Ask a Croc Expert
Read Dr. Britton's responses on the following croc topics:
Crocodile Predators
Crocodile Speed
Croc/Human Interaction
Crocodile Size
Crocodile Hunting
Crocodile Defense
Alligators vs. Crocodiles
Crocodile Mothering
Crocodile Infections
Crocodile Bites
Crocs in Water
Croc Body Heat
The Crocodile Heart
Q: On the Crocodile HeartWhat is the evolutionary advantage of the crocodilian species having a four chambered heart like the human heart? Do all crocodilians have them or just the big ones?
Tracy Marman
A: All crocodilians have a four-chambered heart, very similar in fact to birds and of course mammals. Birds and crocodilians share a common ancestor, so this is perhaps not surprising. Unlike birds and mammals, it is no exaggeration to say that crocodilians have the most advanced heart we know of.
The heart performs two main functions — pumping blood to the lungs (pulmonary circulation), and pumping blood around the body (systemic circulation). The four-chambered heart is the most efficient design because it separates the pulmonary and systemic circulations — there is no mixing of oxygenated blood from the lungs with deoxygenated blood from the body.
This separation also confers another advantage — the ability to grow to much larger sizes than animals with incomplete separation of pulmonary and systemic circulation (e.g. lizards, with three-chambered hearts). Large animals need high pressures from the systemic left ventrical to pump blood around the body, and such pressures would rupture the delicate tissues in the lungs if the pulmonary system were not separate. In the four-chambered heart, the right ventrical is much smaller and provides the correct pressure to send blood to the lungs.
This four-chambered design enabled crocodilians at some stage in their evolution to develop a means of bypassing the pulmonary system altogether. There is a valve called the "foramen of panizza," which connects the pulmonary and systemic outflows from the heart to an actively contracting valve in the heart. This valve shuts off the pulmonary system during diving — in fact, crocodiles are totally unique in that they have actively controlled muscular valves in the heart. This is important for crocodilians because diving essentially makes the pulmonary system redundant until the animal resurfaces and takes a breath. Hence, blood is diverted from the pulmonary system to the systemic to provide more blood and a greater aerobic capacity while submerged. Combine this with remarkable hemoglobin levels in the blood, which carries more oxygen and releases it to oxygen-starved tissues more readily, and you have an animal ideally suited to extended periods of submergence while still remaining active — perfect for hunting in and around water.
