Convergent Acoustic Field of View in Echolocating Bats
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1
University of Southern Denmark, Denmark
Most echolocating bats exhibit a strong correlation between body size and sonar frequency. In general, the smaller the species, the higher the frequency of its echolocation calls. This relationship has been attributed to size-signal allometry or acoustic detection constraints imposed on wavelength by preferred prey size. We propose a novel hypothesis: Smaller bats emit higher frequencies to achieve sufficiently narrow sonar beams. A sound beam is narrower (i.e. more directional) the shorter the wavelength relative to the emitter size. Thus, the smaller the bat and its emitter, the mouth, the shorter wavelength is required to produce a narrow beam, driving up the sonar frequency. We found that in a flight room mimicking closed habitat six aerial hawking vespertilionid species (ranging in size from 4-26 grams) produced sonar beams of extraordinarily similar shape and volume. That is each species had a horizontal and vertical directionality index (DI) of 11±1dB (i.e., half-amplitude angle of approximately 38º) and on-axis sound level of 104±3dB RMS (re. 20 μPa @ 10 cm). Therefore all bats adapted their calls to achieve the same acoustic field of view (FOV). We suggest that a flexible, context-specific FOV has been an overlooked constraint for echolocating bats and that the need for high directionality - not prey size or body size per se - has forced bats to use higher frequencies the smaller they are. Our results reveal echolocation as not just a simple substitute for vision, but as a dynamic system, allowing different species, regardless of body size, to converge upon optimal FOV according to habitat and task.
Figure 1. Vertical (left) and horizontal (right) directionality for P. pygmaeus, M. daubentonii, V. murinus, M. dasycneme, E. serotinus and N. noctula, measured at their respective peak frequencies. Solid grey lines indicate the much narrower directionality of M. daubentonii when flying in open field and the grey line outside the data points indicates the broad directionality this species uses in the last phase of prey pursuit.
Keywords:
BAT,
Directionality,
Echolocation
Conference:
Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.
Presentation Type:
Poster (but consider for student poster award)
Topic:
Sensory: Audition
Citation:
Jakobsen
L,
Ratcliffe
JM and
Surlykke
A
(2012). Convergent Acoustic Field of View in Echolocating Bats.
Conference Abstract:
Tenth International Congress of Neuroethology.
doi: 10.3389/conf.fnbeh.2012.27.00243
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Received:
30 Apr 2012;
Published Online:
07 Jul 2012.
*
Correspondence:
Dr. Lasse Jakobsen, University of Southern Denmark, Odense M, Denmark, lasse@biology.sdu.dk