Biology Reference
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(Fig.  14.6). Since all stags do this, formant dispersion is a
reliable index of body size. How was this situation reached?
Presumably, at a previous point in their evolutionary history,
no stags lowered their larynx, so formant frequency was also a
reliable index of body size. Then some stags evolved the ability
to lower their larynx, allowing them to 'exaggerate' their
signal to their own advantage, but making their signal
unreliable. The increased reproductive success of these males
would have meant that the ability to lower the larynx would
quickly spread through the population, until it was done by all
stags, at which point the signal would become a reliable index
again. We might predict such exaggeration to be a common
feature of indices, as whenever they are able to arise they
should spread rapidly to fixation. An even more extreme
example of an extended larynx is provided by the strange
looking male hammerhead bat, Hypsignathus monstrosus ,
where over half its body is its larynx (Fig. 14.7; Bradbury,
1977). Other possible examples of traits that could have
evolved to exaggerate size include when mammals such as
dogs raise the fur along their neck and back as part of an
aggressive display, or when threatened puffer fish expand by
filling their elastic stomachs with water.
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Fig. 14.6 As red deer stags roar, they retract
their larynx, and hence increase the length of
their vocal tract. This results in the production
of lower formant frequencies. From Fitch and
Reby (2001). With permission of the Royal
Society.
Image not available in this digital edition.
Fig. 14.7 A male hammerhead bat, Hypsignathus monstrosus . Males compete for
mates in leks that contain 30-150 males, by signalling to females with loud honking
calls. These calls appear to have a large influence on mating success, with just 6% of
the males obtaining 79% of the copulations. Males have a bizarre morphology that
was presumably selected because it exaggerates their calls, with an enormous bony
larynx that fills their chest cavity and a head with enlarged cheek pouches, inflated
nasal cavities and a funnel shaped mouth. Photo © CNRS Photothèque/Devez, Alain R.
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