Academic journal article The American Midland Naturalist

Variation in the Echolocation Calls of Little Brown Bats (Myotis Lucifugus) in Response to Different Habitats

Academic journal article The American Midland Naturalist

Variation in the Echolocation Calls of Little Brown Bats (Myotis Lucifugus) in Response to Different Habitats

Article excerpt

ABSTRACT.-

Among different bat species, echolocation call structure varies predictably according to habitat use. Much of this interspecific variation in echolocation calls reflects the physical constraints of sound propagation and echo formation in open versus spatially complex habitats. Bats must use calls that are suitable for detecting obstacles and prey in a particular setting; thus, bats that use similar habitats and catch similar prey produce similar calls. The same can be true for variation in echolocation calls within a single species that uses a variety of habitats. I recorded the echolocation calls and habitat use of a habitat generalist bat, Myotis lucifugus in order to determine whether the echolocation calls of M. lucifugus vary predictably across habitats, and if call variation is the result of individual flexibility in habitat use and echolocation behavior. I found that M. lucifugus used calls with higher frequencies, shorter durations and steeper frequency modulation in cluttered habitats than in open habitats. This type of call is consistent with a short-range target-detection strategy whereas the lower frequencies, longer call durations and shallower frequency modulation of bats in open habitats are consistent with longer-range target detection strategies. Furthermore, radiotelemetry indicated that individuals routinely foraged in different habitats. Taken together, these results suggest that call variation across habitats is the result of individual plasticity in echolocation behavior.

INTRODUCTION

Insectivorous bat species that use similar foraging strategies tend to have acoustically similar echolocation calls (Aldridge and Rautenbach, 1987; Kalko, 1995; Siemers et al., 2001). Experiments and observational studies have shown that particular call types are better suited for specific types of target detection (e.g., Barclay, 1986; Surlykke and Moss, 2000; Jensen et al., 2001; Schnitzler et al., 2003). In particular, species that forage in open areas tend to produce echolocation calls that optimize long-range detection of large insects (Siemers et al., 2001; Schnitzler et al., 2003). These bats use relatively long, low frequency, narrowband calls. On the other hand, species that forage amidst obstacles must use a strategy that allows them to detect both prey and nearby obstacles. Many of these bats use short, high frequency, wideband calls (Aldridge and Rautenbach, 1987; Kalko, 1995; Broders et al., 2004).

What is true for different species in different habitats may also be true for individual species that forage in both open and spatially complex areas: species that use a variety of habitats may also use habitat-specific echolocation calls. Indeed, many bat species are known to use different echolocation calls in different habitats (Kalko and Schnitzler, 1993; Obrist, 1995; Jensen and Miller, 1999; Tibbels, 1999; but see Jensen et al., 2001), and in some instances, this call variation is known to be the result of individual flexibility rather than inter-individual differences in habitat use and call behavior (Zbinden, 1989).

In a field study, I addressed two hypotheses concerning bat behavior. The first is that bat species found in a variety of habitats use calls that favor short-range detection strategies in spatially complex areas and different calls that favor long-range detection of prey in open areas. The alternative hypothesis is that such species use a single echolocation strategy in all situations. In other words, a short-range call strategy that is necessary in cluttered habitats may suffice in open areas as well. The second hypothesis is that individual bats move between habitats, rather than specializing on one habitat type or another. If calls do vary across habitats (hypothesis 1) and individuals do move between habitats (hypothesis 2), then it can be inferred that intraspecific variation in echolocation among habitats is the result of within-individual, rather than between-individual, variation in echolocation. …

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