2. Morphological and Energetic Constraints
A number of different selective forces can act on any one morphological
or behavioral trait, including long-distance communication signals (e.g.,
Wilczynski and Ryan 1999). These forces may act in unison or in opposition to one another. In addition, a trait may be restricted in its ability to
evolve in response to selection either because the necessary genetic variation does not exist or because of the constraints of physical laws. Consequently, the traits that we see are not always at a selective optimum with
respect to one fitness component but instead reflect a compromise between
a number of different selective forces and constraints.
The physical structures used for sound production play a large role in
determining both the temporal and spectral characteristics of acoustic
signals. Although these structures can certainly evolve in response to the
various selective forces acting on long-distance communication signals, phylogenetic or physical constraints on their morphology can also impose limitations on the form of signals. In this section, we discuss how two such
limiting factors, the low energetic efficiency of sound production and the
body size of the sender, constrain the signals used for long-distance communication.
2.1. The Energetics of Signal Production
2.1.1. Energetics and Efficiency
One important element of natural selection is the energetic cost of an
otherwise advantageous trait. The cost associated with the energy required
to perform a display or behavior can outweigh the benefit of the trait, especially when it depletes resources necessary for basic maintenance or other
activities. The energetic cost of acoustic signaling can be estimated in a
number of ways, the best of which is probably the rate of oxygen consumption (V
.
O 2
) during a bout of calling. This measure is most easily procured for animals such as insects and frogs, which will call in a respirometer
(MacNally and Young 1981; Ryan 1988). Less reliable methods must usually
be applied to studies of birds and mammals (e.g., Brackenbury 1979; but
see Eberhard 1994); for this reason, the most well-known studies of calling
energetics have focused on anurans and insects.
These studies have repeatedly shown that acoustic signals are extremely
costly to produce. The rate of oxygen consumption during calling can be
5–30 times that during rest (Stevens and Josephson 1977; MacNally and
Young 1981; Prestwich and Walker 1981; Bucher et al. 1982; Taigen and
Wells 1985; Taigen et al. 1985; Ryan 1988; Prestwich et al. 1989). Indeed,
most studies on insects and frogs demonstrate that calling to attract mates
is one of the most energetically expensive activities in which males engage
5. Selection on Signals
227
A number of different selective forces can act on any one morphological
or behavioral trait, including long-distance communication signals (e.g.,
Wilczynski and Ryan 1999). These forces may act in unison or in opposition to one another. In addition, a trait may be restricted in its ability to
evolve in response to selection either because the necessary genetic variation does not exist or because of the constraints of physical laws. Consequently, the traits that we see are not always at a selective optimum with
respect to one fitness component but instead reflect a compromise between
a number of different selective forces and constraints.
The physical structures used for sound production play a large role in
determining both the temporal and spectral characteristics of acoustic
signals. Although these structures can certainly evolve in response to the
various selective forces acting on long-distance communication signals, phylogenetic or physical constraints on their morphology can also impose limitations on the form of signals. In this section, we discuss how two such
limiting factors, the low energetic efficiency of sound production and the
body size of the sender, constrain the signals used for long-distance communication.
2.1. The Energetics of Signal Production
2.1.1. Energetics and Efficiency
One important element of natural selection is the energetic cost of an
otherwise advantageous trait. The cost associated with the energy required
to perform a display or behavior can outweigh the benefit of the trait, especially when it depletes resources necessary for basic maintenance or other
activities. The energetic cost of acoustic signaling can be estimated in a
number of ways, the best of which is probably the rate of oxygen consumption (V
.
O 2
) during a bout of calling. This measure is most easily procured for animals such as insects and frogs, which will call in a respirometer
(MacNally and Young 1981; Ryan 1988). Less reliable methods must usually
be applied to studies of birds and mammals (e.g., Brackenbury 1979; but
see Eberhard 1994); for this reason, the most well-known studies of calling
energetics have focused on anurans and insects.
These studies have repeatedly shown that acoustic signals are extremely
costly to produce. The rate of oxygen consumption during calling can be
5–30 times that during rest (Stevens and Josephson 1977; MacNally and
Young 1981; Prestwich and Walker 1981; Bucher et al. 1982; Taigen and
Wells 1985; Taigen et al. 1985; Ryan 1988; Prestwich et al. 1989). Indeed,
most studies on insects and frogs demonstrate that calling to attract mates
is one of the most energetically expensive activities in which males engage
5. Selection on Signals
227