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Lessons from Crocodilians on Vertebrate Cardiac Shunting and Exercise pp. 57-80 $100.00
Authors:  (John Eme, Department of Biological Sciences, University of North Texas)
Crocodilians comprise 23 extant species within three Families, Crocodylidae, Alligatoridae and Gavialidae, which are more closely related to endothermic birds and extinct Dinosaurs than to the rest of the ectothermic reptiles - Lepidosaurs and Testudines. Crocodilians posses a unique vertebrate cardiac anatomy - two systemic aortae like all reptiles, and a completely four-chambered heart with two atria and two ventricles, similar to birds and mammals. All reptiles are capable of right-to-left cardiac shunt (R-L shunt), and reptiles regularly bypass the lungs and recirculate systemic blood (R-L shunt). In the past 50 years, it has become a tenet in comparative physiology that R-L shunt may provide adaptive advantage(s) to reptiles, for example, by allowing for prolonged submergence (longer aquatic dives).
The four-chambered crocodilian heart allows for elimination of R-L shunt by surgical occlusion of the aorta (Left Aorta – LAo) that allows for R-L shunt, a procedure not possible in other reptiles. Recent studies have investigated the significance of R-L shunt in American alligator (Alligator mississippiensis) by surgically occluding the LAo and measuring growth, metabolism, digestion and respiration. Three variables were affected that suggest shunt may be important for digestion - secretion of gastric acid together with rate of bone demineralization, and body mass during early ontogenetic growth in a separate study. However, the following variables were not adversely affected by occluding the LAo in alligators - head and snout-to-vent lengths during early ontogenetic growth, oxygen consumption of juveniles and eggs, voluntary apnoea time, breathing frequency, tidal volume and minute ventilation. These unaffected variables support the hypothesis that the crocodilian cardiovascular system is not an adaptive circulatory “design”, but a plesiomorphic character state not selected against.
However, the crocodilian oxygen transport system is quite plastic in response to repeated bouts of exercise, unlike the metabolically inflexible, ‘classic reptilian’ response. Cardiovascular and metabolic responses to exercise training have been investigated for two species, American alligator and Estuarine crocodile (Crocodylus porosus). Exercise-trained crocodiles and alligators showed an increase in maximum oxygen consumption compared to sedentary control animals. In addition, exercise-trained alligators demonstrated a significant increase in ventricular mass, possibly a hypertrophy associated with the increased aerobic capacity. However, similar to previously studied reptiles, aerobic and anaerobic enzymatic activities did not change in response to exercise training. The response of crocodilians to repeated bouts of activity appears to be intermediate between a ‘classic’ mammalian training response and the ‘metabolically inflexible’ training response of other reptiles. 

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Lessons from Crocodilians on Vertebrate Cardiac Shunting and Exercise pp. 57-80