Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 64

Decisions based on the perceived signal (other signals, choice time wasted, reasons
for choice, need for choice, predator risk, parasite risk, physiological state,
quality of signal, quality of different signal components, signal channel use)
As we have already stated, communication requires energy, which represents
fatigue, a physiological stress. In birds this represents a serious constraint that in
part can be avoided with a varied repertoire that engages different muscles at a time.
Signaling can be restricted to a specific time of season by an environmental
constraint such as water availability in deserts for frogs. Signaling activity in this
case will be confined to the period of water availability.
The signal transmission through the medium represents a further evolutive step
in the communication theory. In fact, signals should reduce the effect of back-
ground noise and interfering signals from other species. Under a selective constraint
signals should avoid attenuation, blocking, absorption, reflection, and refraction.
We have already discussed this aspect in Chap. 3 in the section devoted to the
acoustic adaptive hypothesis.
The intensity of the signal faces attenuation with distance, and a signal that
reverberates can arrive with a rate of degradation that reduces its efficiency if
poorly designed. These effects are more evident if the signal has a high content
of information or when such information is transmitted at a high rate. Thus, the
complexity of the signals that we can observe is the result of a trade-off between the
maximum possible of information that can be transmitted and all the environmental
and biological constraints that limit this efficiency. Signaling behavior is under such
evolutionary constraint with preferences for time and space to minimize the distor-
tion; this is the case of animal choruses that probably are concentrated in the most
favorable period of the day when the air is calm and noises are at a minimum.
It is reasonable to admit that explosive breeders and microhabitat specialists use
more predictable signal design than species that are continuous breeders and are
microhabitat generalists.
In signal generation, many constraints affect complexity and quality of the
signal, and the same occurs in signal reception: there are biophysical, biochemical,
and energetic limits in receptor design.
For instance, if the background noise is predictable enough this noise can be
filtered out or the place or time could be selected to reduce the effect of the noise.
The use of multiple systems of signals, for instance, visual combined with
acoustic, could solve problems of noise interferences or discrimination. The reduc-
tion of the effects of noise in a signal can be solved with the use of a filter or with a
signal design easy to distinguish.
In evolutionary terms, wrong receivers have to be reduced, as subjects with the
capacity of eavesdropping can use the information of the emitter or the receiver for
personal benefit. Such as a case is documented in cicadas that are parasitized by
sarcophagid flies. These flies are attracted by the cicada male courtship song. In this
way flies exploit cicadas, and fly larva destroy the sound generation organ of the
cicada, which reduces the risk that another fly can parasitize the same individual,
and fly behavior definitely prevents further exploitation of the cicada's signal.