic processing of a few amino acids that are only biosynthesized by autotrophs, which means that they confer little shift in δ15N values across trophic levels[20,23] and integrate the δ15N values of the autotrophs eaten by consumers in food webs
Assuming similar turnover times for trophic and source amino acids, the trophic positions (TP) of an organism can be estimated from its δ15N values of glutamic acid (δ15NGlu) and phenylalanine (δ15NPhe) as TP = (δ15NGlu − δ15NPhe + β)/trophic discrimination factor (TDF) + 1 (1)

Summary

Introduction

Bats exhibit a diversity of feeding strategies, including nectarivory, frugivory, sanguivory, piscivory, carnivory, and insectivory In doing so they carry out ecosystem services of ecological and socioeconomic importance, such as pollination and insect predation (e.g.1,2). Stable isotope ratios of carbon and nitrogen (δ13Cbulk and δ15Nbulk, respectively) in animal tissues provide a more spatiotemporally integrated and inexpensive assessment of diet, and have become an important tool to enhance understanding of the prey items and trophic positions (TP) of wildlife, including bats (e.g.4,13–16). We measured δ13Cbulk and δ15Nbulk, along with δ15NAA, from nine bat species with relatively well-characterized and specialized diets and which represent a variety of feeding guilds We use these data to assess the precision of δ15NAA-based estimates of TP relative to δ15Nbulk and estimates of TP inferred from known dietary information

Objectives

Methods

Results

Conclusion