be a suite of neuroanatomical specializations for low-frequency hearing in
the inner ear, which either increase sensitivity to low-frequency sounds or
extend the range of hearing into the low frequencies. These include the
largest number of cochlear neurons for any mammal, the second-highest
density of cochlear neurons for any mammal, and three peaks of cochlear
neuron density rather than the two peaks typical of other bats or the one
typical of all other mammals. The third peak is in the apical portion of the
cochlea, where low-frequency sounds are detected (Bruns et al. 1989).
Although there are a number of striking cases of predator- and parasitegenerated selection on long-distance acoustic signals, the impression from
being in the field with these calling animals is that we probably know of
only a relatively small proportion of these cases. Furthermore, it seems that
there will be many more cases of adaptations on the part of eavesdroppers
to detect and localize long-distance cues of prey. We predict that many of
these interactions between signalers and unintended receivers will be found
in the tropics, where the diversity of predator–prey interactions tends to be
greater.
5. Long-Distance Signals, Speciation, and
Sexual Selection
It is not only the difficulties of production and transmission of long-distance
signals that have made them a central focus of so many research programs.
Their involvement in the reproductive biology of many species has made
these communication systems critical to the development of evolutionary
theories of speciation and sexual selection.
5.1. Speciation
The middle of the twentieth century saw a rebirth of Darwinism referred
to as the Modern Synthesis. This synthesis combined research traditions
from paleontology, systematics, population genetics, and behavior to accomplish a grand, unified theory of evolution by natural selection (Mayr 1982).
A critical issue in accomplishing this synthesis was understanding the origin
of species. It was in this realm that behavior made its most lasting contribution to evolutionary theory. If a biological species is defined as a group
of potentially reproductively interacting individuals, then there must be
mechanisms that draw conspecifics toward each other and away from
heterospecifics when searching for mates.
Despite the large number of acoustically signaling insects, frogs, and
birds, most of the species that produce long-distance signals produce a
species-typical one. We make this statement without implying species typology and lack of meaningful geographic variation within a species (Foster
1999; Foster and Endler 1999). By classifying a signal as “species-typical,”
we are making a statement about how signal variation is partitioned within
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M.J. Ryan and N.M. Kime
the inner ear, which either increase sensitivity to low-frequency sounds or
extend the range of hearing into the low frequencies. These include the
largest number of cochlear neurons for any mammal, the second-highest
density of cochlear neurons for any mammal, and three peaks of cochlear
neuron density rather than the two peaks typical of other bats or the one
typical of all other mammals. The third peak is in the apical portion of the
cochlea, where low-frequency sounds are detected (Bruns et al. 1989).
Although there are a number of striking cases of predator- and parasitegenerated selection on long-distance acoustic signals, the impression from
being in the field with these calling animals is that we probably know of
only a relatively small proportion of these cases. Furthermore, it seems that
there will be many more cases of adaptations on the part of eavesdroppers
to detect and localize long-distance cues of prey. We predict that many of
these interactions between signalers and unintended receivers will be found
in the tropics, where the diversity of predator–prey interactions tends to be
greater.
5. Long-Distance Signals, Speciation, and
Sexual Selection
It is not only the difficulties of production and transmission of long-distance
signals that have made them a central focus of so many research programs.
Their involvement in the reproductive biology of many species has made
these communication systems critical to the development of evolutionary
theories of speciation and sexual selection.
5.1. Speciation
The middle of the twentieth century saw a rebirth of Darwinism referred
to as the Modern Synthesis. This synthesis combined research traditions
from paleontology, systematics, population genetics, and behavior to accomplish a grand, unified theory of evolution by natural selection (Mayr 1982).
A critical issue in accomplishing this synthesis was understanding the origin
of species. It was in this realm that behavior made its most lasting contribution to evolutionary theory. If a biological species is defined as a group
of potentially reproductively interacting individuals, then there must be
mechanisms that draw conspecifics toward each other and away from
heterospecifics when searching for mates.
Despite the large number of acoustically signaling insects, frogs, and
birds, most of the species that produce long-distance signals produce a
species-typical one. We make this statement without implying species typology and lack of meaningful geographic variation within a species (Foster
1999; Foster and Endler 1999). By classifying a signal as “species-typical,”
we are making a statement about how signal variation is partitioned within
242
M.J. Ryan and N.M. Kime