130
T.w. Cranford
the tip of the rostrum (Evans 1973). He suggested that sounds might be
made by mechanical action of the nasal plugs against the edge of the bony
nares in a series of "relaxation oscillations." Even though his "frictionstiction" mechanism is appealing because of "efficient energy conversion,"
it is difficult to imagine how it could account for rapid repetition rates (800
or more per second) and the consistent waveforms and precise rhythms
produced by dolphins. Over and above the potential for damaging effects
by friction on soft tissue, the Evans model would seem to require one of
two unlikely scenarios. The sounds would have to be produced either at
a single localized sound generation site using rapid reciprocating muscle
actions, or at multiple sites using a kind of stridulatory motion.
In hindsight, a single site and rapid reciprocating contractions could
not account for dolphin pulse repetition rates in excess of 800 per second
(Lilly 1962), since the fastest muscles yet recorded can attain only 500
contractions per second. These rapid firing rates are, interestingly enough,
those of the stridulous sound generation muscles in cicadas (Josephson and
Young 1985).
The multisited stridulation scenario is unlikely for two reasons. First,
it would require several resilient and, more or less, matched sources in
order to account for the consistent nature of successive pulse waveforms
(Kamminga 1988; Au 1990,1993). Second, it is difficult to imagine how a
dolphin could maintain the ramped timing and precise spacing observed
between pulses (in trains consisting of several hundred clicks), since this
would require the animal to actuate the multisited mechanism several times
to produce a single click train.
Later, Evans and Maderson (1973) altered the earlier notion to include
rubbing of the nasal plugs and adjacent tissues upon the blowhole ligament,
again in a series of relaxation oscillations, but this suggested mechanism
is also stridulatory and suffers from the same objections raised in the
previous paragraph. These friction-based mechanisms are also questionable because repeated agitation should produce considerable wear upon
the soft tissue sources or stimulate the build up of a protective keratinized
epithelium. I have seen no evidence of either. Evans and Maderson were
apparently aware of the potential for mechanical damage or wear upon
the soft tissues from this kind of mechanism and they described a glandular region that provides lubrication (which should counteract their frictionstiction mechanism). Finally, the stated primary reason for their affinity
to this friction-based mechanism is for its efficient energy conversion.
However, there are other potentially efficient mechanisms and there is
no evidence that dolphins or other odontocetes are energy limited when
it comes to sound generation. There is evidence that the energy investment
in sound generation activity is not trivial (Cole 1995), but there is no
indication that it is, or could become, a significant fraction of an animal's
daily budget, as it is in roosting bats (Speakman et a1.1989) or in some frogs
(Ryan 1988).
T.w. Cranford
the tip of the rostrum (Evans 1973). He suggested that sounds might be
made by mechanical action of the nasal plugs against the edge of the bony
nares in a series of "relaxation oscillations." Even though his "frictionstiction" mechanism is appealing because of "efficient energy conversion,"
it is difficult to imagine how it could account for rapid repetition rates (800
or more per second) and the consistent waveforms and precise rhythms
produced by dolphins. Over and above the potential for damaging effects
by friction on soft tissue, the Evans model would seem to require one of
two unlikely scenarios. The sounds would have to be produced either at
a single localized sound generation site using rapid reciprocating muscle
actions, or at multiple sites using a kind of stridulatory motion.
In hindsight, a single site and rapid reciprocating contractions could
not account for dolphin pulse repetition rates in excess of 800 per second
(Lilly 1962), since the fastest muscles yet recorded can attain only 500
contractions per second. These rapid firing rates are, interestingly enough,
those of the stridulous sound generation muscles in cicadas (Josephson and
Young 1985).
The multisited stridulation scenario is unlikely for two reasons. First,
it would require several resilient and, more or less, matched sources in
order to account for the consistent nature of successive pulse waveforms
(Kamminga 1988; Au 1990,1993). Second, it is difficult to imagine how a
dolphin could maintain the ramped timing and precise spacing observed
between pulses (in trains consisting of several hundred clicks), since this
would require the animal to actuate the multisited mechanism several times
to produce a single click train.
Later, Evans and Maderson (1973) altered the earlier notion to include
rubbing of the nasal plugs and adjacent tissues upon the blowhole ligament,
again in a series of relaxation oscillations, but this suggested mechanism
is also stridulatory and suffers from the same objections raised in the
previous paragraph. These friction-based mechanisms are also questionable because repeated agitation should produce considerable wear upon
the soft tissue sources or stimulate the build up of a protective keratinized
epithelium. I have seen no evidence of either. Evans and Maderson were
apparently aware of the potential for mechanical damage or wear upon
the soft tissues from this kind of mechanism and they described a glandular region that provides lubrication (which should counteract their frictionstiction mechanism). Finally, the stated primary reason for their affinity
to this friction-based mechanism is for its efficient energy conversion.
However, there are other potentially efficient mechanisms and there is
no evidence that dolphins or other odontocetes are energy limited when
it comes to sound generation. There is evidence that the energy investment
in sound generation activity is not trivial (Cole 1995), but there is no
indication that it is, or could become, a significant fraction of an animal's
daily budget, as it is in roosting bats (Speakman et a1.1989) or in some frogs
(Ryan 1988).