dispositions could bias the types of sounds incorporated into signal complexity; some species, such as mockingbirds, might be less constrained. This
view of signal complexity, in general, and bird song repertoires, specifically,
shifts the focus from any message that might be encoded by parts of the
song to the value of complexity per se. Marler (1998), for example, has
recently suggested that “. . . the song functions as affective rather than symbolic signals, and the variety is generated, not to diversify meaning, but
rather to maintain the interest of anyone who is listening, and to alleviate
habituation” (p. 12). Music rather than language might be the preferred
analog for some types of animal communication. Such a view is consistent
with the ideas of sensory biases and sensory exploitation discussed here. In
fact, some recent studies in musicology have considered the proposition
that characteristics of the human cochlea might dictate some aspects of
music appreciation (Zentner and Kagan 1996).
6.3.3. Sensory Exploitation in Túngara Frogs
The call of the túngara frog, Physalaemus pustulosus, has two components:
a whine and a chuck. The whine initiates the call, is always present, and may
be followed by one or several chucks or can be produced alone. When
chucks are added to the call, they are appended near the end of the whine.
Up to six chucks can be added, although one to three chucks is the more
common occurrence. In controlled laboratory experiments, a whine is both
necessary and sufficient to elicit phonotaxis in female frogs. When females
are given a choice between a whine only and a whine with chucks, females
prefer the latter. Thus, the túngara frogs exhibit both chucks and preference
for chucks (reviewed in Ryan 1985b; Ryan and Rand 1999).
Phylogenetic analysis combined with behavioral experimentation can
sometimes provide insights into the historical pattern by which signals and
receivers evolved. Pleiotropy and the genetic correlation hypotheses suggest that signals and receivers evolve in concert, whereas sensory exploitation suggests that signals exploit preexisting biases. These approaches were
used to investigate the manner in which the chuck and the preference for
chuck evolved.
The species group to which the túngara frog belongs is the Physalaemus
pustulosus species group. This group consists of two smaller monophyletic
groups, one west of the Andes mountains and the other in Middle America
and east of the Andes (Cannatella et al. 1998; Fig. 5.9). Only species in the
eastern group add suffixes to the call; they are lacking not only in the species
in the western group but also in the more than 20 species of the genus that
have been studied. This suggests that the chuck evolved after the two
smaller groups within the species group diverged; examination of laryngeal
correlates of the chuck support this interpretation (Ryan and Drewes 1990).
If females in the western group preferred chucks added to their conspecific whine, even though their males are incapable of producing them, it
would suggest that preference for chucks was a preexisting bias exploited
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M.J. Ryan and N.M. Kime
view of signal complexity, in general, and bird song repertoires, specifically,
shifts the focus from any message that might be encoded by parts of the
song to the value of complexity per se. Marler (1998), for example, has
recently suggested that “. . . the song functions as affective rather than symbolic signals, and the variety is generated, not to diversify meaning, but
rather to maintain the interest of anyone who is listening, and to alleviate
habituation” (p. 12). Music rather than language might be the preferred
analog for some types of animal communication. Such a view is consistent
with the ideas of sensory biases and sensory exploitation discussed here. In
fact, some recent studies in musicology have considered the proposition
that characteristics of the human cochlea might dictate some aspects of
music appreciation (Zentner and Kagan 1996).
6.3.3. Sensory Exploitation in Túngara Frogs
The call of the túngara frog, Physalaemus pustulosus, has two components:
a whine and a chuck. The whine initiates the call, is always present, and may
be followed by one or several chucks or can be produced alone. When
chucks are added to the call, they are appended near the end of the whine.
Up to six chucks can be added, although one to three chucks is the more
common occurrence. In controlled laboratory experiments, a whine is both
necessary and sufficient to elicit phonotaxis in female frogs. When females
are given a choice between a whine only and a whine with chucks, females
prefer the latter. Thus, the túngara frogs exhibit both chucks and preference
for chucks (reviewed in Ryan 1985b; Ryan and Rand 1999).
Phylogenetic analysis combined with behavioral experimentation can
sometimes provide insights into the historical pattern by which signals and
receivers evolved. Pleiotropy and the genetic correlation hypotheses suggest that signals and receivers evolve in concert, whereas sensory exploitation suggests that signals exploit preexisting biases. These approaches were
used to investigate the manner in which the chuck and the preference for
chuck evolved.
The species group to which the túngara frog belongs is the Physalaemus
pustulosus species group. This group consists of two smaller monophyletic
groups, one west of the Andes mountains and the other in Middle America
and east of the Andes (Cannatella et al. 1998; Fig. 5.9). Only species in the
eastern group add suffixes to the call; they are lacking not only in the species
in the western group but also in the more than 20 species of the genus that
have been studied. This suggests that the chuck evolved after the two
smaller groups within the species group diverged; examination of laryngeal
correlates of the chuck support this interpretation (Ryan and Drewes 1990).
If females in the western group preferred chucks added to their conspecific whine, even though their males are incapable of producing them, it
would suggest that preference for chucks was a preexisting bias exploited
260
M.J. Ryan and N.M. Kime