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

Abstract

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 vocalizations. 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.

Keywords: breathing; crocodilian; electromyography; uncinate processes.

Figure 1.

(a) Diagram of the axial musculature of A. mississippiensis illustrating the iliocostalis (ic), deep external oblique (eop), rectus abdominis (ra) and truncocaudalis (tc). Star indicates EMG electrode placement in the ic. (b) Muscles have been removed to highlight the uncinate processes (arrows, ucp). Cranial is to the left.

Figure 2.

Electromyography (EMG) of the iliocostalis muscle. The top trace in each panel is a representative raw unprocessed EMG signal (mV), the middle trace is processed, rectified EMG activity for all alligators (mV, band-pass filtered 60–250 Hz) and the bottom trace indicates; inspiration and expiration for (a–c) and denotes alarm distress vocalization events for (d). All traces are from one alligator. The iliocostalis is active during expiration when alligators were at rest and breathing at 28°C (a), demonstrates no activity during either expiration or inspiration when at rest and breathing either rapidly (b) or more slowly (c) with the body submerged in water at 18°C and demonstrated a marked increase in activity during expiration associated with alarm vocalizations (d). Alarm vocalization events (bottom trace, d) were indicated by manually depressing a 5 V trigger upon each vocalization shown in the bottom trace, hence the slight offset caused by the delay in reaction time to the alarm vocalization event.