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Shunting and survival in alligators Gardner, Manuela Natascha

Abstract

In terms of cardiac anatomy, crocodilians occupy an intermediate position between the avian and reptilian circulations. They have a completely divided heart but have retained the ability to shunt deoxygenated right ventricular (RV) blood to the body via the left aorta (LAo), which arises from the right ventricle. The LAo continues posteriorly and supplies the digestive system with right ventricular blood during shunting. The central hypothesis of my study was that the ability to shunt RV blood to the digestive system via the LAo aids in the secretion of gastric acid during digestion and/or in reducing the alkaline tide following feeding in alligators (Alligator mississippiensis). I used two groups of alligators to investigate the role of the ability to shunt RV blood to the digestive system. Alligators in one group had their LAo surgically tied off and cut (LAo cut; unable to shunt via the LAo) and alligators in the other group had sham surgeries performed on them (LAo intact). I measured growth rates, food intake, water intake, digestion rates, conversion rates, metabolic rates and acid-base status during feeding. Metabolic rates, mass changes, blood variables and acid-base status were also measured during long-term fasting. In addition, I studied the effect of changing body temperature on acid-base status in the two groups of alligators. During fasting, mass changes, blood variables, oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory exchange ratios (RERs) and acid-base status of the two groups of alligators were not significantly different. During feeding, growth rates, digestion rates and conversion rates were similar in the two groups, as were VO2 values. Average VCO2 and RER values, however, were significantly higher in alligators with their LAo cut. Changes in acid-base status after feeding were similar between the two groups of alligators. However, when temperature was decreased from 30° C to 20° C alligators with their LAo cut exhibited larger acid-base disturbances during feeding, with significantly higher average pH values. Also, differences observed in acid-base status were reduced or disappeared when feeding was stopped. In summary, the ability to shunt RV blood to the gut after feeding did not benefit digestion or aid in ameliorating the alkaline tide. However, the acid-base status was significantly affected in alligators unable to shunt RV blood to the digestive system via the LAo after feeding and when body temperature was decreased. Thus, during times of additional physiological challenges such as temperature changes, acid-base regulation may become compromised in alligators unable to shunt RV blood to the body. Even so, ligating the LAo did not affect survival in laboratory reared alligators. If shunting RV blood to the body benefits alligators in a natural setting, its role has yet to be determined.

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