60
Heiner Romer
on acoustic signals. Parasitoid tachinid flies and passively listening bats use the
acoustic signals to find their hosts (Cade 1975, 1981; Belwood and Morris 1987;
Lakes-Harlan and Heller 1992,). We would thus expect a trade-off between the
costs and benefits of signaling, and the degree of conspicuousness of a given signal
would indicate the point of balance between the counteracting selective forces
driving the evolution of the signal.
Leaving these costs aside and assuming that there are only benefits from a
conspicuous advertisement call of a frog, bird or insect, how can the active space
of the signal be increased (the space where the signal is above detection threshold
of intended receivers)? This active space depends on: (1) the amplitude of the
broadcast signal, (2) the properties of the transmission channel, (3) the amplitude
and spectral features of ambient noise, and (4) the sensory performance of the
receiver to detect signals and to discriminate between them. The main focus of this
chapter is on the role of the transmission channel, and by comparing animals of
very different sizes and in the different transmission media of air and water, I wish
to demonstrate some of the general rules for sound transmission that apply to all of
them.
2 Properties of the Transmission Channel
Natural habitats are not sound-proof rooms; they exhibit hard or soft boundaries,
reflecting surfaces, scattering by objects and ambient noise which all affect the
signal-to-noise ratio, temporal pattern, and frequency spectrum of the signal at the
position of the receiver(s). There are several reviews dealing with physical and
other aspects on sound propagation and acoustic communication outdoors, which
are highly recommended for a deeper insight into the mechanisms relevant for
sound propagation outdoors (Michelsen 1978; Wiley and Richards 1982;
Michelsen and Larsen 1983; Spiesberger and Fristrup 1990; Embleton 1996;
Romer 1998). In this chapter only some aspects will be briefly covered which are
important for a signaler to increase the active space of a signal.
2.1 Excess Attenuation Through Scattering
If there is a selection pressure on a sender to be more conspicuous we would
expect that either the signal or signaling behavior is adapted in a way which
reduces any source for excess attenuation, in order to increase the signal-to-noise
ratio at the site of the receiver. Atmospheric absorption, scattering by objects in
the transmission channel, reflective layers such as the ground, and strata of
vegetation can result in the attenuation of the signal in excess to that predicted
from the spherical spread of sound alone, given by the 6 dB decrease for each
doubling of distance. Absorption is not a major source of attenuation for most
animals, but for frequencies as high as I 00 kHz, attenuation due to absorption may
Heiner Romer
on acoustic signals. Parasitoid tachinid flies and passively listening bats use the
acoustic signals to find their hosts (Cade 1975, 1981; Belwood and Morris 1987;
Lakes-Harlan and Heller 1992,). We would thus expect a trade-off between the
costs and benefits of signaling, and the degree of conspicuousness of a given signal
would indicate the point of balance between the counteracting selective forces
driving the evolution of the signal.
Leaving these costs aside and assuming that there are only benefits from a
conspicuous advertisement call of a frog, bird or insect, how can the active space
of the signal be increased (the space where the signal is above detection threshold
of intended receivers)? This active space depends on: (1) the amplitude of the
broadcast signal, (2) the properties of the transmission channel, (3) the amplitude
and spectral features of ambient noise, and (4) the sensory performance of the
receiver to detect signals and to discriminate between them. The main focus of this
chapter is on the role of the transmission channel, and by comparing animals of
very different sizes and in the different transmission media of air and water, I wish
to demonstrate some of the general rules for sound transmission that apply to all of
them.
2 Properties of the Transmission Channel
Natural habitats are not sound-proof rooms; they exhibit hard or soft boundaries,
reflecting surfaces, scattering by objects and ambient noise which all affect the
signal-to-noise ratio, temporal pattern, and frequency spectrum of the signal at the
position of the receiver(s). There are several reviews dealing with physical and
other aspects on sound propagation and acoustic communication outdoors, which
are highly recommended for a deeper insight into the mechanisms relevant for
sound propagation outdoors (Michelsen 1978; Wiley and Richards 1982;
Michelsen and Larsen 1983; Spiesberger and Fristrup 1990; Embleton 1996;
Romer 1998). In this chapter only some aspects will be briefly covered which are
important for a signaler to increase the active space of a signal.
2.1 Excess Attenuation Through Scattering
If there is a selection pressure on a sender to be more conspicuous we would
expect that either the signal or signaling behavior is adapted in a way which
reduces any source for excess attenuation, in order to increase the signal-to-noise
ratio at the site of the receiver. Atmospheric absorption, scattering by objects in
the transmission channel, reflective layers such as the ground, and strata of
vegetation can result in the attenuation of the signal in excess to that predicted
from the spherical spread of sound alone, given by the 6 dB decrease for each
doubling of distance. Absorption is not a major source of attenuation for most
animals, but for frequencies as high as I 00 kHz, attenuation due to absorption may