Biomedical Engineering Reference
In-Depth Information
produced by a single source, i.e., by one pair of oscillating labia) was the wide
spanning of the vibration frequency. The same set of measurements used to
identify the muscles that govern the active opening of the labia also made it
possible to recognize the existence of a correlation between the activity of a
ventral muscle (siringealis ventralis, vS) and the frequency of the vocalization.
The activity of this muscle increases with the frequency. This muscle controls
the tension of the labia by means of the separation of the cartilaginous rings
that support them.
The role played by these muscles seem to be widespread among oscines
[Suthers 2001], although some evidence exists that vS might also play a
role in the gating of the sounds produced by zebra finches [Vicario 1991,
Goller and Suthers 1996a].
3.2.3 Vocal Learners and Intrinsic Musculature
Songbirds have intrinsic and extrinsic syringeal muscles; the former originate
and insert completely within the syrinx, and the latter originate or insert
outside the syrinx. Intrinsic muscles probably control the relative position
of syringeal elements, while extrinsic muscles affect the syrinx as a whole
[Larsen and Goller 2002, Gaunt 1983].
The distinction based on syrinx morphology has survived, although nowa-
days we are more interested in other aspects of birdsong, namely its relevance
as a learned complex behavior. From this viewpoint, independent control of
syringeal components seems to be a necessary (although not su
cient) con-
dition for vocal plasticity, since it releases the constraints of a simple syrinx
[Gaunt 1983]. Extrinsic muscles are common to all birds, but not all birds
have in addition intrinsic muscles which directly alter the syringeal configura-
tion. Indeed, vocal learning seems to be confined to species possessing intrinsic
musculature, independent of the number of intrinsic muscles. Among vocal
learners, parrots and hummingbirds have two pairs, while some songbirds
have at least four pairs, such as the mockingbird (
Mimus polyglottos
).
As has already been mentioned, not every avian syrinx is a bipartite struc-
ture. In contrast to songbirds, parrots and doves have only one vocal valve.
Direct endoscopic observation of syringeal movements during phonation have
shown that oscines and nonoscines also differ in the source of vocalizations: pi-
geons and parrots use the lateral tympaniform membranes to generate sound,
in contrast to the medial and lateral labia used by the oscines [Suthers 2001].
In both cases the sound generator is a vibrating membrane.
3.3 The Nonoscine Syrinx
The syringeal anatomy of the nonoscines presents a wide diversity. In some
cases, the syrinx is tracheobronchial as in songbirds. In others, the syrinx is
entirely tracheal, and there are cases in which the syrinx is bronchial. In all
