A consistent acoustic feature to discriminate Myotis species

Joseph M. Szewczak University of California White Mountain Research Station, Bishop, California 93514

Acoustic species recognition of many bats has focused on the terminal, or minimum frequency of calls. For example, with this method a Myotis californicus is described as a 50-kHz bat, and a M. ciliolabrum as a 40-kHz bat. However, high-resolution full-spectrum sonograms of calls from these bats reveal that the terminal portion of these calls consists of a downward hooked tail that fades in amplitude, and thus the minimum detected frequency can vary considerably according to the orientation and distance of the bat from the detector. For 151 M. californicus calls analyzed from 18 sequences recorded from four locations, the average minimum apparent frequency was 43.5 kHz with a range of 35.4–48.4 kHz. For 86 M. ciliolabrum calls analyzed from 11 sequences recorded from five locations, the average minimum apparent frequency was 34.8 kHz with a range of 29.7–40.9 kHz [view plot]. The substantial overlap of the minimum apparent frequency from these two species raises concern regarding the use of this characteristic for species discrimination. For this data set, 13.9% of M. californicus calls overlapped within the range of M. ciliolabrum calls, while 34.9% of M. ciliolabrum calls overlapped within the range of M. californicus calls. This suggests that recordings within the overlapping range (21.5% of all calls) should be rejected as ambiguous. However, high-resolution sonograms reveal a separate, more consistent distinguishing characteristic. For many Myotid calls, the terminal tail is preceded by a frequency modulated sweep of greater amplitude that is consistently resolved by ultrasonic recordings, even if the terminal tail is not. The frequency at which this sweep turns down to the tail, herein termed the "lower characteristic frequency," is a consistent morphological characteristic of these calls, and is not prone to variation from recording sensitivity. For the same calls described above, the lower characteristic frequency for M. californicus averaged 50.3 kHz with a range of 46.2–53.3 kHz, and for M. ciliolabrum averaged 41.8 kHz with a range of 38.6–44.7 kHz [view plot]. Thus, while the apparent minimum frequency ranges overlapped by 5.4 kHz, the lowest characteristic frequency ranges were separated by 1.5 kHz. By using the lower characteristic frequency, calls from these species can be readily and unambiguously recognized. Furthermore, many other Myotids exhibit this characteristic, suggesting a general approach for acoustic Myotid recognition.