nation of conspecifics from heterospecifics (Hödl 1977; Drewry and Rand
1983).
As with habitat acoustics, however, these community-level analyses are
subject to a number of different confounds, such as differences in body size
and evolutionary history. These confounds render the evidence for acoustic
niche partitioning of frog and insect calls inconclusive.
4. Predator- and Parasite-Generated Selection
The most fundamental communication interaction is dyadic, involving a
signal and a receiver. Most communication channels, however, are not
private, and there can be eavesdropping from unintended receivers. When
these unintended receivers are conspecifics, they can use the information
gained to compromise the fitness of the communicators. When the eavesdroppers are parasites or predators, the fitness consequence can be even
greater. The parasites are usually searching for a host on which their young
can develop while predators are looking for food. Long-distance signals
are the most risky because they are more likely to be detected by eavesdroppers than are close-range signals.
Zuk and Kolluru (1998) recently reviewed the literature on exploitation
of sexual signals by predators and parasites. They list 19 known cases. In
nine of the cases, the eavesdroppers are insects; seven of those cases involve
a tachinid fly. In all but one case, the insect eavesdroppers are attracted to
signals produced by other insects, usually crickets. The one exception is a
chaoborid fly being attracted to a tree frog. Zuk and Kolluru (1998) cite ten
examples of vertebrates that eavesdrop on signals. Often these are bats
homing in on signals of frogs and insects, although turtles, lizards, birds, and
other mammals do so as well. Acoustic cues are more likely to be used by
parasites and predators to find a victim than are visual cues, although they
are used less commonly than olfactory cues.
4.1.1. Acoustically Orienting Parasites and Calling in Crickets
The best-known example of parasite/predator-generated selection on
acoustic signaling comes from the classic studies of Cade (1975, 1981) on a
tachinid fly, Euphasiopteryx depleta (= Ormia depleta), that parasitizes the
field crickets Gryllus rubens, G. integer, and G. lineaticeps.As do many crickets, males of these species use long-distance acoustic cues to attract females
for the purpose of mating. Female crickets are not the only ones attending
to these cues, however; the tachinid flies orient toward these calls and
deposit larvae on the singing male. The larvae develop inside the male, use
the male as a source of nutrition, and emerge in 7–10 days (Cade 1975).
Selection on male calling from these parasites has resulted in at least two
adaptations by the crickets. The most dramatic is the evolution of variation
5. Selection on Signals
239
1983).
As with habitat acoustics, however, these community-level analyses are
subject to a number of different confounds, such as differences in body size
and evolutionary history. These confounds render the evidence for acoustic
niche partitioning of frog and insect calls inconclusive.
4. Predator- and Parasite-Generated Selection
The most fundamental communication interaction is dyadic, involving a
signal and a receiver. Most communication channels, however, are not
private, and there can be eavesdropping from unintended receivers. When
these unintended receivers are conspecifics, they can use the information
gained to compromise the fitness of the communicators. When the eavesdroppers are parasites or predators, the fitness consequence can be even
greater. The parasites are usually searching for a host on which their young
can develop while predators are looking for food. Long-distance signals
are the most risky because they are more likely to be detected by eavesdroppers than are close-range signals.
Zuk and Kolluru (1998) recently reviewed the literature on exploitation
of sexual signals by predators and parasites. They list 19 known cases. In
nine of the cases, the eavesdroppers are insects; seven of those cases involve
a tachinid fly. In all but one case, the insect eavesdroppers are attracted to
signals produced by other insects, usually crickets. The one exception is a
chaoborid fly being attracted to a tree frog. Zuk and Kolluru (1998) cite ten
examples of vertebrates that eavesdrop on signals. Often these are bats
homing in on signals of frogs and insects, although turtles, lizards, birds, and
other mammals do so as well. Acoustic cues are more likely to be used by
parasites and predators to find a victim than are visual cues, although they
are used less commonly than olfactory cues.
4.1.1. Acoustically Orienting Parasites and Calling in Crickets
The best-known example of parasite/predator-generated selection on
acoustic signaling comes from the classic studies of Cade (1975, 1981) on a
tachinid fly, Euphasiopteryx depleta (= Ormia depleta), that parasitizes the
field crickets Gryllus rubens, G. integer, and G. lineaticeps.As do many crickets, males of these species use long-distance acoustic cues to attract females
for the purpose of mating. Female crickets are not the only ones attending
to these cues, however; the tachinid flies orient toward these calls and
deposit larvae on the singing male. The larvae develop inside the male, use
the male as a source of nutrition, and emerge in 7–10 days (Cade 1975).
Selection on male calling from these parasites has resulted in at least two
adaptations by the crickets. The most dramatic is the evolution of variation
5. Selection on Signals
239