A novel accessory respiratory muscle in the American alligator (Alligator mississippiensis)

The muscles that effect lung ventilation are key to understanding the evolutionary constraints on animal form and function. Here, through electromyography, we demonstrate a newly discovered respiratory function for the iliocostalis muscle in the American alligator (Alligator mississippiensis). The iliocostalis is active during expiration when breathing on land at 28C and this activity is mediated through the uncinate processes on the vertebral ribs. There was also an increase in muscle activity during the forced expirations of alarm distress vocalisations. Interestingly we did not find any respiratory activity in the iliocostalis when the alligators were breathing with their body submerged in water at 18C, which resulted in a reduced breathing frequency. The iliocostalis is an accessory breathing muscle that alligators are able to recruit in to assist expiration under certain conditions.调控肺通气的肌肉是解析动物形态与功能演化制约机制的关键所在。本研究通过肌电描记法（electromyography），首次证实美洲短吻鳄（Alligator mississippiensis）的髂肋肌（iliocostalis）具备此前未被发现的呼吸功能。在28℃的陆地环境中呼吸时，髂肋肌会在呼气阶段被激活，该激活过程由椎肋上的钩突（uncinate processes）介导。当短吻鳄发出警戒应激鸣叫并进入强制呼气阶段时，该肌肉的活动同样显著增强。值得注意的是，当短吻鳄在18℃的水下呼吸时，我们未观测到髂肋肌存在任何呼吸相关活动，此时其呼吸频率也随之降低。综上，髂肋肌属于辅助呼吸肌，短吻鳄可在特定条件下募集该肌肉以辅助完成呼气动作。