All crocodilians have complete anatomical separation between the right and left ventricles, which is similar to birds and mammals and unlike all other non-avian reptiles. However, the crocodilian heart retains two systemic aortae (left aorta and right aortic arch), a feature that is common to all non-avian reptiles. In crocodilians, the left aorta (LAo) emerges from the right ventricle (RV) alongside the pulmonary artery (PA), and the right aortic arch (RAo) emerges from the left ventricle (LV) (Webb, 1979). This anatomical arrangement results in the capacity for a 'right-to-left' (R-L) cardiac shunt, a 'pulmonary bypass' cardiac shunt, in which a fraction of systemic venous blood recirculates into the systemic arterial circulation (Hicks, 1998).The crocodilian RAo and LAo communicate at two distinct points. As the aortae emerge from the heart, they run side-by-side, sharing a common wall for several centimetres. Near the base of this common wall and just downstream of the aortic valves, there is a small opening called the foramen of Panizza (FoP) (Panizza, 1833), which is of variable calibre and allows for potential blood flow between the RAo and LAo (Grigg and Johansen, 1987;Axelsson et al., 1996;Axelsson and Franklin, 2001). The second point of communication between the aortae is an arterial anastomosis in the abdominal cavity, caudal to the liver. Beyond this anastomosis, the RAo continues as the dorsal aortal, and the LAo becomes the coeliac artery, which gives rise to smaller arteries that supply most of the blood flow to the gastrointestinal tract. Consequently, the LAo is the primary source of blood for the splanchnic circulation, although blood from the RAo can also enter the splanchnic bed via the anastomosis (Axelsson et al., 1991).R-L cardiac shunt has been hypothesised to be important in various activities and physiological functions (Hicks, 1998;Hicks, 2002). For semi-aquatic reptiles like crocodilians, generation of a R-L cardiac shunt has often been observed during breath-holds associated with diving (White, 1969;Grigg and Johansen, 1987;Hicks and Wang, 1996). The reduction in pulmonary blood flow during apnoea has been hypothesised to conserve lung O 2 stores and sequester CO 2 away from the lung, possibly extending aerobic dive times (White, 1978;White, 1985;Grigg and Johansen, 1987). The development of a R-L cardiac shunt also results in arterial desaturation through admixture of venous blood, and the resulting systemic hypoxemia can trigger tissue hypometabolism, which could contribute to the prolongation of aerobic dives (Hicks and Wang, 1999;Platzack and Hicks, 2001).Crocodilians provide an opportunity to investigate experimentally the proximate functions of reptilian cardiac shunting. Their cardiac anatomy lends itself to surgical modification that prevents R-L cardiac shunt while maintaining the integrity of the ventricular chambers, a procedure that is not possible in other reptiles. The purpose of the present study was to test the hypotheses that removal of shunt (occlusion of LAo) woul...