Crocodile hearts

Ever wondered how some animals stay submerged underwater for hours at a time? University of Queensland zoologist Craig Franklin studies saltwater crocodiles - the Earth’s largest living reptiles and owners of the most complex heart in the animal kingdom…

Craig - Crocodiles are mostly aquatic. They spend the majority of their life in river systems or sometimes out in the ocean and they are divers. They can spend periods of time underwater, they can catch food underwater like fish, but they can also remain submerged at the water’s edge and ambush prey that come down to drink. Our recent research has shown that they can spend many hours on a single lungful of air, and the record is round about 7 hours.

Chris - That’s a long, long time isn’t it? How do you think they’re doing that?

Craig - The key to diving is how to manage oxygen. For the crocodiles we think a large part of it is their cardiovascular system and their heart is, I think, the most sophisticated, most complex heart in the animal kingdom.

Chris - Quite surprising given they’re quite ancient beings, aren’t they - they’ve been around for hundreds of millions of years?

Craig - Yeah. We can track back to the jurassic and protosuchus is what’s regarded as the first crocodilian. We can jump forward in time into a hundred million years, and in Queensland, Australia a discovery was made of a crocodile that essentially is what we have today in shape and appearance.

Chris - So they're clearly well adapted and very successful. So when you look inside one what is the anatomy that you think gives them this ability?

Craig - Imagine a four chambered heart, just like a human heart - two atria, two ventricles. Blood returns from the body deoxygenated and it enters the right atrium. It then gets ejected into the right ventricle and, as that ventricle contracts, that ejects that blood to the lungs.

Chris - So far that's just like you and me?

Craig - Just like you and me. That blood then returns to the left atrium, then to the left ventricle, and then it’s ejected out to the body, so to the head and to the lungs.

Chris - That’s still just like you and me?

Craig - Exactly. Just like you and me. But this is when it gets a bit more complicated. If we just focus on the right side of the heart, that side of the heart typically ejects blood to the lungs through the pulmonary arteries but, in the crocodile, there’s an extra vessel. It’s called the left aorta, and that vessel allows blood to be pumped to the body so the right ventricle almost has a choice. It can either send blood to the lungs or it can send blood to the body.

Chris - Why would that help?

Craig - Our hypothesis is that if the crocodile can control the amount of blood that goes to the lungs, and perhaps limit it and re-divert it to the body, it can save the oxygen in the lungs. So it kind of meters the amount of blood that goes to the lungs, a little bit like a reservoir, a scuba tank in the crocodiles that it can use from time to time.

Chris - Your theory would be then that the crocodile is going to dive so it gets a lung full of air, then submerges. And then periodically diverts the blood, instead of round its body, through the lungs, grabs a bit of that oxygen, shoves that oxygenated blood round the body for a while until it needs another taste of oxygen and then it opens and closes this choice in the right side of the heart? Is that born out by observation though - do we know that crocodiles do go down with a big lungful of air because diving mammals like whales classically and characteristically breath out before they submerge?

Craig - There’s been experimental studies to show that crocodiles will go down with a lungful of air and use that as a key source of oxygen. But really the trick to this heart, and this flow pattern is how that blood can either go to the lungs or to the body. And what we have discovered is a very special type of valve that sits at the base of the vessel that goes to the lungs. We call it cog teeth because it looks like cog teeth or two sets of knuckles that come together. What we have shown is that when it does close it diverts the blood to the body.

Chris - But have you been able to go in and look at an awake behaving crocodile in order to see if what you think is happening is true because these are not small animals! It’s not like a mouse to study, it’s pretty challenging.

Craig - We have. The great thing these days is that physiologists like myself used to bring the animal into the lab; now we take the lab to the animal. This is the field of telemetry where things like physiological devices are being miniaturised to such an extent that you can implant them in animals and you can actually measure their physiology while they're just going about their daily business.

Chris - So you’ve done that - you’ve got devices you’re implanting in crocodiles to measure these heart functions?

Craig - Yes. We’ve implanted a number of animals and we’ve been recording flows and pressures and there is evidence to say that during diving they do divert blood away from the lungs back into the body.

Chris - It’s ingenious, isn’t it? One quick question I can’t resist though - how on earth do you get the croc to cooperate with the implantation of a device?

Craig - Carefully! Of importance to us is the welfare of the animal. The whole thing is that we want to observe natural behaviours. And so we look after the animal and we make sure that there’s little impact and that way we can record these natural behaviours.

Chris - How do you catch them?

Craig - Catching’s easy. Catchings reasonably easy if you have a very experienced team and I’ve been very lucky in that Australia Zoo, Steve Irwin originally and Terri Irwin now, provides the capacity, the capability to capture these animals and for us to put tracking devices on.