[ot] tree frog's use of acoustics

["j.frede" <jfrede@xxxxxxxxxxxxxxxxxx>]

some one posted this to the phonography list, its really interesting,

j.frede
current location:los angeles
http://ritualdocument.com/jfrede
po.box 292045 los angeles, ca 90029
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Los Angeles Times
December 7, 2002

SCIENCE FILE / IN BRIEF

Male Frogs Make the Most of Acoustics

Like a teenager hoping his car stereo will attract the attention of girls,
male tree frogs in Borneo appear to take advantage of acoustics to attract
mates.

Researchers have found that males of a species of inch-long tree hole frogs
tune their high-pitched mating calls to take maximum advantage of the
acoustics of the holes in which they live and sing to fe- males.

Frogs placed in artificial holes changed the pitch of their singing to
sound attractive to females, according to the study published in this
week's issue of Nature.

****

Nature 420, 475 (2002); doi:10.1038/420475a

Animal communication: Tree-hole frogs exploit resonance effects

These anurans know a trick or two when it comes to the romantic powers of
song.

Animal mating calls that exert a comparatively high sound pressure
propagate over greater distances and generally have greater attractive
power (1, 2). Here we show that calling male Bornean tree-hole frogs
(Metaphrynella sundana) actively exploit the acoustic properties of
cavities in tree trunks that are partially filled with water and which are
primarily used as egg-deposition sites. By tuning their vocalizations to
the resonant frequency of the hole, which varies with the amount of water
that it contains, these frogs enhance their chances of attracting females.

In a simulated tree-hole experiment, we placed a calling male in an opaque
plastic tube that was partially filled with water. The frog's vocalizations
were recorded the following night as the water level was slowly reduced
(increasing the air column from 50 to 144 mm) over a period of 28 min. We
measured the change in air-column depth from a graduated cylinder connected
to the tube, and later sampled time segments from the recording at regular
intervals.

The frog's call is a simple tonal pulse with little or no frequency
modulation; the fundamental frequency is the dominant one. We analysed the
call's pitch in relation to the air-column depth in dynamic resonance units
(DRU), calculated as the call pitch at any point divided by the resonant
frequency at that point. Resonance was predicted to occur at 1 DRU
(corresponding to an air-column depth of one-quarter of the wavelength of
the current pitch).

A peak in sound pressure was evident at about 0.95 DRU (Fig. 1a). The pulse
duration peaked when the resonance effect was strongest (Fig. 1b).
Inspection of oscillograms indicated that this was not merely due to an
echo effect but to the production of longer call pulses (results not
shown). Also, the interval between call pulses was shortest when the sound
pressure peaked (Fig. 1b). Both of these call alterations are energetically
costly (3) and are known to increase male attractiveness in many amphibians
(4-6).

In addition to its flexibility in calling effort, the frog initially called
with a varying pitch until it found the frequency that resulted in maximum
amplitude. It then managed to track closely the falling water level in its
resonance-matching over a period of several minutes, gradually lowering its
pitch by about 115 Hz (Fig. 1c). The frog then apparently lost track of the
optimal call pitch and resumed calling with an erratic pitch. Although this
result relates to only one individual, the most likely explanation for this
striking pattern is that the frog was actively tracking the resonant
frequency of the tube.

Our field recordings of males calling from natural tree holes also show
this correlation between pulse duration and interpulse interval, and
between pulse duration and call pitch variability (Pearson correlation
coefficient of -0.245, n = 238, P < 0.001; and correlation coefficient of
-0.237, n = 237, P < 0.001, respectively; pitch variability is given as
pitch coefficient of variance based on 10  4 (mean  s.d.) consecutive call
pulses). This indicates that frogs in natural tree holes also increase
their calling effort if they can adjust their call pitch to match the
resonant properties of the hole.

Several crickets (7-9) and burrowing frogs (10, 11) benefit from sound
amplification by calling from baffles or burrows. However, to our knowledge
this is the first evidence of an animal not only sampling resonance
properties (12) but also facultatively adjusting its call pitch and calling
strategy in what seems to be an adaptive manner. Our findings indicate that
animals may be better at exploiting signal-enhancing structures than was
previously appreciated, and that flexibility in related, but not obligately
linked, traits may also follow from such adaptive strategies.

* Division of Amphibians and Reptiles, Field Museum of Natural History,
Chicago, Illinois 60605, USA
? Research and Education Division, Sabah Parks, PO Box 10626, 88806 Kota
Kinabalu, Sabah, Malaysia
e-mail: bjorn.lardner@xxxxxxxxxxxxx

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Competing financial interests: declared none.

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