Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 93

To study in depth this interesting point, Snell-Rood ( 2012 ) has investigated
North American wood warblers (50 species), which represent a diverse and well-
studied monophyletic clade of birds. This group is geographically distributed across
a wide range of climates. The song repertoire ranges from 3 to 9 kHz. Divergence in
song is connected to the speciation process, and the climatic conditions can affect
this process. In the same study Snell-Rood investigated the bats of the American
Southwest that represent the richest collection of all North America north of
Mexico. This region has a great variety of elevation gradients characterized by
different temperatures and humidity.
In bats, the signal process is largely based on a self-hearing process, although
eavesdropping in some species of bats has been observed (see in this chapter the
section devoted to eavesdropping). Considering that bats emit sounds at a frequency
from 12 to 100 kHz and considering also the strong influence of absorption of the
high frequencies, bats remain good candidates to study the effects of climatic
changes on signaling systems.
At least two hypotheses can be presented on this matter:
1. Frequency characteristics of sounds should be linked to sound absorption.
2. Signal duration should increase with the increase of signal absorption because
detectability is connected to duration.
A significant variation in sound absorption has been found across the geographic
range of the warblers and a negative correlation has been found between frequency
and atmospheric absorption. This result has been confirmed also in the signals of
bats in which the echolocation call frequency is inversely connected with sound
absorption.
The Snell-Rood work suggests that organisms under a variable environment are
able to change the signal structure to optimize the communication. In particular, in
bats has been recognized the capacity to change the duration and frequency
according to wet and dry seasons. As illustrated in Fig. 4.13 , during the wet season
the frequency is lower and the duration is higher. Considering that the absorption is
higher during the wet season of 0.58 dB/m and that bats use echolocation in a range
of 20 m away, during the wet season the signal will be affected by an absorption of
24 dB. These mammals have the ability to change call structure also in relationship
to a change in atmospheric pressure.
4.17 Song and Reproductive Timing
The song repertoire is an adaptive character, and the richness of such repertoire is a
more effective signal. Considering that male parental ability, age, genetic quality,
and physiological condition may be correlated with song complexity, a female will
achieve benefits by mating with a male that expresses high complexity in singing
repertoire. During the time of female fertility the song appears extremely important
for the choice that females will make. Males will benefit for female choice and also