Shizuko Hiryu a,b,1, Mary E. Bates c, James A. Simmons b,d,1, and Hiroshi Riquimaroux a,b
-Author Affiliations
aFaculty of Life and Medical Sciences and
bBio-navigation Research Center, Doshisha University, Kyotanabe 610-0321, Japan; and
cDepartment of Psychology and
dDepartment of Neuroscience, Brown University, Providence, RI 02912
Edited by Nobuo Suga, Washington University, St. Louis, MO, and approved February 19, 2010 (received for review January 12, 2010)
Abstract
Sonar broadcasts are followed by echoes at different delays from objects at different distances. When broadcasts are emitted rapidly in cluttered surroundings, echo streams from successive broadcasts overlap and cause ambiguity in matching echoes to corresponding broadcasts. To identify reactions to ambiguity in clutter, echolocating bats that emit multiple-harmonic FM sounds were trained to fly into a dense, extended array of obstacles (multiple rows of vertically hanging chains) while the sonar sounds the bat emitted were recorded with a miniature radio microphone carried by the bat. Flight paths were reconstructed from thermal-infrared video recordings. Successive rows of chains extended more than 6 m in depth, so each broadcast was followed by a series of echoes from multiple rows of chains that lasted up to 40 ms. Bats emitted sounds in pairs (“strobe groups”) at short (20–40 ms) interpulse intervals (IPIs) alternating with longer IPIs (>50 ms). For many short IPIs, the stream of echoes from the first broadcast was still arriving when the second broadcast was emitted. This overlap caused ambiguity about matching echoes with broadcasts. Bats shifted frequencies of the first sound in each strobe group upward and the second sound downward by 3–6 kHz. When overlap and ambiguity ceased, frequency shifts ceased also. Frequency differences were small compared with the total broadcast band, which was 75–80 kHz wide, but the harmonic structure of echoes enhances the differences in spectrograms. Bats could use time–frequency comparisons of echoes with broadcasts to assign echoes to the corresponding broadcasts and thus avoid ambiguity.
bat sonar clutter interference echo delay FM sounds
Footnotes
1To whom correspondence may be addressed. E-mail:shiryu@mail.doshisha.ac.jp or james_simmons@brown.edu.
Author contributions: S.H., M.E.B., J.A.S., and H.R. designed research; S.H., M.E.B., J.A.S., and H.R. performed research; S.H. contributed new reagents/analytic tools; S.H., M.E.B., J.A.S., and H.R. analyzed data; and S.H., M.E.B., J.A.S., and H.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
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