Evolution of vertebrate motor systems for acoustic and electric communication: peripheral and central elements

Among vertebrates, there are a number of different neuromuscular systems specialized for the production of acoustic and electric social communication signals. Each system involves distinct sets of striated muscles that are derived from paraxial mesodermal somites and are components of a peripheral effector or 'communication organ': the larynx and syrinx of tetrapods, the sonic swim bladder and electric organ of fishes. Each of these systems further exhibits species-typical characters ranging from the number of muscles involved to myofibrillar architecture. Given that communication uses striated muscle, its circuitry is seen to represent a modification of existing motor systems. Thus, it is not surprising that these systems share several features suggesting that common mechanical or developmental factors influence their evolution. Of prime importance in comparing central communication circuitry between fishes and tetrapods is that among tetrapods the sound-generating organs are a part of the ventilatory system and so their activity must be coordinated with other sets of neuromuscular units. Differences in the central pattern of circuitry are thus expected between fishes and tetrapods, as well as among tetrapods themselves, since ventilation mechanisms differ between taxa. Moreover, birds have independently evolved a vocal syrinx which is distinctly separate from their larynx.