4. Acoustic Communication in Whales and Dolphins
167
signals initially produced for echolocation have no communicative functions (Tyack 1997). In fact, these kinds of buzzes could be viewed as
preadaptations for a food call, should ecological conditions render such a
call beneficial.
2.4.3 Attributes of Fish as Targets
Most studies of echolocation in dolphins have used geometric targets made
of metal. Yet one of the keys to advances in the study of bat echolocation
came from focusing on the acoustic properties of the natural targets for
which bat echolocation evolved. For example, focus on the fluttering wings
of moths directed attention to processing of amplitude modulation and
Doppler-induced frequency modulation in bats. Specialized auditory processing in bats appears to have evolved through selection to optimize
echolocation for the specific acoustic target properties of the most important natural targets. This suggests that anyone interested in the possible
evolution of specialized auditory mechanisms in cetaceans might benefit
from analyzing the acoustic properties of the natural targets of dolphin
echolocation as well.
The optimal frequency of a sound used for echolocation depends upon
the expected size and composition of the target. Absorption imposes a
significant penalty for higher frequencies, but small targets can best be
detected by short wavelength').., or high-frequency signals. Many discussions
of the optimal frequency for dolphin echolocation have modeled reflection
of sound energy from a spherical rigid target by Rayleigh scattering (e.g.,
section 8.1 of Au 1993), but Clay and Medwin (1977, 1998) emphasize that
bubbles in water provide even better targets at much lower frequencies. A
rigid spherical target of radius a reflects maximum energy when the wavelength of the sound impinging on it equals the circumference of the sphere,
or ' ). . , = 21ta. When the ratio of 21ta/').." often referred to as ka by acousticians,
equals one, there is maximum reflection from the rigid sphere. While this
relation holds to a first approximation for most rigid targets, gas-filled
bubbles create a very different sonar target in water. When a bubble is
exposed to sound with a wavelength longer than the bubble, it oscillates
and reradiates sound. Figure 4.3 illustrates the amount of energy backscattered from a rigid sphere (right) and a bubble (left) as a function of ka,
which is proportional to frequency. The echo strength from a rigid sphere
drops off sharply from signals with ka < 1, or with a wavelength').., > 21ta.
By contrast, a bubble of the same size scatters about 400 times more sound
energy at a frequency about 11100 the resonant frequency of the sphere.
What do bubbles and rigid spheres have to do with echolocation in
dolphins? Most dolphins are presumably less interested in rigid spheres
than targets such as fish, squid, sharks, conspecifics, and large obstacles. The
high-frequency echolocation system of dolphins clearly can detect echoes
167
signals initially produced for echolocation have no communicative functions (Tyack 1997). In fact, these kinds of buzzes could be viewed as
preadaptations for a food call, should ecological conditions render such a
call beneficial.
2.4.3 Attributes of Fish as Targets
Most studies of echolocation in dolphins have used geometric targets made
of metal. Yet one of the keys to advances in the study of bat echolocation
came from focusing on the acoustic properties of the natural targets for
which bat echolocation evolved. For example, focus on the fluttering wings
of moths directed attention to processing of amplitude modulation and
Doppler-induced frequency modulation in bats. Specialized auditory processing in bats appears to have evolved through selection to optimize
echolocation for the specific acoustic target properties of the most important natural targets. This suggests that anyone interested in the possible
evolution of specialized auditory mechanisms in cetaceans might benefit
from analyzing the acoustic properties of the natural targets of dolphin
echolocation as well.
The optimal frequency of a sound used for echolocation depends upon
the expected size and composition of the target. Absorption imposes a
significant penalty for higher frequencies, but small targets can best be
detected by short wavelength').., or high-frequency signals. Many discussions
of the optimal frequency for dolphin echolocation have modeled reflection
of sound energy from a spherical rigid target by Rayleigh scattering (e.g.,
section 8.1 of Au 1993), but Clay and Medwin (1977, 1998) emphasize that
bubbles in water provide even better targets at much lower frequencies. A
rigid spherical target of radius a reflects maximum energy when the wavelength of the sound impinging on it equals the circumference of the sphere,
or ' ). . , = 21ta. When the ratio of 21ta/').." often referred to as ka by acousticians,
equals one, there is maximum reflection from the rigid sphere. While this
relation holds to a first approximation for most rigid targets, gas-filled
bubbles create a very different sonar target in water. When a bubble is
exposed to sound with a wavelength longer than the bubble, it oscillates
and reradiates sound. Figure 4.3 illustrates the amount of energy backscattered from a rigid sphere (right) and a bubble (left) as a function of ka,
which is proportional to frequency. The echo strength from a rigid sphere
drops off sharply from signals with ka < 1, or with a wavelength').., > 21ta.
By contrast, a bubble of the same size scatters about 400 times more sound
energy at a frequency about 11100 the resonant frequency of the sphere.
What do bubbles and rigid spheres have to do with echolocation in
dolphins? Most dolphins are presumably less interested in rigid spheres
than targets such as fish, squid, sharks, conspecifics, and large obstacles. The
high-frequency echolocation system of dolphins clearly can detect echoes