1998). In this chapter, we offer a general survey of various factors that influence the evolution of long-distance signals.
The focus by researchers on long-distance signals is due in part to the
conspicuousness of these signals. Except for those secluded in the most
urban and sterile environments, most of us have probably heard a dawn
chorus of birds, the evening chirping of crickets, and the nocturnal serenading of frogs. In all of these cases, the sounds we hear are not merely
random fluctuations in ambient pressure derived from some inconsequential movement of an animal’s body parts, as if one happened to step in a
puddle and caused a complicated and intricate but rather meaningless
pattern of waves and troughs on the water’s surface. These sounds are
acoustic signals. Thus, by definition, they have evolved under selection to
serve a communication purpose.
The purpose of long-distance signals, defined by Littlejohn (2001) as
signals that function over a distance of more than several body lengths from
the receiver, is to advertise the presence of the sender to a receiver. In many
of the cases we consider, the sender is a male who is advertising his presence in a context linked in some way to reproduction, usually to receivers
that are potential mates or potential competitors. The evolution of longdistance acoustic signals involves the modification of the animal’s morphology and physiology to couple a mechanical displacement to pressure
fluctuations in the external environment; the production of these signals
sometimes stresses the physical and energetic limitations imposed on bioacoustic production. Furthermore, not all pressure fluctuations will be
favored by selection. They must have the temporal and spectral properties
that allow them to transmit over a functional distance; that is, to encounter
the intended receiver. Nor is efficient transmission through the environment the sole criterion of selection. The signal must interact effectively with
the receiver. At a minimum, it must be detected. Thus, the sounds are also
constrained to function within temporal and spectral limitations relative to
the intended receiver. But this pattern of pressure fluctuations emanating
from the sender must have meaning; thus, it is also constrained by the
higher-order neural processing and cognitive abilities of the intended
receiver, and to complicate matters, evolution of these long-distance communication systems is not one-sided. Whether viewed as an intricate evolutionary dance or an arms race, the properties of the signal and receiver
have the potential to influence each other’s evolution—thus, the signals and
receivers evolve but the communication system coevolves.
Only when all of these criteria are met do we have communication, and
when we do, it is an amazing phenomenon. As Pinker (1994) eloquently
stated for human language: “Simply by making noises with our mouths, we
can reliably cause precise new combinations of ideas to arise in each other’s
minds. The ability comes so naturally that we are apt to forget what a
miracle it is” (p. 15).
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M.J. Ryan and N.M. Kime
The focus by researchers on long-distance signals is due in part to the
conspicuousness of these signals. Except for those secluded in the most
urban and sterile environments, most of us have probably heard a dawn
chorus of birds, the evening chirping of crickets, and the nocturnal serenading of frogs. In all of these cases, the sounds we hear are not merely
random fluctuations in ambient pressure derived from some inconsequential movement of an animal’s body parts, as if one happened to step in a
puddle and caused a complicated and intricate but rather meaningless
pattern of waves and troughs on the water’s surface. These sounds are
acoustic signals. Thus, by definition, they have evolved under selection to
serve a communication purpose.
The purpose of long-distance signals, defined by Littlejohn (2001) as
signals that function over a distance of more than several body lengths from
the receiver, is to advertise the presence of the sender to a receiver. In many
of the cases we consider, the sender is a male who is advertising his presence in a context linked in some way to reproduction, usually to receivers
that are potential mates or potential competitors. The evolution of longdistance acoustic signals involves the modification of the animal’s morphology and physiology to couple a mechanical displacement to pressure
fluctuations in the external environment; the production of these signals
sometimes stresses the physical and energetic limitations imposed on bioacoustic production. Furthermore, not all pressure fluctuations will be
favored by selection. They must have the temporal and spectral properties
that allow them to transmit over a functional distance; that is, to encounter
the intended receiver. Nor is efficient transmission through the environment the sole criterion of selection. The signal must interact effectively with
the receiver. At a minimum, it must be detected. Thus, the sounds are also
constrained to function within temporal and spectral limitations relative to
the intended receiver. But this pattern of pressure fluctuations emanating
from the sender must have meaning; thus, it is also constrained by the
higher-order neural processing and cognitive abilities of the intended
receiver, and to complicate matters, evolution of these long-distance communication systems is not one-sided. Whether viewed as an intricate evolutionary dance or an arms race, the properties of the signal and receiver
have the potential to influence each other’s evolution—thus, the signals and
receivers evolve but the communication system coevolves.
Only when all of these criteria are met do we have communication, and
when we do, it is an amazing phenomenon. As Pinker (1994) eloquently
stated for human language: “Simply by making noises with our mouths, we
can reliably cause precise new combinations of ideas to arise in each other’s
minds. The ability comes so naturally that we are apt to forget what a
miracle it is” (p. 15).
226
M.J. Ryan and N.M. Kime