Biology Reference
In-Depth Information
3.1
Introduction
Biophonies are integral and substantial aspects of the soundscape, and their variety
and dynamics concur with overall environmental biodiversity. In this chapter we
illustrate and discuss some hypotheses originated by empirical indoor/outdoor
research in different fields of the physiology, behavioral, and cognitive bioacoustics
disciplines devoted to explaining nature and the role of acoustic performances of
vocal animals. At least four hypotheses contribute to the bioacoustics theoretical
framework: the morphological adaptation hypothesis (MAH), the acoustic adapta-
tion hypothesis (AAH), the acoustic niche hypothesis (ANH), and the species
recognition hypothesis (SRH). Testing and discussing these hypotheses contributes
to better understanding of how natural selection operates and what strategies are
used by animals when performing acoustic signals (indeed, the majority of cases of
these hypotheses have been tested on birds). Briefly, the morphological adaptation
hypothesis (MAH) refers to the embodiment constraints, the acoustic adaptation
hypothesis (AAH) considers the effects of environmental stressors, the acoustic
niche hypothesis (ANH) is formulated under a competitive context, and the species
recognition hypothesis (SRH) refers to a behavioral context (Fig. 3.1 ). All these
phenomenological contexts represent concurrent proxies that shape acoustic
performances and which often operate in tandem, as argued by Seddon ( 2005 ).
3.2 The Morphological Adaptation Hypothesis (MAH)
Different forces in action in natural selection and vocalizations (song, call, alarms,
voice) are phenotypical characters with high plasticity used to better define species-
specific competencies.
Most vocal animals have complex organs of vocalization; for instance, in
mammals the sound is produced by the vocal folds and modulated by the tongue.
In songbirds the sound is produced by the syrinx and then modulated by the vocal
tract and beak (after Fletcher 2007 ).
Larger animals such as elephants communicate at a greater distance than smaller
animals such as wildcats, and the same occurs for birds such as cockatoos when
compared with the distance traveled by the songs of smaller birds. However, the
acoustic energy produced in proportion by larger species is only one component of
the vocal signal. It is reasonable to speculate that for long-distance communication
animals utilize also the best range of frequencies to reduce sound degradation, but it
remains unknown what are the evolutive proxies
that produce specific
differentiation.
The morphological adaptation hypothesis (MAH) refers to the role of body size
as a biological constraint of the vocalization organs and their acoustic performance.
According to this hypothesis, organisms with large body mass utter vocaliza-
tions with lower frequency than smaller species. For instance, in many species of
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