6. Auditory eNS of Dolphins
289
map an extensive surrounding area of the environment with specific
acoustic parameters. According to Suga (1984), the moustached bat, P.
parnellii, maps an area of only about 3m, and target distances of 50 to
140cm are best represented. On the other hand, dolphins, with an echolocation range of more than 100m, probably map a large area of their
surroundings for specific acoustic parameters. Since sound travels over four
times as fast in water, neurons performing equal echo delays of X ms will
represent Y m in a bat and 4 Ym in a dolphin. Specific neurons forming
such axes may take up considerable space in the cortex and could be a
major reason for the great expansion of neocortex in the dolphin. In
addition, I have mentioned various other findings. For example, the observation of large-diameter auditory nerve fibers, the short latency and the
relatively flat latency-intensity function of the dolphin ABR waves, and
rapid temporal resolution of successive sounds. These observations all lend
support to my view that much of the great hypertrophy of the dolphin
auditory system-and perhaps of the entire brain-results from the
animal's need for great precision and speed in processing sound.
9. Summary
Cetaceans are born in the water and live their entire lives there. Even
before birth, we suspect that the fetal auditory system is receiving sound
from the environment to a much greater extent than terrestrial mammals
at the same stage of life due to the excellent match between body tissues
of the mother and the water in which she swims. This early auditory input
may be the basis for the very high state of development of the auditory
central nervous system. When compared with the brains of terrestrial
animals, the size of auditory structures such as the auditory nerve and inferior colliculus are greatly enlarged. There is a greatly increased number of
ganglion cells in the cochlea, suggesting a greater need for acoustic detail
for processing in the central nervous system. Further, the larger diameter
of the myelinated fibers within the auditory nervous system suggests the
need for rapid and detailed processing of acoustic information in the
aquatic environment, which transports sound almost five times as fast as
does air.
Acknowledgments. I thank C.S. Johnson, w.E. Evans, T.H. Bullock, and IG.
McCormick as well as the late E.G. Wever and the late EG. Wood for kindling and encouraging my early interest in the auditory sense of marine
mammals. I thank Michelle Reddy for assistance in preparation of the manuscript. And, thanks to Cynthia Smith and D.L. Woods for reviewing the
manuscript and making helpful suggestions.
289
map an extensive surrounding area of the environment with specific
acoustic parameters. According to Suga (1984), the moustached bat, P.
parnellii, maps an area of only about 3m, and target distances of 50 to
140cm are best represented. On the other hand, dolphins, with an echolocation range of more than 100m, probably map a large area of their
surroundings for specific acoustic parameters. Since sound travels over four
times as fast in water, neurons performing equal echo delays of X ms will
represent Y m in a bat and 4 Ym in a dolphin. Specific neurons forming
such axes may take up considerable space in the cortex and could be a
major reason for the great expansion of neocortex in the dolphin. In
addition, I have mentioned various other findings. For example, the observation of large-diameter auditory nerve fibers, the short latency and the
relatively flat latency-intensity function of the dolphin ABR waves, and
rapid temporal resolution of successive sounds. These observations all lend
support to my view that much of the great hypertrophy of the dolphin
auditory system-and perhaps of the entire brain-results from the
animal's need for great precision and speed in processing sound.
9. Summary
Cetaceans are born in the water and live their entire lives there. Even
before birth, we suspect that the fetal auditory system is receiving sound
from the environment to a much greater extent than terrestrial mammals
at the same stage of life due to the excellent match between body tissues
of the mother and the water in which she swims. This early auditory input
may be the basis for the very high state of development of the auditory
central nervous system. When compared with the brains of terrestrial
animals, the size of auditory structures such as the auditory nerve and inferior colliculus are greatly enlarged. There is a greatly increased number of
ganglion cells in the cochlea, suggesting a greater need for acoustic detail
for processing in the central nervous system. Further, the larger diameter
of the myelinated fibers within the auditory nervous system suggests the
need for rapid and detailed processing of acoustic information in the
aquatic environment, which transports sound almost five times as fast as
does air.
Acknowledgments. I thank C.S. Johnson, w.E. Evans, T.H. Bullock, and IG.
McCormick as well as the late E.G. Wever and the late EG. Wood for kindling and encouraging my early interest in the auditory sense of marine
mammals. I thank Michelle Reddy for assistance in preparation of the manuscript. And, thanks to Cynthia Smith and D.L. Woods for reviewing the
manuscript and making helpful suggestions.