3. Impulse Sound Sources
139
important as the combined configuration of the air sacs, which more likely
determines the composite reflective function.
5.3 Connective Tissue Theca
The connective tissue theca, a high-density capsule that embraces the posterior region of the fatty melon, has only recently been described as an
integral structural unit (Cranford 1992a). Its functional significance is still
a matter of speculation, although its proposed role as an acoustic "megaphone" or wave guide (Cranford 1992a; Cranford et al. 1996) has found
some support (Aroyan et aI.1992;Aroyan 1996). The morphological variety
in the connective tissue theca across the Odontoceti is as rich as that found
in melon morphology, although we have only begun to conceive of the
theca's role in biosonar beam formation (Degollada et al. 1998).
The connective tissue theca appears to be thicker and more extensive in
those odontocetes whose nasal diverticula are less complex or elaborate.
For example, the ziphiids have a thick mass of connective tissue that caps
the dorsal roof of the forehead. Ziphiids are also noted for their lack of
elaborate air sacs, in contrast to delphinoids. By way of illustration, the air
space that passes between the blowhole and the phonic lips in Mesoplodon
is broad laterally and dorsoventrally flattened, without the extensive outpocketing or folding seen in dolphins and porpoises (Heyning 1989;
Cranford 1992a). It would be interesting to ascertain whether the groupwise differences between these characteristics indicate a primitive condition or a derived state. The answer to that puzzle would lead to another
intriguing question. Does the increased development of connective tissue
interfaces in ziphiids belie the primitive condition and indicate an advanced
state of acoustic reflectivity for deep-diving species, where compressive
forces reduce the air volume and therefore its potential reflective function?
5.4 Melon
Norris (1964) credits EG. Wood and Paul Asa-Dorian with the first suggestion that the fatty melon could function as an impedance matching
device between soft tissue and the aqueous environment. Since then, Norris
and a host of other workers have demonstrated impedance matching and
suggested that the melon also functions as an "acoustic lens." Considerable
progress has also been made in describing the characteristics of "acoustic
fats" within the odontocete forehead (Norris 1968; Malins and Varanasi
1975; also see a review in Morris 1986).
Norris and Harvey (1974) offered the most dramatic demonstration of
the focusing effect of the melon. They found that the melon was capable of
focusing sound both along and across its major axis. There is a tantalizing
array of morphological and topological differences in the melon of odontocetes. There is a central low-density core that runs thorough the melon of
139
important as the combined configuration of the air sacs, which more likely
determines the composite reflective function.
5.3 Connective Tissue Theca
The connective tissue theca, a high-density capsule that embraces the posterior region of the fatty melon, has only recently been described as an
integral structural unit (Cranford 1992a). Its functional significance is still
a matter of speculation, although its proposed role as an acoustic "megaphone" or wave guide (Cranford 1992a; Cranford et al. 1996) has found
some support (Aroyan et aI.1992;Aroyan 1996). The morphological variety
in the connective tissue theca across the Odontoceti is as rich as that found
in melon morphology, although we have only begun to conceive of the
theca's role in biosonar beam formation (Degollada et al. 1998).
The connective tissue theca appears to be thicker and more extensive in
those odontocetes whose nasal diverticula are less complex or elaborate.
For example, the ziphiids have a thick mass of connective tissue that caps
the dorsal roof of the forehead. Ziphiids are also noted for their lack of
elaborate air sacs, in contrast to delphinoids. By way of illustration, the air
space that passes between the blowhole and the phonic lips in Mesoplodon
is broad laterally and dorsoventrally flattened, without the extensive outpocketing or folding seen in dolphins and porpoises (Heyning 1989;
Cranford 1992a). It would be interesting to ascertain whether the groupwise differences between these characteristics indicate a primitive condition or a derived state. The answer to that puzzle would lead to another
intriguing question. Does the increased development of connective tissue
interfaces in ziphiids belie the primitive condition and indicate an advanced
state of acoustic reflectivity for deep-diving species, where compressive
forces reduce the air volume and therefore its potential reflective function?
5.4 Melon
Norris (1964) credits EG. Wood and Paul Asa-Dorian with the first suggestion that the fatty melon could function as an impedance matching
device between soft tissue and the aqueous environment. Since then, Norris
and a host of other workers have demonstrated impedance matching and
suggested that the melon also functions as an "acoustic lens." Considerable
progress has also been made in describing the characteristics of "acoustic
fats" within the odontocete forehead (Norris 1968; Malins and Varanasi
1975; also see a review in Morris 1986).
Norris and Harvey (1974) offered the most dramatic demonstration of
the focusing effect of the melon. They found that the melon was capable of
focusing sound both along and across its major axis. There is a tantalizing
array of morphological and topological differences in the melon of odontocetes. There is a central low-density core that runs thorough the melon of