Enormous crocodile

Crocodiles have the most complex hearts in the whole of the animal kingdom.

That was the argument made by Professor Craig Franklin, head of the Eco-Lab research group at the University of Queensland, Australia, during his talk at the British Cardiovascular Society annual Conference in Manchester.

Professor Franklin studies vertebrates that spend long periods of time underwater. Some of the most interesting animals he works with are crocodiles, and his research has been trying to discover how these fearsome creatures manage to stay submerged for so long.

The record for a human holding their breath under water is around 12 minutes. Elephant seals can get up to around 2 hours, and a Cuvier’s beaked whale once clocked a dive of over 137 minutes. But the longest recorded dive for a crocodile is 387 minutes, and Professor Franklin believes they can manage up to 8 hours in colder waters before resurfacing for air.

How can an animal with lungs, which has to resurface to breathe and recover from the last dive, manage such a feat of endurance? Professor Franklin puts it down to the evolution of their remarkable hearts.

In a while, crocodile

Crocodiles are members of the archosaurs, a wider family of animals that live on land and lay eggs. This means they are part of the same ancestral family as both birds and dinosaurs. The earliest known crocodilians, a species called Protosuchus, lived around 200 million years ago.

Compared to other diving vertebrates, such as seals, turtles or hippos, the crocodilians have had much longer to adapt to their surroundings and become the apex predators that they are today. When it comes to life underwater, one of their key adaptations is now thought to be in the heart and circulatory system.

To uncover the secrets of their cardiovascular quirks, Professor Franklin and his colleagues fitted five Nile crocodiles with matchbox-sized heart monitors. They were then studied over a number of months and the data was used to fill in some of the missing gaps from previous research and imaging of crocodile hearts.

Breath-taking complexity

Crocodiles are the only reptiles to have a four-chambered heart, much like our own. Unlike us, however, they have two aortas — the blood vessel in humans that carries blood from the heart to the rest of the body. They have a right aorta that comes out of the left ventricle and a left aorta from the right ventricle.

Adding to the anatomical complexity is a valve found at the top of the right ventricle that looks like a pair of cog-teeth, which the animals can actively control to redirect the flow of blood. This set-up effectively allows crocodiles to stop blood flowing from the heart to the lungs, and instead just cycle it continually around the body with each beat.

In humans, every heartbeat circulates blood to both the lungs and the body. This means the lungs are emptied of oxygen very quickly. However, Professor Franklin’s research suggests that the nifty little ‘cog-teeth valve’ in crocodile hearts can be actively opened and closed to let a small amount of blood flow to the lungs. This allows the animals to occasionally dip into the store of oxygen in their lungs, just to top up the cells that need it most.

When the croc prepares for a long dive, it fills its lungs, dives and then slows its heartbeat down to between 7–10 beats per minute. It then uses the valve to close off the lungs and, with just one half of the heart now working, the blood is also channelled to a few vital organs. This all helps the crocodiles to use less energy, and only send blood to the lungs to pick up oxygen when it is absolutely necessary.

Have a heart for crocodiles

It’s a breath-takingly complicated system, and there is still more to learn. But it has allowed crocodiles to stay underwater longer than any other vertebrate, and is a beautiful example of cardiology at the extremes — the theme for 2017 BCS conference.

It’s also important to remember that catching these animals is no mean feat. The safest techniques — for both the crocs and the researchers — were perfected by the legendary conservationist Steve Irwin, whose wildlife reserve in northern Australia has been the base of much of Professor Franklin’s work.

As you might imagine, the research is unlikely to have a direct impact on heart patients. While it doesn’t change medical practice, it does reveal another side to these much-maligned creatures and is a poignant reminder of the need for conservation efforts to protect their habitats.