Biology Reference
In-Depth Information
(a)
(b)
75
75
74
74
73
73
72
72
71
71
70
70
69
69
68
68
67
67
66
65
66
1.9
2
2.1
2.2
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
Log body weight
Reproductive success
Fig. 14.5
Formant frequency, body size and reproductive success in male red deer.
The length of the vocal tract can be estimated by the formant frequencies, as longer
tracts lead to lower frequencies. The estimated vocal tract length is greater in (a) larger
deer and (b) deer with a higher lifetime reproductive success. From Reby and McComb
(2003). With permission from Elsevier.
with lower formant frequencies also had greater reproductive success, in terms
of number of offspring fathered (Fig. 14.5b).
While, these results show that formant frequency is a reliable index of body size, they
do not show that other deer use this information as a signal. Reby
et al
. (2005) tested
this by playing synthesized vocalizations to harem holding stag holders to simulate the
intrusion of a new challenger stag during the rutting season. They used recordings
made from four mature stags in the 1980s whose roars had never been heard by the
males on which the experiments were run, and re-synthesized them to produce roars
with formant frequencies characteristic of a small, medium or large male. When these
were played at close range to harem-holding stags, the stags paid more attention to, and
roared more frequently, in response to roars suggesting larger opponents. Furthermore,
when harem-holding stags are challenged with roars of larger opponents, they produce
roars with lower formant spacing, suggesting a more fully extended vocal tract. So not
only do stags roar more in response to the challenge of a larger male, but they make
themselves sound bigger. More recently, Charlton
et al
. (2007) have shown that females
also use formant frequencies as a signal of body size. When they were played the
synthesized roars of small and large stags, females preferentially moved towards the
speakers that were playing the roar of a large male.
A puzzling feature of the roars produced by stags is that they are of much lower
frequency than those produced by females, or by other similar sized animals. Fitch and
Reby (2001) showed that this is because stags have a low resting larynx position relative
to other mammals, and that they retract their larynx further when roaring. This
retraction elongates the vocal tract and lowers the formant frequencies of their roar
Formant
frequency is used
as a signal of
body size by
stags …
… and female
deer
Stags have
dropped their
larynx and
elongated their
vocal tract.
