112
T.w. Cranford
Clark, Chapter 4), but this notion is difficult to substantiate and there are
alternative explanations for the function of whistles (Norris et al. 1994;
McCowan 1995; McCowan and Reiss 1995; Moore and Ridgway 1995,1996).
To my knowledge there is no evidence that odontocete whistles are used in
echolocation, although narrow bandwidth, frequency-modulated sounds are
used by some bats for this purpose (Neuweiler 1984,1990). The production
of whistles apparently requires the movement of a larger volume of air
than is required in the production of clicks (Ridgway and Carder 1988).
This probably makes odontocete whistles unsuitable for echolocation
because air volume is drastically reduced by hydrostatic pressure during
diving. A few odontocete species have been reported to whistle and click
simultaneously (Lilly and Miller 1961; Evans and Prescott 1962; Brill
and Harder 1991). The simultaneous production of whistles and clicks
may indicate different generation mechanisms, although the question
of whether or not they are produced at different locations is open for
discussion.
Bangs are less well known from the literature, where they have been
referred to as pops, jaw-claps, cracks, or bangs (Caldwell et al. 1962; Lilly
1962; Overstrom 1982; dos Santos et al. 1990). These bangs are apparently
not produced by clapping the jaws together (Cranford et al. 1993) or slapping portions of the body against the water surface or by powerful movements of the tail flukes against prey items as, for instance, reported by
Smolker and Richards (1988). Bangs have not been systematically studied
or described, but they are generally rich in low-frequency energy, with
primary peaks in the neighborhood of 1 to 8kHz, depending upon the
species. The exact frequency range is probably a function of the size of the
sound generation structures involved and, consequently, the size of
the animal. Bangs generally last for several milliseconds and are thought to
function in prey capture (Norris and Mli>hl 1983; Norris et al. 1994, p. 280)
and/or in social facilitation (Caldwell et al. 1962; Overstrom 1982; Connor
and Smolker 1996), but they may have a myriad of different functions. Little
is known about the generation mechanism for bangs (Marten et al. 1988;
Cranford et al. 1993).
Only one study focused directly upon the site and mechanism for the generation of bangs (Cranford et al. 1993). From that work, we concluded that
the generation site was in the supracranial nasal region (i.e., the nose). In
addition, we postulated that the mechanism might be similar to that used
for echolocation pulses, except that the tissue region involved should be
larger and the air pressures attained, considerably higher.
Pulses will be the focus of the remainder of this chapter. This type of
sound has been most often studied. Pulses can be subdivided into broad
categories based upon repetition rate. Those produced at slow rates, referred
to as clicks, often sound like a rusty hinge on a creaky door and are unequivocally used by odontocetes during echolocation. When pulses are produced
at very rapid rates, they are often referred to as bursts or burst pulses.
T.w. Cranford
Clark, Chapter 4), but this notion is difficult to substantiate and there are
alternative explanations for the function of whistles (Norris et al. 1994;
McCowan 1995; McCowan and Reiss 1995; Moore and Ridgway 1995,1996).
To my knowledge there is no evidence that odontocete whistles are used in
echolocation, although narrow bandwidth, frequency-modulated sounds are
used by some bats for this purpose (Neuweiler 1984,1990). The production
of whistles apparently requires the movement of a larger volume of air
than is required in the production of clicks (Ridgway and Carder 1988).
This probably makes odontocete whistles unsuitable for echolocation
because air volume is drastically reduced by hydrostatic pressure during
diving. A few odontocete species have been reported to whistle and click
simultaneously (Lilly and Miller 1961; Evans and Prescott 1962; Brill
and Harder 1991). The simultaneous production of whistles and clicks
may indicate different generation mechanisms, although the question
of whether or not they are produced at different locations is open for
discussion.
Bangs are less well known from the literature, where they have been
referred to as pops, jaw-claps, cracks, or bangs (Caldwell et al. 1962; Lilly
1962; Overstrom 1982; dos Santos et al. 1990). These bangs are apparently
not produced by clapping the jaws together (Cranford et al. 1993) or slapping portions of the body against the water surface or by powerful movements of the tail flukes against prey items as, for instance, reported by
Smolker and Richards (1988). Bangs have not been systematically studied
or described, but they are generally rich in low-frequency energy, with
primary peaks in the neighborhood of 1 to 8kHz, depending upon the
species. The exact frequency range is probably a function of the size of the
sound generation structures involved and, consequently, the size of
the animal. Bangs generally last for several milliseconds and are thought to
function in prey capture (Norris and Mli>hl 1983; Norris et al. 1994, p. 280)
and/or in social facilitation (Caldwell et al. 1962; Overstrom 1982; Connor
and Smolker 1996), but they may have a myriad of different functions. Little
is known about the generation mechanism for bangs (Marten et al. 1988;
Cranford et al. 1993).
Only one study focused directly upon the site and mechanism for the generation of bangs (Cranford et al. 1993). From that work, we concluded that
the generation site was in the supracranial nasal region (i.e., the nose). In
addition, we postulated that the mechanism might be similar to that used
for echolocation pulses, except that the tissue region involved should be
larger and the air pressures attained, considerably higher.
Pulses will be the focus of the remainder of this chapter. This type of
sound has been most often studied. Pulses can be subdivided into broad
categories based upon repetition rate. Those produced at slow rates, referred
to as clicks, often sound like a rusty hinge on a creaky door and are unequivocally used by odontocetes during echolocation. When pulses are produced
at very rapid rates, they are often referred to as bursts or burst pulses.