Crocodile on the sand.

Crocodiles are amazing animals. I admit that I am partial as a comparative physiologist who has studied them for many years, but I’m not alone in finding their interesting history and uncommon characteristics fascinating.

Crocodilians have a unique heart that shares anatomical features with reptiles, birds and mammals. The crocodilian heart has four chambers with two atria and two ventricles, the same as birds and mammals. However, birds and mammals have only a single aorta—the main artery that supplies blood to the circulatory system, but crocodilians and reptiles have two. In crocodilians and reptiles the right aorta pumps oxygen-rich blood from the lungs to the body. Meanwhile, the left aorta exits the heart separately, alongside the pulmonary artery. The left aorta can send oxygen-poor blood back out to the body at the same time that the separate pulmonary artery sends oxygen-poor blood to the lungs to eliminate carbon dioxide and take up oxygen.

The crocodilian pulmonary artery has a special valve composed of cartilage-like “teeth” similar to cogs on a gear called a “cog tooth” valve. The cog tooth valve is controlled by the hormone adrenaline, and when it contracts, blood is diverted from the lungs into the left aorta. This bypass or aortic shunt mixes oxygen-poor blood coming back from the body with oxygen-rich blood coming from the lungs. The result is that less oxygen is carried to the body through arterial circulation.

When a right-to-left aortic shunt occurs in the hearts of mammals (including humans) and birds, it is almost always caused by a developmental defect and is fatal if left uncorrected because their hearts can’t pump enough oxygen-rich blood to keep them alive. Yet, a right-to-left aortic shunt is a normal part of crocodilian anatomy.  In my research I am exploring how crocodilians survived for millions of years with a seemingly abnormal heart. Does the anatomy of the crocodilian heart give them a “special” physiological advantage? If so, what is it?

Recently, the usefulness of research like mine has been called into question, but studying crocodilian circulation is important because it helps us to understand:

• how the species has evolved and thrived with a distinctive mix of anatomical features,
• the complex form and function of the vertebrate circulatory system, and
• human health concerns such as developmental defects in human hearts.

Many questions still remain about why crocodilians have the physiology that they do, but we have learned that these are wonderful creatures neither reptile nor bird. They are uniquely crocodilian. Check out National Geographic, the Discovery Channel website and crocodilian.com to learn more.

James W. Hicks, PhD

James W. Hicks, PhD, is interim vice chancellor for research and a professor of ecology and evolutionary biology in the School of Biological Sciences at University of California, Irvine.