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Original Paper: Eme, J., J. Gwalthney, T. Owerkowicz, J. M. Blank and J. W. Hicks. 2010. Turning crocodilian hearts into bird hearts: Growth rates are similar for alligators with and without right-to-left cardiac shunt. Journal of Experimental Biology 213: 2673−2680.

Crocodilian hearts are four-chambered, just like those of birds and mammals. But crocodiles have two aortas—one from each ventricle—and the flow into each is controlled by a valve. The two aortas are connected by a shunt after they leave the heart. Thus, the right ventricle of the crocodile heart can pump blood to the lungs or to the rest of the body. If the crocodile is breathing, the resistance in the pulmonary circuit is low, and blood flows through the pulmonary artery because high backpressure from the left ventricle exerted through the shunt keeps the right aortic valve shut. When the animal is not breathing, the pulmonary resistance increases, and blood from the right ventricle flows into the right aorta. Researchers tried to demonstrate an adaptive advantage to the aortic shunt in crocodiles.

Working with young American alligators (5–7 months of age), the researchers surgically closed the right aorta near the right ventricle so that blood from the right ventricle could not flow through it to the body but had to flow through the lungs, as in an avian or mammalian circulatory system. The researchers performed sham operations on a similar group of animals, as controls. Sham operations were carried out exactly as the experimental operations, except that the aortas were not actually closed. After the crocodiles recovered from surgery, the researchers divided them into two experimental groups and two control groups that would be either sedentary (no exercise) or that would run on a treadmill to exhaustion three times a week. Over the next 21 months, the researchers measured each animal’s body mass and body length every 2 weeks. Animals were provided enough food to satisfy their hunger at each feeding.

Data for each group were subjected to least squares analysis. Bars represent mean values determined for each group, and error bars are the standard errors of the means. The probabilities that the means in each comparison are due to chance alone are shown above the graphs.

Questions

Question 1

Question 2

When an alligator is diving and spending time underwater, it is not actively breathing. In these instances (which can extend for long periods of time), the animal does not need to expend energy to pump blood through the lungs. An adaptive advantage is the ability to conserve energy by not pumping blood to the lungs when there is no benefit to be gained from doing so.

Question 3

If bypassing the pulmonary circuit conserves energy in these animals, an indication might be that growth rates of control animals would have been higher than those of experimental animals, since control animals would have more energy available to divert to growth. However, if we require a P value < 0.01 to conclude that there is a significant difference between the control and experimental groups for both sedentary and exercised animals, there are no significant differences in body mass or body length. This indicates that growth rates between control and experimental animals were similar, suggesting that the pulmonary bypass capability confers no adaptive advantage in terms of growth of juvenile alligators, even when they are stressed by exercise. Therefore these data alone do not allow us to say that the shunt affords a significant conservation of energy related to growth.

Question 4

An investigator could compare the duration of breath-hold dives in experimental and control animals. This indicator would provide a measure of the time animals can spend not breathing, which would be an advantage in their ability to survive underwater for extended periods of time. Animals with greater dive times could evade predators more easily and also gain greater predatory advantages themselves by their ability to lie in wait for prey for extended times underwater.
A second indicator could be the amount of food consumed and waste produced by experimental and control animals under otherwise identical conditions. This indicator could be used to compute calories burned and would support the hypothesis if experimental animals burned more calories than control animals, since experimental animals would need more energy to go about their daily activities.
A third indicator would be the number of eggs produced by female alligators in experimental and control groups. To support the hypothesis, the results from this study would have to show that experimental females produce fewer eggs over a defined time period than control females. The quantity of eggs is related to the amount of energy the animals have available to put toward reproduction, and loss of the shunt may cause experimental animals to have less energy to expend in producing eggs.