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Plasticity along the cardiac-gastrointestinal axis in carnivorous reptiles

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Abstract

Digestion of meals and assimilation of nutrients are energetically-costly endeavors for all animals, and especially for intermittently-feeding ectothermic predators. The costs of digestion are attributable to a suite of aerobically-demanding processes, and interactions of digestive organs with the functionally and anatomically diverse hearts of non-avian reptiles have been hypothesized to support these processes. Specifically, postprandial cardiac hypertrophy is thought to increase convective oxygen transport to meet the aerobic demands of digestion and assimilation, and the right-to-left shunt (R-L) of crocodilians is thought to aid in acid secretion and provision of metabolic substrates.

In Chapter 1, I examined the renowned rapid postprandial cardiac hypertrophy of Burmese pythons, predicting that because it likely exists to augment oxygen delivery during digestion, it should be triggered by an oxygen supply-demand mismatch. To test this prediction, I experimentally manipulated the magnitude of the O2 supply-demand mismatch by rendering animals anemic prior to feeding. I argued, because we found cardiac hypertrophy only in this group of manipulated animals, that an autonomically-mediated response to a “low O2 signal” resulted in altered postprandial hemodynamics and subsequent hypertrophy.

In Chapter 2, I utilized a similar experimental protocol on alligators to determine whether they normally exhibited postprandial cardiac hypertrophy, and if not, whether it could be artificially induced by rendering animals anemic prior to feeding. I found only modest postprandial increases in cardiovascular function, enlargement of only stomach and liver in fed animals, and no other differences in visceral organ mass. I argued that there are neither down-regulations following fasting nor up-regulations following feeding of most visceral organs in alligators.

In Chapter 3, I tested the hypothesis that the R-L shunt of crocodilians facilitates rate of net energy gain in crocodilians. Despite significant changes in ventricular hemodynamics and a near doubling of ventricular mass in animals with surgically-occluded left aortae, I found no consequences of elimination of the shunt. This and other findings support the idea that the R-L shunt is an evolutionary remnant which has never been selected against.

Together these results illustrate the complex and plastic interactions between the digestive and circulatory systems.

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