2. Cetacean Ears
67
peribullar cavity spaces not occupied by conventional soft tissues normally
found in the temporo-mandibular space. Up to five sets of ligaments extend
from the periotic bulla to the sinus walls, suspending the temporal bone
complex in the center of the cavity. Except in physeterids and some ziphiids there are no distinct connections between the odontocete tympanoperiotic complex and the skull. In older animals, bony adhesions (and glue
in museum displays) may connect the periotic bulla to the surrounding
skull bones, but this is not the usual case. This complex set of straps and
foam effectively aligns the tympanic with the mandibular and lateral fatty
channels and allows differential motion of both elements of the tympanoperiotic complex (Figs. 2.2A, 2.3) (Ketten 1998a).
Fraser and Purves (1960) speculated that the enlarged peribullar spaces
were an adaptation for the mechanical stress of high ambient pressures and
were correlated with diving ability. Oelschlager (1986) showed, however,
that peribullar and pterygoid sinuses were best developed in ultra-high frequency dolphins, like the Amazonian I. geoffrensis, but are poorly developed in pelagic mysticetes. He argued that the peribullar plexus and
spacious sinuses act primarily to acoustically isolate the ear for echolocation. It is now generally accepted that in odontocetes the mixed tissue
plexus and suspensory ligaments in lieu of bony skull attachments are
indeed effective acoustical isolators. A threadlike zygomatic arch that
borders the peribullar space minimizes sound conduction from the frontal
and premaxillary sound-producing regions (Cranford, Chapter 3), completing the picture of a rather sophisticated tissue-based acoustic isolation
chamber with the odontocete ear at its core.
In mysticetes the picture is very different. The peribullar space is smaller
and is occupied largely by a thick, fibrous peribullar capsule that pads the
ventral and posterior surfaces of the tympanic bulla. Long flanges of spongy
bone that project medially and posteriorly from the periotic bulla interdigitate with the skull, wedging the periotic tightly against the squamosal and
occipital wings (Figs. 2.2B, 2.4). This strongly suggests bony sound conduction to the ear in baleen whales. Mysticete tympanics are typically twice the
volume of the periotics. They are hemispherical, resembling a truncated
ostrich egg with exceptionally thick (>2cm) walls of compact bone. Like the
tympanics in odontocetes, they are partly fused to the periotic on their
lateral and posterior faces. Whether differences in size and shape of the
periotic and tympanic and its associated tissues strongly influence hearing abilities of cetaceans has not been directly investigated. The wide range
of tympanic sizes, bony attachments, and middle ear volumes suggest,
however, that based on the tympanic bulla range alone there are large
acoustical differences within even the mysticete ears.
5.1.3 Underwater Ears: Another Place Theory
Sound localization is an important aspect of hearing on which the medium
has a profound impact. In land mammals, two binaural cues are important
67
peribullar cavity spaces not occupied by conventional soft tissues normally
found in the temporo-mandibular space. Up to five sets of ligaments extend
from the periotic bulla to the sinus walls, suspending the temporal bone
complex in the center of the cavity. Except in physeterids and some ziphiids there are no distinct connections between the odontocete tympanoperiotic complex and the skull. In older animals, bony adhesions (and glue
in museum displays) may connect the periotic bulla to the surrounding
skull bones, but this is not the usual case. This complex set of straps and
foam effectively aligns the tympanic with the mandibular and lateral fatty
channels and allows differential motion of both elements of the tympanoperiotic complex (Figs. 2.2A, 2.3) (Ketten 1998a).
Fraser and Purves (1960) speculated that the enlarged peribullar spaces
were an adaptation for the mechanical stress of high ambient pressures and
were correlated with diving ability. Oelschlager (1986) showed, however,
that peribullar and pterygoid sinuses were best developed in ultra-high frequency dolphins, like the Amazonian I. geoffrensis, but are poorly developed in pelagic mysticetes. He argued that the peribullar plexus and
spacious sinuses act primarily to acoustically isolate the ear for echolocation. It is now generally accepted that in odontocetes the mixed tissue
plexus and suspensory ligaments in lieu of bony skull attachments are
indeed effective acoustical isolators. A threadlike zygomatic arch that
borders the peribullar space minimizes sound conduction from the frontal
and premaxillary sound-producing regions (Cranford, Chapter 3), completing the picture of a rather sophisticated tissue-based acoustic isolation
chamber with the odontocete ear at its core.
In mysticetes the picture is very different. The peribullar space is smaller
and is occupied largely by a thick, fibrous peribullar capsule that pads the
ventral and posterior surfaces of the tympanic bulla. Long flanges of spongy
bone that project medially and posteriorly from the periotic bulla interdigitate with the skull, wedging the periotic tightly against the squamosal and
occipital wings (Figs. 2.2B, 2.4). This strongly suggests bony sound conduction to the ear in baleen whales. Mysticete tympanics are typically twice the
volume of the periotics. They are hemispherical, resembling a truncated
ostrich egg with exceptionally thick (>2cm) walls of compact bone. Like the
tympanics in odontocetes, they are partly fused to the periotic on their
lateral and posterior faces. Whether differences in size and shape of the
periotic and tympanic and its associated tissues strongly influence hearing abilities of cetaceans has not been directly investigated. The wide range
of tympanic sizes, bony attachments, and middle ear volumes suggest,
however, that based on the tympanic bulla range alone there are large
acoustical differences within even the mysticete ears.
5.1.3 Underwater Ears: Another Place Theory
Sound localization is an important aspect of hearing on which the medium
has a profound impact. In land mammals, two binaural cues are important