1. Overview
5
greater than 10 kHz the absorption losses increase dramatically with range.
The sounds of many baleen whales do not extend much above 1kHz and
therefore experience very little absorption loss. The sounds of dolphins are
generally higher than 20kHz and are therefore limited to relatively shortrange propagation.
2. Hearing in Cetaceans
Almost all of our knowledge of hearing in cetaceans comes from studies
performed with small odontocetes. The most studied species is the Atlantic
bottlenose dolphin (Tursiops truncatus). Despite the amount of research
performed with T truncatus, our understanding of auditory processes in
dolphins lags considerably behind that for humans and other terrestrial
mammals. There are still many large gaps in our knowledge of various auditory processes occurring within the most studied odontocete. Much of
the auditory research with dolphins has followed along the lines of human
auditory research, but at a much slower pace and with considerably
less intensity.
2.1 Hearing in Dolphins
Our discussion of the hearing sensitivity and capabilities of dolphins will
necessarily be brief since Nachtigall et al. (Chapter 8) devote an entire
chapter on this topic. Johnson (1967) produced the first audiogram of an
odontocete, an Atlantic bottlenose dolphin (T truncatus), and since then
audiograms have been measured for the harbor porpoise (P phocoena) by
Andersen (1970), the killer whale (0. orca) by Hall and Johnson (1971) and
Bain and Dahlheim (personal communication), the beluga whale (Delphinapterus leucas) by White et al. (1978), the Pacific bottlenose dolphin (Tursiops gilli) by Ljungblad et al. (1982), the false killer whale (Pseudorca
crassidens) by Thomas et al. (1988), the Chinese river dolphin (Lipotes vexilliter) by Wang et al. (1992), Risso's dolphin (Grampus griseus) by Nachtigall et al. (1995), and the tucuxi (Sotafia fluviatilis) by Sauerland and
Dehnhardt (1998). The audiograms of these odontocetes are shown in
Figure 1.2. The curve for 0. orca is from unpublished data of Bain and
Dalheim and is the average from two animals, one having a high-frequency
cutoff at 104 kHz and the other at 120 kHz.
Several interesting features of the data in Figure 1.2 include the similarity in shape, levels of maximum sensitivity, and high-frequency limits of all
the different audiograms. This seems quite remarkable in light of the different sizes of the various animals, their different habitats, and the different prey that they consume. The one common feature of all the odontocetes
covered in Figure 1.2 is their use of echolocation for foraging, orientation,
and navigation purposes. All of these animals have a very wide frequency
range of hearing covering over nine octaves! All the animals (except the
5
greater than 10 kHz the absorption losses increase dramatically with range.
The sounds of many baleen whales do not extend much above 1kHz and
therefore experience very little absorption loss. The sounds of dolphins are
generally higher than 20kHz and are therefore limited to relatively shortrange propagation.
2. Hearing in Cetaceans
Almost all of our knowledge of hearing in cetaceans comes from studies
performed with small odontocetes. The most studied species is the Atlantic
bottlenose dolphin (Tursiops truncatus). Despite the amount of research
performed with T truncatus, our understanding of auditory processes in
dolphins lags considerably behind that for humans and other terrestrial
mammals. There are still many large gaps in our knowledge of various auditory processes occurring within the most studied odontocete. Much of
the auditory research with dolphins has followed along the lines of human
auditory research, but at a much slower pace and with considerably
less intensity.
2.1 Hearing in Dolphins
Our discussion of the hearing sensitivity and capabilities of dolphins will
necessarily be brief since Nachtigall et al. (Chapter 8) devote an entire
chapter on this topic. Johnson (1967) produced the first audiogram of an
odontocete, an Atlantic bottlenose dolphin (T truncatus), and since then
audiograms have been measured for the harbor porpoise (P phocoena) by
Andersen (1970), the killer whale (0. orca) by Hall and Johnson (1971) and
Bain and Dahlheim (personal communication), the beluga whale (Delphinapterus leucas) by White et al. (1978), the Pacific bottlenose dolphin (Tursiops gilli) by Ljungblad et al. (1982), the false killer whale (Pseudorca
crassidens) by Thomas et al. (1988), the Chinese river dolphin (Lipotes vexilliter) by Wang et al. (1992), Risso's dolphin (Grampus griseus) by Nachtigall et al. (1995), and the tucuxi (Sotafia fluviatilis) by Sauerland and
Dehnhardt (1998). The audiograms of these odontocetes are shown in
Figure 1.2. The curve for 0. orca is from unpublished data of Bain and
Dalheim and is the average from two animals, one having a high-frequency
cutoff at 104 kHz and the other at 120 kHz.
Several interesting features of the data in Figure 1.2 include the similarity in shape, levels of maximum sensitivity, and high-frequency limits of all
the different audiograms. This seems quite remarkable in light of the different sizes of the various animals, their different habitats, and the different prey that they consume. The one common feature of all the odontocetes
covered in Figure 1.2 is their use of echolocation for foraging, orientation,
and navigation purposes. All of these animals have a very wide frequency
range of hearing covering over nine octaves! All the animals (except the