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After the application of this model these two authors have found that the early
morning is the most favorable period for sound propagation when compared with
other times of day and night. The biophonies produced at that time are 20 fold more
effective than the same signal with the same amplitude produced at midday. The
height above the substrate at which a sound is produced increases the area of
broadcast coverage. The frequency of the signal is another important element.
Lower frequencies increase the area of broadcast coverage, although in open
areas where wind is frequent and also there is higher turbulence the signals are
transmitted better at high frequency. This last fact is discussed in terms of commu-
nication theory by which the transmission in a turbulent medium may be enhanced
by the use of higher frequencies.
7.3 Choruses in Amphibians
In amphibians, choruses are very common phenomena. The phonotaxy of some
species of amphibians has been demonstrated, but the chorus seems to represent a
complex system of communication and not just a stochastic system of interindivid-
ual acoustic competition, overlap, and masking. Especially in species that breed in
ephemeral ponds the necessity to breed in a short time before the water disappears
obliges males and females to concentrate, which produces a high level of intra- and
interspecific acoustic interference: at first sight, the amphibian chorus seems a
paradox. Many males singing simultaneously create a background noise that
masks the individuals, which are obliged to increase their acoustic performances.
Anuran males produce at least two different calls: an aggressive call used to
interact with males and calls directed to females. The aggressive calls generally are
less attractive for females. In some species there is a gradual shift from one type to
another accordingly. In a few species it has been observed that the same call is
composed of two parts: the first is directed to males and the second to the female.
This two-note system appears, from an energetic point of view, extremely wasteful.
When a chorus is analyzed at fine temporal resolution, different patterns can be
described (Grafe
2005
). In general the degree of overlap between calling males
increases with the number of contemporarily calling males, but the level of overlap
remains lower when compared with a random comparison. This observation means
that males have the capacity to place their calls in silent gaps, reducing the masking
effect of other males. In some species the capacity to adjust the call timing has been
observed. For example, in the neotropical treefrog (
Hyla microcephala
), Schwatz
(
1993
) observed that a male when interrupted by the call of a neighboring male
increases the spacing between calls to reduce overlap of the subsequent note.
Different strategies can be observed to reduce the competition: males can repeat
their calls several times, may increase the complexity of their calls, and can protect
the calling site and the sonic space around it, preventing the intrusion of other
males. Increase of the complexity of the acoustic repertoire has been observed in
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