Tecumseh Fitch on vocal length & larynx size
- Because the length of the vocal folds determines the lowest frequency at
which the folds can vibrate, and long folds are thus able to produce lower
frequencies, one might expect that a low fundamental would provide a
reliable indication of large body size.
This is indeed true if we look across a large range of spp (mice have high
fundamentals relative to dogs, elephants very much lower), but the size of
the larynx is not tightly constrained by body size. Thus, a huge larynx has
independently evolved in many mammal spp, probably in response to selection
for low-pitched voices, eg, in howler monkeys Alouatta, the larynx & hyoid
have grown to fill the space between mandible & sternum, giving these small
monkeys remarkably impressive low-pitched voices. The most extreme example
is seen in the hammerhead bat Hypsignathus monstrosus, in which the larynx
of males expands to fill the entire thoracic cavity, pushing the heart,
lungs & trachea down into the abdomen.
A similar though less impressive increase in larynx dimensions is observed
in human males and is responsible for their voice change at puberty.
Even without greatly enlarging the larynx, a transformation of the vocal
folds into massive vocal pads in large cats & other mammals makes the
production of very low-pitched roars possible in some large mammals.
Not surprisingly, the lowest pitched voices are possessed by the largest
animals, which can produce infra-sound. Because low frequencies can travel
further than high frequencies in some environments, infrasonic vocalizations
probably represent the most widely broadcast animal signals in existence.
Elephants produce & hear extremely low frequency rumbles & other infrasonic
vibrations. In certain environments (eg, open savannah with Tp inversions),
such sounds are able to carry more than 10 km, leading to unusu.large
vocally mediated communication networks in African elephants. But even these
long-distance communication systems are dwarfed by those of the large baleen
whales : both fin- & blue whale males Balaenoptera physalis & musculus
produce low-frequency vocalizations with 20 Hz fundamental frequencies ;
under ideal conditions a vocalization from a single animal can fill an
entire ocean basin. The mechanism by which these low-frequency sounds are
produced is unknown, but baleen whales possess a large larynx with a massive
vocal fold & a laryngeal sac that allows air recirculation, making it
possible that the aero-dynamic/myo-elastic theory applies even to these