Roland Frey, Alban Gebler, Guido Fritsch, Kaarlo Nygrén and Gerald E.
Nordic rattle: the hoarse vocalization and the inflatable laryngeal air sac of
reindeer (Rangifer tarandus)
Journal of Anatomy 210 (2), 131?159.
Laryngeal air sacs have evolved convergently in diverse mammalian lineages
including insectivores, bats, rodents, pinnipeds, ungulates and primates, but
their precise function has remained elusive. Among cervids, the vocal tract of
reindeer has evolved an unpaired inflatable ventrorostral laryngeal air sac.
This air sac is not present at birth but emerges during ontogenetic
development. It protrudes from the laryngeal vestibulum via a short duct
between the epiglottis and the thyroid cartilage. In the female the growth of
the air sac stops at the age of 2?3 years, whereas in males it continues to
grow up to the age of about 6 years, leading to a pronounced sexual dimorphism
of the air sac. In adult females it is of moderate size (about 100 cm3),
whereas in adult males it is large (3000?4000 cm3) and becomes asymmetric
extending either to the left or to the right side of the neck. In both adult
females and males the ventral air sac walls touch the integument. In the adult
male the air sac is laterally covered by the mandibular portion of the
sternocephalic muscle and the skin. Both sexes of reindeer have a double
stylohyoid muscle and a thyroepiglottic muscle. Possibly these muscles assist
in inflation of the air sac. Head-and-neck specimens were subjected to
macroscopic anatomical dissection, computer tomographic analysis and
skeletonization. In addition, isolated larynges were studied for comparison.
Acoustic recordings were made during an autumn round-up of semi-domestic
reindeer in Finland and in a small zoo herd. Male reindeer adopt a specific
posture when emitting their serial hoarse rutting calls. Head and neck are kept
low and the throat region is extended. In the ventral neck region, roughly
corresponding to the position of the large air sac, there is a mane of longer
hairs. Neck swelling and mane spreading during vocalization may act as an
optical signal to other males and females. The air sac, as a side branch of the
vocal tract, can be considered as an additional acoustic filter. Individual
acoustic recognition may have been the primary function in the evolution of a
size-variable air sac, and this function is retained in mother?young
communication. In males sexual selection seems to have favoured a considerable
size increase of the air sac and a switch to call series instead of single
calls. Vocalization became restricted to the rutting period serving the
attraction of females. We propose two possibilities for the acoustic function
of the air sac in vocalization that do not exclude each other. The first
assumes a coupling between air sac and the environment, resulting in an
acoustic output that is a combination of the vocal tract resonance frequencies
emitted via mouth and nostrils and the resonance frequencies of the air sac
transmitted via the neck skin. The second assumes a weak coupling so that
resonance frequencies of the air sac are lost to surrounding tissues by
dissipation. In this case the resonance frequencies of the air sac solely
influence the signal that is further filtered by the remaining vocal tract.
According to our results one acoustic effect of the air sac in adult reindeer
might be to mask formants of the vocal tract proper. In other cervid species,
however, formants of rutting calls convey essential information on the quality
of the sender, related to its potential reproductive success, to conspecifics.
Further studies are required to solve this inconsistency.
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