Mechanics of the Syrinx in Gallus Gallus. I. a Comparison of Pressure Events in Chickens to Those in Oscines

A recent study (Gaunt et al. 1973) of gas pressure and flow in a vocalizing oscine (Starling, Sturnus vulgaris) produced unexpected results. First, while pressure in the air sacs during a distress call usually rose to 40 times that of ventilatory exhalation, pressure in the trachea, “downstream” from the syrinx, was commonly lower than during exhalation; indeed, it often scarcely exceeded atmospheric pressure. Second, despite the increased pressure head, which tended to persist for twice as long as an exhalation, the volume of gas exhaled increased by no more than a third, and often was similar to that seen in excited ventilation. Third, if pressure within the air sacs (especially the interclavicular) was allowed to drop by flow through a cannula, both vocalization and flow through the trachea ceased, even in the presence of relatively high air sac pressures. These combined observations suggest that the oscine syrinx somehow imparts a high resistance to flow during vocalization. In effect, the syrinx acts as a nozzled valve. Although the column of moving air that activates the membrane(s) may have a high velocity, the cross-sectional diameter of the column is small, and remarkably little air is consumed by the vocalizing process. We have hypothesized that such a valve would provide a strong selective advantage during vocalization by increasing both the loudness (related to air velocity) and duration (related to air consumption) of the call. The exact nature of the valve in oscines is not known. However, our experiments indicate that it probably involves a structure that is controlled by muscular activity rather than pressure events. Hence, it is probably a more substantial structure than a vibrating membrane. The studies of Chamberlain et al. ( 1968)) Greenewalt ( 1968)) and Stein ( 1968) suggest that an appropriate structure would be the external labium (EL), and the models we have proposed (Gaunt and Wells 1973, fig. 10) assume that to be the case. Oscines generally (always?) have an EL, while non-passerines generally lack it. It is certainly absent from chickens, and the labia associated with the bullae of some male anatids show a quite different structure. The syrinx of chickens also differs from that of ostines in the nature and position of the vibrating membranes. A chicken’ s syrinx is tracheobronchial (fig. 1). Just anterior to the syrinx the final four rings of the trachea are fused into a “drum.” The pessulus is a dorsoventrally-oriented, cartilaginous bar in the medial plane marking the divergence of the brouchi from the trachea. The ends of this bar are expanded into triangular plates at the dorsal and ventral surfaces, giving it a dumbbell-like appearance. Between the posterior end of the drum and the pessulus, the trachea is strongly laterally compressed. Its walls are essentially membranous, comprising the external tympaniform membranes, the anterior portions of which are invested by cartilaginous partial rings. Hence, the external tympaniform membranes are directly opposed to each other in this region. They continue posteriad onto the bronchi where they are also opposed by the internal tympaniform membranes. Experiments by Gross (196413) indicated that the external tympaniform membranes are the major source of sound production in chickens. In contrast, the sound-producing membranes of oscines are the internal tympaniform membranes lying at the anterior end of the medial surface of each bronchus. They are not opposed by another membrane, but by either the EL or the outer walls of the bronchi, depending upon the species. Although pressure changes in the air sacs during vocalizations of chickens have been investigated (Brackenbury 1972, Gross 1964a), no study deals with simultaneous pressure events on either side of the syrinx. Such a study would be of dual interest. It would provide basic information of pressure events in a simpler (i.e., fewer muscles, no EL) and presumably more primitive syrinx than that of