CROCODILES can hold their breath underwater for more than an hour. Researchers in Cambridge have now shown that this ability depends on a tiny fragment of the animal’s haemoglobin the protein that carries oxygen from the lungs to the rest of the body.
Crocodiles use a waste product of metabolism – the bicarbonate ions formed when carbon dioxide dissolves in water – as the trigger for haemoglobin to unload the oxygen it carries. The more energy the crocodile’s cells burn up, the more carbon dioxide they produce. So the bicarbonate triggering mechanism ensures that large quantities of oxygen are supplied only to the hard-working tissues that need it the most, thus conserving a submerged crocodile’s limited supplies. This efficient system of oxygen allocation, combined with the reptile’s very slow metabolism, explains how it is able to survive underwater for so long.
To find out which part of the crocodile’s haemoglobin performs this trick, Hennakao Komiyama, Kiyoshi Nagai and colleagues at the Medical Research Council’s Laboratory of Molecular Biology in Cambridge made haemoglobin molecules that were part-crocodile and part-human. They looked for those that were triggered to release oxygen by bicarbonate ions, and found that human haemoglobin behaved like its counterpart in crocodiles if it contained a particular sequence of just 12 amino acids from the crocodile’s haemoglobin.
The research is not going to lead to the development of genetically engineered humans who can stay underwater for over an hour. But Nagai says that the discovery could be used to develop haemoglobin-based “artificial blood” for emergency transfusions. If oxygenated haemoglobin is injected directly into the bloodstream – rather than as a component of red blood cells in a whole-blood transfusion – it will not release the oxygen unless triggered to do so artificially. Haemoglobin engineered to include the crocodile’s triggering mechanism, however, would release the oxygen in the presence of bicarbonate in the bloodstream.
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Most studies on diving have been done with Alligator mississippiensis, but the general principal probably applies to most if not all species of crocodile. Studies in the 1960's showed that an American alligator could remain submerged for up to two hours. During this time, heart rate can drop to 2 or 3 beats per minute to help reduce oxygen consumption. It has also been shown that during such prolonged dives, oxygen consumption immediately after diving actually falls as the dive continues. In essence, the animal adjusts its oxygen consumption to enable it to dive for longer. Although the lungs can be used as a reserve air supply, in most cases when the crocodile dives quickly it expels the majority of air from its lungs. The crocodilian heart comes into play here, with valves closing to redirect blood to essential areas, and restrict its flow into non-essential areas - thus contributing to the reduced oxygen consumption that is seen. Crocodilian blood is also remarkable in its affinity for oxygen, carrying more in oxygen rich areas, and releasing it more quickly in oxygen deprived areas.
SALTWATER CROCODILE BEFORE DIVE
But there's a problem with the oft-quoted figure of two hours - under normal circumstances, crocodilians do not remain submerged for such periods. A voluntary dive normally lasts no longer than 15 minutes, and usual dive times are around 4 to 6 minutes. Studies measuring maximum dive duration before death involved holding alligators underwater - hardly a natural behaviour. Why is there such a high degree of tolerance, then? The experiments looking at dive time in alligators were concerned with resting animals. Therefore, active and stressed animals in real-life situations will be using oxygen up more quickly and reducing their maximum submergence time. A struggling crocodile underwater can actually drown in 20 to 30 minutes, as some people attempting to capture crocodiles have found to their cost. The smaller the crocodile, the lower its capacity to survive without oxygen underwater.
In times of extreme stress, extended dives can be employed by the crocodile. Such instances have been recorded in alligators where the water they are resting in starts to freeze over (e.g. near the northern limit of their range in the US) where forced submergence is caused by the surface of the pond freezing over. There is one observation of an alligator surviving submerged in near-freezing water for over 8 hours. This also illustrates the importance of temperature in crocodilian physiology - in this case, the colder the animal is the lower its metabolic and oxygen consumption rates, and so a colder crocodilian should be able to hold its breath for longer.