Greetings from Scaly Slimy Spectacular! We are celebrating Croctober here at Zoo Atlanta, featuring our two west African slender-snouted crocodiles (Mecistops cataphractus) Meci and Babu. They are the first species you encounter upon entering Scaly Slimy Spectacular (as long as you keep your eyes peeled). The reason you may have trouble spotting them is because they spend a lot of time submerged underwater. In fact, crocodilians can spend hours underwater without coming up for a breath of air. I have personally observed a female saltwater crocodile spend one hour and 45 minutes submerged. So how are crocodilians (which include every species of crocodile, alligator, caiman, and gharial) able to perform this amazing feat? A handful of physiological adaptations allow crocodilians to remain submerged underwater – including a specialized heart and a unique metabolic system. Let’s go over each of these adaptations to get a better understanding.
Crocodilians have a four-chambered heart – just like people! And just like the circulatory system in people, the heart takes in deoxygenated blood from the body, sends it to the lungs to become oxygenated, and the blood comes back to the heart, where it will then be pumped to the rest of the body. Pretty straightforward. But if you’re a crocodile submerged underwater, who cares if you have blood going to the lungs? After a short period of time underwater, the oxygen in the lungs is gone (crocodilians will typically expel air in order to sink, anyways). Crocodilians have a small opening called the Foramen of Panizza between their left and right aorta of the heart.1 Skipping all the technical jargon – it means that blood does NOT have to unnecessarily flow to the lungs when the crocodilian is underwater. The heart rate can fall to just 2-3 beats/minute, thus saving energy in the form of reduced cellular respiration. This same adaptation is interestingly also linked to efficient digestion.2 Oxygen-rich blood is also carried to areas of the body that need it the most.3
As explained previously, cellular respiration (a type of metabolic reaction) is reduced due to a decrease in heart rate and the elimination of unnecessary blood flow to the lungs. However, crocodilians can carry out their most strenuous physical exertions by using no oxygen at all – a process called anaerobic respiration.3 Imagine you’re a hungry crocodile holding your breath underwater, and just when you need to resurface to breathe so that the cells in your body can be re-oxygenated, here comes an unsuspecting tasty monkey hanging down from a tree above your head. You are going to need to leap up out of the water to get the monkey, using all the power in your oxygen-deprived muscular tail. And once you have the monkey, you’ll need to bring it back underwater to ensure success. No time for more oxygen. Do you have the energy to do it? Fortunately, the energy used in this event does not require oxygen if you are a crocodile. The energy used will be from glycolysis. Imagine the Epi-Pen scene in Pulp Fiction. Crocodilians have the ability to give an all-out burst even when their musculature cells are running on empty.
All of these awesome adaptations allow crocodilians to stay submerged underwater for extended amounts of time. There may even be additional evolutionary adaptations that have yet to be discovered in crocodilians, which is why Zoo Atlanta has an entire Research Team that works closely with animal care professionals to investigate topics just like this. So next time you see Meci and Babu underwater, you can tell your friends how they are able to stay submerged for so long.
1 Axelsson, M., Franklin, C.E., Lofman, C.O., Nilsson, S., and Grigg. G. C. 1996. Dynamic anatomical study of cardiac shunting in crocodiles using high-resolution angioscopy. The Journal of Experimental Biology 199, 359–365
2 Farmer, C.G., Uriona, T., Olsen, D., Steenblik, M., and Sanders, R. 2008. The Right-to-Left Shunt of Crocodilians Serves Digestion. Physiological and Biochemical Zoology 81: 125-137
3 Koniyama, N.H., Miyazaki, G., Tame, J., and Nagai, K. 1995. Transplanting a unique allosteric effect from crocodile into human haemoglobin. Nature. 1995 Jan 19;373(6511):244-6.
4 Seymour, R.S. 2013. Maximal Aerobic and Anaerobic Power Generation in Large Crocodiles versus Mammals: Implications for Dinosaur Gigantothermy. PLoS One 8(7): e69361
(Photo: Noah C.)
Keeper III, Herpetology