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The structure of echolocation calls, and the distance over which bats perceive their environment, varies with the amount of structural clutter through which they are flying. Clutter and species had significant effects on the frequency-time characteristics of search-phase echolocation calls of northern long-eared (Myotis septentrionalis) and little brown bats (M. lucifugus). We tested an a priori derived model that predicted the pattern of differences in echolocation call variable values among clutter categories would provide insight into the relative maximum distances that bat species could perceive using echolocation. Specifically, the model predicted that species adapted to flying and foraging in cluttered habitats would have a shorter maximum perceptual distance than species adapted to flying and foraging in uncluttered habitats. The results supported this model and suggest the clutter-adapted M. septentrionalis had a shorter maximum perceptual distance than M. lucifugus, a species known to forage in a variety of habitats but mainly in uncluttered areas (i.e., over water). Using calls as the sampling unit, a neural network correctly classified >94% of the echolocation calls to species in high clutter. In medium and low clutter, >82% of the calls were correctly classified to species; however >90% correct classification was achieved by leaving <30% of calls unclassified. Researchers should develop clutter-specific call libraries to improve species classification accuracy for echolocation calls.
Key words: clutter, echolocation, holographic neural networks, identification, Myotis lucifugus, Myotis septentrionalis, study design
Order Chiroptera contains approximately 25% of the world's mammalian species richness with over 1,000 species worldwide (Fenton 1983; Findley 1993). A characteristic of the more speciose suborder, Microchiroptera (approximately 700 species-Koopman 1993), is their use of echolocation for spatial orientation and target discrimination. The structure of echolocation calls in this group is highly variable and probably a function of species' body size and foraging strategy (Aldridge and Rautenbach 1987; Barclay and Brigham 1991; Fenton 1990; Norberg and Rayner 1987). Species that forage in open, uncluttered habitats use long duration and narrow frequency bandwidth calls, whereas species that forage in cluttered habitats use shorter duration and broad frequency bandwidth calls that are better for precise localization and discrimination of targets from the background (Schnitzler and Kalko 2001). In addition to interspecific variation in echolocation call structure, significant variation can also exist intraspecifically. Such variation...