effort at a constant level by decreasing the rate at which these longer
calls are produced (Wells and Taigen 1986; Klump and Gerhardt 1987).
This trade-off between call rate and length has been taken as evidence
that overall calling effort is constrained by the energetic cost of signal
production.
Energy limitations may also restrict calling behavior over a longer time
scale, such as the amount of time that a male can spend calling over one or
several nights. Some studies on the calling behavior of frogs have suggested
that energetic demands limit male calling to a certain proportion of nights
(Murphy 1994a; Marler and Ryan 1995) or to a restricted period during a
given night (Wells and Taigen 1986; but see Murphy 1999). Male frogs
usually call on less than 30% of available nights, and most males of species
with prolonged breeding seasons appear in chorus only a few times during
the season (Bevier 1997). Choruses usually do not last long after midnight,
and large proportions of glycogen reserves of the trunk muscles can be
depleted after only a few hours of calling (Wells et al. 1995; Bevier 1997).
The relative mating success of male frogs and toads is strongly tied to the
number of nights in attendance at a chorus (Murphy 1994b; Wagner and
Sullivan 1995). In addition, females often prefer to mate with males who
produce longer mating calls or calls produced at faster rates (reviewed in
Ryan and Keddy-Hector 1992). Such preferences can extend beyond the
normal range of male variation (Gerhardt 1991). The origin for such preferences is unclear—it may be because of inherent sensory biases for greater
neural stimulation or because the preference results in matings with males
in better condition (Ryan and Keddy-Hector 1992). Whatever the cause
of the preference, the response of male calling behavior to sexual selection
may be constrained by the energetic cost of calling.
2.2. Body Size and Wavelength
The mass of the sound-producing structure plays a large role in determining
the frequency of communication signals; structures with greater mass can
produce lower-frequency signals more efficiently. Consequently, in many
frogs, birds, and mammals, the frequency of communication signals is correlated with body size (Fig. 5.2). This relationship between body size and
signal frequency often holds for comparisons among groups of species (e.g.,
frogs: Ryan 2001; birds: Morton 1977; Wallschager 1980; Bowman 1983;
Ryan and Brenowitz 1985;Wiley 1991; mammals: Fitch and Hauser, Chapter
3). The same relationship holds among individuals within a single species
or population for many frogs (e.g., Ryan 1985b; Gerhardt 1994; Howard and
Young 1998). But Fitch and Hauser (Chapter 3) suggest that within species
of birds and mammals, the expected correlation of body size and sound frequency is not as strong as expected.
One of the most comprehensive and enlightening studies on this subject
was conducted by Martin (1972). He dissected various components of the
230
M.J. Ryan and N.M. Kime
calls are produced (Wells and Taigen 1986; Klump and Gerhardt 1987).
This trade-off between call rate and length has been taken as evidence
that overall calling effort is constrained by the energetic cost of signal
production.
Energy limitations may also restrict calling behavior over a longer time
scale, such as the amount of time that a male can spend calling over one or
several nights. Some studies on the calling behavior of frogs have suggested
that energetic demands limit male calling to a certain proportion of nights
(Murphy 1994a; Marler and Ryan 1995) or to a restricted period during a
given night (Wells and Taigen 1986; but see Murphy 1999). Male frogs
usually call on less than 30% of available nights, and most males of species
with prolonged breeding seasons appear in chorus only a few times during
the season (Bevier 1997). Choruses usually do not last long after midnight,
and large proportions of glycogen reserves of the trunk muscles can be
depleted after only a few hours of calling (Wells et al. 1995; Bevier 1997).
The relative mating success of male frogs and toads is strongly tied to the
number of nights in attendance at a chorus (Murphy 1994b; Wagner and
Sullivan 1995). In addition, females often prefer to mate with males who
produce longer mating calls or calls produced at faster rates (reviewed in
Ryan and Keddy-Hector 1992). Such preferences can extend beyond the
normal range of male variation (Gerhardt 1991). The origin for such preferences is unclear—it may be because of inherent sensory biases for greater
neural stimulation or because the preference results in matings with males
in better condition (Ryan and Keddy-Hector 1992). Whatever the cause
of the preference, the response of male calling behavior to sexual selection
may be constrained by the energetic cost of calling.
2.2. Body Size and Wavelength
The mass of the sound-producing structure plays a large role in determining
the frequency of communication signals; structures with greater mass can
produce lower-frequency signals more efficiently. Consequently, in many
frogs, birds, and mammals, the frequency of communication signals is correlated with body size (Fig. 5.2). This relationship between body size and
signal frequency often holds for comparisons among groups of species (e.g.,
frogs: Ryan 2001; birds: Morton 1977; Wallschager 1980; Bowman 1983;
Ryan and Brenowitz 1985;Wiley 1991; mammals: Fitch and Hauser, Chapter
3). The same relationship holds among individuals within a single species
or population for many frogs (e.g., Ryan 1985b; Gerhardt 1994; Howard and
Young 1998). But Fitch and Hauser (Chapter 3) suggest that within species
of birds and mammals, the expected correlation of body size and sound frequency is not as strong as expected.
One of the most comprehensive and enlightening studies on this subject
was conducted by Martin (1972). He dissected various components of the
230
M.J. Ryan and N.M. Kime