1. The
!
!
Honesty of Red
Deer
Introduction
Red deer stags (Cervus Elaphus) communicate their
intentions using ‘roars’. These roars consist of
oscillations of the larynx’s vocal folds which produce
a wave with a fundamental frequency, which then
pass through the vocal tract, this amplifies certain
frequencies depending on unique resonances. This
shapes the signal’s envelope, producing ‘formant’
peaks.
It has been predicted that the fundamental
frequency should be a reliable cue to body size, with
larger species being lower pitched.
However this has not appeared to be the case thus
far. Whereas because the larynx is directly attached
to the base of the skull and can only extend as far as
the sternum, vocal tract length is anatomically
constrained, creating a limit to the lowest formant
frequency that can be produced, making formants a
more reliable measure of body size.
Method
Roars were recorded from 57 red deer on the
Island of Rum, aged 5 - 13 (m=8.3) and weighing
89 - 150kg (m=125). Reproductive success was
calculated based on mean number of mating days
during the season averaged over 4 years.
The first roar of each bout was measured as it
represents the highest fundamentals and lowest
formants. Maximum vocal tract length was
estimated based on the lowest formant of each
stag.
Reby, D., & McComb, K. (2003). Anatomical constraints generate honesty: acoustic cues to age
and weight in the roars of red deer stags.
Animal behaviour, 65(3), 519-530.
Results
Unlike fundamental frequencies which were not
correlated with age nor
weight, formant
frequencies were
negatively correlated with
both age and weight (fig.1 -
left). Also as minimum
formant frequency
decreased, both
reproductive success and
estimated maximal vocal
tract length (maxVTL)
increased.
MaxVTL was also
positively correlated with both age and weight,
with a 6.8 cm average difference between adults
and sub-adults.
Discussion
The results suggest that a roar’s fundamental
frequencies do not present a reliable cue to the
weight or age of the stag producing it. However it
is suggested that formant frequencies, (due to
being anatomically constrained by their maximal
vocal tract length), do present an honest cue as to
weight and age, and by extension reproductive
fitness, of the stag producing it. This suggests that
such constraints may have developed precisely to
allow a deer to predict an opponent’s strength or
partner’s reproductive abilities; something which
is not found in the majority of the animal kingdom
due to not having this degree of control over their
vocalisations.
Luke Costley-White
References
Fant, G. (1960) Acoustic Theory of Speech Production. The
Hague:Mouton.
Fitch, W. T. & Reby, D. (2001) The descended larynx is not uniquely
human. Proceedings of the Royal Society of London, Series B, 268,
1669–1675.
Morton, E. S. (1977). On the occurrence and significance of motivation-
structural rules in some bird and mammal sounds. American
Naturalist, 855-869.