Biology Reference
In-Depth Information
Fig. 3.11
Example of
acoustic shift between two
interacting species,
represented by power
spectra of the white-banded
treefrog before (
dark grey
)
and during (
pale grey
) the
stimulus of a broadcast
American bullfrog
advertisement call (
white
)
(Reproduced with
permission from Both and
Grant
2012
)
The necessity to have distinct and well-recognized sounds is essential in the
mating system of many songbirds. In many species the presence of masking by
biological origins can produce an increase of energy to counteract the sonic level. In
cases of nonbiological masking, such as produced by anthropogenic noise, some
species can move in silence, waiting for a quiet moment. In other cases animals can
shift their vocal output to time periods in which the acoustic masking is lower.
Brumm (
2006
) has proved that male nightingales (
Luscinia megarhynchos
) when
exposed to the playback of heterospecific song are able to start to sing in the silent
intervals of the interspecific songs (Fig.
3.12
). This flexibility enables birds to insert
their songs in a silent interval, to save energy, and to improve the efficiency of
signal transmission, as observed during frog choruses. There are two possible
strategies to reduce masking effects: increase vocal sound pressure or avoid con-
current songs. In the nightingale Brumm (
2006
) has observed that a male starts
singing 880 ms after the last male has finished singing. The same was observed in
the coqui treefrog (
Eleutherodactylus coqui
) by Zelick and Narins (
1985
): in this
species males started calling within a 750-ms interval when exposed to a playback
noise. This interval is necessary for a male to be sure that the other male has
completed his acoustic performance. In other cases the voluntary overlap of two
songs by two competitive species is an aggressive signal, as argued by Naguib and
Mennill (
2010
).
Anuran choruses are very complex soundtopes created by contemporary calls of
several males at the same time. In a neotropical frog,
Hyla microcephala
, Schwartz
(
1993
) has conducted experiments of playback by using an interactive computer-
based system able to simulate the call interruptions of the males. A male when
interrupted by the call of another male increases the spacing between his notes.
Using synthetic calls with a silent gap, this author has found a vocal reaction of
males to sound intensity. The acoustic overlap between males was less than 10 %.
When males were calling in groups, the amount of sound interference between the
males was highly variable, ranging from 34 to 92 %.
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