138
T.w. Cranford
Aroyan et al. 1992; Au 1993). More specifically, in delphinids the flattened
shelves of the premaxillary bones are covered by the highly (acoustically)
reflective air sacs (Griffin 1980). The skull evidently plays its traditional supporting (skeletal) role and determines some overall geometry but may only
be directly effective in acoustic reflection at depth when the available air
volume is compressed.
5.2 Nasal Air Sacs
The complex three-dimensional morphology and acoustic reflectivity of air
sacs are undoubtedly central to the function of projecting biosonar sounds
forward (Giro and Dubrovskii 1975). These supracranial diverticula, especially the premaxillary sacs, likely comprise the most influential stage in
beam formation. In fact, it may be that small errors or omissions in air sac
models, in a specimen of Delphinus de/phis (the short-beaked common
dolphin), account for the small discrepancy in source location between computer simulations and anatomic predictions for the right side of the head
(Aroyan 1996). It has been shown that these premaxillary sacs are inflated
with some air just before the onset of sound generation (Norris et al. 1971;
Dormer 1979).
Other characters, such as the inflatable and malleable nature of the nasal
diverticula, may be partially responsible for the intractable search for the
sound sources. One reason the sound generator(s) have been so difficult to
locate is that they are shrouded in these acoustically reflective air spaces,
making them difficult to observe with imaging techniques like ultrasound
(Mackay 1980).
Much of the facial musculature forms fanlike insertions upon the margins
of these air spaces, suggesting considerable dexterity in the ability to manipulate their shapes and sizes. The clearest demonstration of this ability is seen
in the cineradiographic movies produced by Norris and colleagues (1971)
and by Dormer (1974,1979). These films reveal the rapid movement, shifting, and adjustment of air between the various sacs. They also show that air
escapes into the vestibular sacs during the production of whistles and bursts
of pulses, after which it is recycled before another bout of generating
sounds. The vestibular sacs (and their ubiquitous homologues in all other
odontocetes) function, at least partially, in the capture and storage of the
air used in sound production so that it can be recycled between bouts of
making sound (Norris et al. 1971).
In addition to the manipulation of air sac morphology, it is interesting
that the sizes and shapes of static air sacs, as might be studied in postmortem specimens, are also variable. In fact, air sac morphology is more
variable than any other aspect of forehead anatomy, both within and
between species (Mead 1975). We might be tempted to attribute this
characteristic to a decrease in their functional importance. However, it may
instead indicate that the size and shape of any individual sac is not as
T.w. Cranford
Aroyan et al. 1992; Au 1993). More specifically, in delphinids the flattened
shelves of the premaxillary bones are covered by the highly (acoustically)
reflective air sacs (Griffin 1980). The skull evidently plays its traditional supporting (skeletal) role and determines some overall geometry but may only
be directly effective in acoustic reflection at depth when the available air
volume is compressed.
5.2 Nasal Air Sacs
The complex three-dimensional morphology and acoustic reflectivity of air
sacs are undoubtedly central to the function of projecting biosonar sounds
forward (Giro and Dubrovskii 1975). These supracranial diverticula, especially the premaxillary sacs, likely comprise the most influential stage in
beam formation. In fact, it may be that small errors or omissions in air sac
models, in a specimen of Delphinus de/phis (the short-beaked common
dolphin), account for the small discrepancy in source location between computer simulations and anatomic predictions for the right side of the head
(Aroyan 1996). It has been shown that these premaxillary sacs are inflated
with some air just before the onset of sound generation (Norris et al. 1971;
Dormer 1979).
Other characters, such as the inflatable and malleable nature of the nasal
diverticula, may be partially responsible for the intractable search for the
sound sources. One reason the sound generator(s) have been so difficult to
locate is that they are shrouded in these acoustically reflective air spaces,
making them difficult to observe with imaging techniques like ultrasound
(Mackay 1980).
Much of the facial musculature forms fanlike insertions upon the margins
of these air spaces, suggesting considerable dexterity in the ability to manipulate their shapes and sizes. The clearest demonstration of this ability is seen
in the cineradiographic movies produced by Norris and colleagues (1971)
and by Dormer (1974,1979). These films reveal the rapid movement, shifting, and adjustment of air between the various sacs. They also show that air
escapes into the vestibular sacs during the production of whistles and bursts
of pulses, after which it is recycled before another bout of generating
sounds. The vestibular sacs (and their ubiquitous homologues in all other
odontocetes) function, at least partially, in the capture and storage of the
air used in sound production so that it can be recycled between bouts of
making sound (Norris et al. 1971).
In addition to the manipulation of air sac morphology, it is interesting
that the sizes and shapes of static air sacs, as might be studied in postmortem specimens, are also variable. In fact, air sac morphology is more
variable than any other aspect of forehead anatomy, both within and
between species (Mead 1975). We might be tempted to attribute this
characteristic to a decrease in their functional importance. However, it may
instead indicate that the size and shape of any individual sac is not as