Abbot S. Gaunt

Syringeal Structure and Avian Phonation

Studies of syringeal function have historically been hampered by two difficulties, one technical and one perceptual. The technical difficulty is that because the syrinx is at the base of a long trachea and because its functioning is distorted if the surrounding interclavicular airsac is ruptured, direct observation of natural syringeal function has so far proved impossible. Hence, all analyses of syringeal function are based on indirect evidence. Such evidence may be obtained from dissections, manipulations of extracted syrinxes, models, analyses of physiological events associated with phonation, or analyses of the sounds produced.

Serially arranged myofibers : an unappreciated variant in muscle architecture

Our comparative studies suggest that the length of myofibers in tetrapods is subject to an unappreciated degree of variability. Many mammalian strap muscles are composed of short, overlapping myofibers. This arrangement and its associated distribution pattern of motor endplates (neural control) appear to be general in birds and widespread in other tetrapods. Contrariwise, most muscles of primates appear to be composed of long myofibers. The implications of this variation for studies of development, neuromuscular control, and muscle function are largely unexplored.

Theoretical models of the avian syrinx

The acoustic spectra of most birdsongs contain either a fundamental tone without overtones, a fundamental with harmonic overtones, or broadband noise with all frequencies present. The classic functional model of the avian syrinx suggests that vibrating medial tympaniform membranes generate these spectra. It is shown here that, while vibrating, these membranes should generate spectra that contain many partial overtones. As partials are not present in most birdsongs, the vibrating membrane model appears inadequate to explain syringeal function. Two new models are presented: the vibrating string model and the aerodynamic model. In the former, membrane-like medial tympaniform membranes are stretched into the shape of a string. Vibrating string-like medial tympaniform membranes could have acoustic spectra with either a fundamental alone or a fundamental plus harmonics. In the latter case, the medial tympaniform membranes function only to create an orifice through which respiratory air must pass. Periodic, trailing vortices are created off the downstream side of the membranes. The periodicity of vortex shedding determines the frequency of the sound. Acoustic spectra generated by this mechanism contain either a fundamental alone, a fundamental plus harmonics, or broadband noise. As the acoustic spectra postulated by the vibrating string model and the aerodynamic model correlate well with the acoustic spectra found in birdsong, they appear to be better models of avian syringeal function.

The effects of tracheal coiling on the vocalizations of cranes (Aves; Gruidae)

SummaryIt is generally supposed that the elongated, often coiled tracheae of many species of birds are adaptations for the production of loud, penetrating calls. A corollary supposition is that the acoustic effects are produced by the resonant properties of the elongated tube, with the birds being analogized to a wind instrument. We have experimented with several species of cranes possessing different degrees of tracheal coiling. Regardless of the degree of coiling, all cranes can utter extremely loud calls using remarkably low driving pressures. Neither surgical modifications of the trachea nor changing the respiratory gases to helium-oxygen produced consistent changes of voice that could be unambiguously attributed to changes of tubal resonances. However, shortening the trachea markedly reduced vocal intensity, the degree of reduction being roughly proportional to the degree of shortening. Although some of that reduction may derive from an increased impedance mismatch at the external aperture of the tube, and some from a decreased radiation directly from the hard walls of the trachea, these explanations scarcely account for the dramatic effects we observed. We, therefore, hypothesize a more unusual mechanism: The tracheal coils that are embedded in the sternum serve a function analogous to the bridge of a stringed instrument, transmitting the vibrations of a tiny sound source to a large radiating surface, the sternum. The sternum then vibrates against the large internal air reservoir of the avian airsac system. As it has a complex shape, the sternum will have many resonances and will respond to many frequencies; as a solid oscillator, its resonances will not be greatly affected by low density gases. Hence, we suggest that cranes and other birds with enlarged windpipes are more properly analogized with a violin than a trombone.

MECHANICS OF THE SYRINX IN GALLUS GALLUS. I. A COMPARISON OF PRESSURE EVENTS IN CHICKENS TO THOSE IN OSCINES

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 mem

Variations in the distribution of motor end‐plates in the avian pectoralis

Most avian muscles consist of serially arranged, overlapping fibers that do not extend the length of the muscle. This condition appears to be plesiomorphic with respect to diapsid reptiles. The presence of this serialfibered architecture is evidenced by bands of stained motor end‐plates (meps) perpendicular to the columns of fibers and dividing each column into a series of “segments.” The avian pectoralis was chosen for a study of variation in the distribution of meps within a single muscle. We report the interspecific variation for 158 specimens in 63 species. We also use additional specimens to examine intraspecific variation.

Muscle Architecture and Control Demands

Muscles effect locomotion, and their gross architecture still poses analytical problems. These problems involve the arrangement of myofibers and motor units within muscles and that of muscles around joints. The arrangement of fibers may involve a range of considerations from the equivalence or nonequivalence of sarcomeres to placement, attachment, and angulation of fascicles and entire muscles; consequently, these levels and their development and coordination overlap. Many problems at the macroscopic level require clarification of how an animal uses a compartment of suite of muscles and whether morphological differences reflect functional ones. The understanding of intermediate architecture, including issues of compartmentation, pinnation, and concatenation, remains more elusive, as some morphologically distinct muscles may be functionally equivalent. As yet we have inadequate appreciation of the opportunities or limitations provided to the control system by a particular arrangement of fibers, or vice versa. Exploration of the rules that govern these conditions provides abundant opportunities for cooperation among neurobiologists, developmental biologists, physiologists and morphologists.

Mechanics of glochidial attachment (Mollusca: Bivalvia: Unionidae)

Glochidia are third‐class levers in which the valves form the lever arms and the single adductor muscle produces the force. In this study the lengths of the lever arms and the areas of glochidial valves and adductor muscles were determined for 57 species of unionid glochidia. The position of the adductor muscle relative to the dorsal margin of the larval valve was also determined for each species. From these data and an analysis of the possible configurations of adductor muscle and valve dimensions, we determined that most of the glochidia within the Unionidae emphasize area of sweep during valve adduction. These glochidia possess long resistance arms and short force arms and generally had small‐diameter adductor muscles. Other glochidia, however, were found to possess one or all of the following: short resistance arms, long force arms, and large‐diameter adductor muscles. It is suggested that these glochidia are adapted for strength of valve adduction and that for these larvae a trade‐off exists between strength of valve adduction and acceptable valve gape. Furthermore, this study suggests that the mode of attachment employed by glochidia has played a major role in the development of these bivalve larvae and has produced convergence in valve shape and adductor muscle size.

Respiratory responses to acute heat stress in cranes (Gruidae): the effects of tracheal coiling

Some species of cranes have extensive coiling of their trachea that substantially increases their anatomical dead space. We subjected individuals of four species of cranes (Anthropoides virgo, Balearica regulorum, Grus grus and Grus japonensis) to acute heat stress to investigate the effectiveness of this trait as a thermoregulatory adaptation. We measured cloacal temperature, respiratory flow and frequency and arterial pH during normothermic breathing and thermal panting. Extra tracheal length appears to be a helpful but nonessential adaptation to prevent cranes from becoming alkalotic while panting. Cranes in our study had relatively lower panting frequencies and greater tidal volumes than have been reported for other birds subjected to heat stress. Tracheal coiling is probably more important to vocalization than to respiration or thermoregulation.

Biology of the Reptilia Volume : 19, Morphology G. Visceral Organs.

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