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A controversial hypothesis has proposed that lizards are subject to a speed-dependent axial constraint that prevents effective lung ventilation during moderate- and high-speed locomotion. This hypothesis has been challenged by results demonstrating that monitor lizards (genus Varanus) experience no axial constraint. Evidence presented here shows that, during locomotion, varanids use a positive pressure gular pump to assist lung ventilation. Disabling the gular pump reveals that the axial constraint is present in varanids but it is masked by gular pumping under normal conditions. These findings support the prediction that the axial constraint may be found in other tetrapods that breathe by costal aspiration and locomote with a lateral undulatory gait.

SUMMARYRecent palaeoatmospheric models suggest large-scale fluctuations in ambient oxygen level over the past 550 million years. To better understand how global hypoxia and hyperoxia might have affected the growth and physiology of contemporary vertebrates, we incubated eggs and raised hatchlings of the American alligator. Crocodilians are one of few vertebrate taxa that survived these global changes with distinctly conservative morphology. We maintained animals at 30°C under chronic hypoxia (12% O 2 ), normoxia (21% O 2 ) or hyperoxia (30% O 2 ). At hatching, hypoxic animals were significantly smaller than their normoxic and hyperoxic siblings. Over the course of 3 months, post-hatching growth was fastest under hyperoxia and slowest under hypoxia. Hypoxia, but not hyperoxia, caused distinct scaling of major visceral organs -reduction of liver mass, enlargement of the heart and accelerated growth of lungs. When absorptive and post-absorptive metabolic rates were measured in juvenile alligators, the increase in oxygen consumption rate due to digestion/absorption of food was greatest in hyperoxic alligators and smallest in hypoxic ones. Hyperoxic alligators exhibited the lowest breathing rate and highest oxygen consumption per breath. We suggest that, despite compensatory cardiopulmonary remodelling, growth of hypoxic alligators is constrained by low atmospheric oxygen supply, which may limit their food utilisation capacity. Conversely, the combination of elevated metabolism and low cost of breathing in hyperoxic alligators allows for a greater proportion of metabolised energy to be available for growth. This suggests that growth and metabolic patterns of extinct vertebrates would have been significantly affected by changes in the atmospheric oxygen level.

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Aspiration breathing is the dominant mechanism of lung inflation among extant amniotes. However, aspiration has two fundamental problems associated with it: paradoxical visceral translation and partial lung collapse. These can constrain the inspiratory tidal volume, reduce the effective lung ventilation, and ultimately curtail the aerobic capacity of an animal. Separation of the pleural and peritoneal cavities by an intracoelomic septum can restrict the cranial shift of abdominal viscera and provide structural support to the caudal lung surface. A muscular septum, such as the diaphragm of mammals or the diaphragmaticus of crocodilians, can exert active control over visceral translation and the degree of lung inflation. To a lesser degree, a nonmuscular septum can also function as a passive barrier when stretched taut by rib rotation. Studies of the posthepatic septum in teiid lizards and the postpulmonary septum in varanid lizards underscore the importance of nonmuscular septa in aspiration. These septa provide plausible functional models that help us infer the evolution of mammalian and avian lung ventilatory systems, respectively.

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...

SUMMARYCrocodilians use a combination of three muscular mechanisms to effect lung ventilation: the intercostal muscles producing thoracic movement, the abdominal muscles producing pelvic rotation and gastralial translation, and the diaphragmaticus muscle producing visceral displacement. Earlier studies suggested that the diaphragmaticus is a primary muscle of inspiration in crocodilians, but direct measurements of the diaphragmatic contribution to lung ventilation and gas exchange have not been made to date. In this study, ventilation, metabolic rate and arterial blood gases were measured from juvenile estuarine crocodiles under three conditions: (i) while resting at 30°C and 20°C; (ii) while breathing hypercapnic gases; and (iii) during immediate recovery from treadmill exercise. The relative contribution of the diaphragmaticus was then determined by obtaining measurements before and after transection of the muscle. The diaphragmaticus was found to make only a limited contribution to lung ventilation while crocodiles were resting at 30°C and 20°C, and during increased respiratory drive induced by hypercapnic gas. However, the diaphragmaticus muscle was found to play a significant role in facilitating a higher rate of inspiratory airflow in response to exercise. Transection of the diaphragmaticus decreased the exercise-induced increase in the rate of inspiration (with no compensatory increases in the duration of inspiration), thus compromising the exercise-induced increases in tidal volume and minute ventilation. These results suggest that, in C. porosus, costal ventilation alone is able to support metabolic demands at rest, and the diaphragmaticus is largely an accessory muscle used at times of elevated metabolic demand.