Pelvic aspiration in the American alligator (Alligator mississippiensis)

The pelvis of crocodilians is highly derived in that the pubic bones are is olated from the acetabulum and are attached to the ischia via moveable join ts. We examined the possible role of this unusual morphology in lung ventil ation by measuring ventilation, abdominal pressure and the electrical activ ity of several abdominal and pelvic muscles in the American alligator (Alli gator mississippiensis). We found that the activity of two pelvic muscles, the ischiopubis and ischiotruncus muscles, was correlated with inspiration; these muscles rotate the pubes ventrally and thereby increase abdominal vo lume. During expiration, contraction of the rectus abdominis and transversu s abdominis rotates the pubes dorsally, We suggest that this mechanism faci litates diaphragmatic breathing by creating space for caudal displacement o f the viscera during inspiration. Because birds also use a dorso-ventral mo vement of the pelvis to effect ventilation, some form of pelvic aspiration may be plesiomorphic for archosaurs.