How can marine mammals survive for so long underwater?
Charlotte Hitch (Vet News Exotics Editor)
Research at the University of Liverpool into oxygen-carrying proteins in the muscles of marine mammals has recently revealed how these animals are able to hold their breath for up to 90 times longer than humans.
The protein myoglobin, which gives muscle its red colour, is responsible for supplying muscle tissue with the oxygen required for respiration. In seals and other marine mammals, muscles are often almost black in colour due to their high myoglobin content, of which the concentration is about ten times that of bovine muscle. More myoglobin means more oxygen can be stored within the muscle itself, so oxygen from the last breath before a dive can be retained in the blood and used by the vital organs, such as the brain and heart.
By storing their own oxygen, the muscle cells are still capable of carrying out respiration to produce the ATP energy needed for movement (i.e. flipper and tail motion) during the dive, so marine mammals can still be active even when they haven’t inhaled for over half an hour. This is what enables sperm whales to dive 2-3km underwater to catch giant squid, a major constituent of their diet.
The problem with having a high muscle myoglobin content is that the protein has self-associating properties when packed tightly together. When proteins stick together, their function is impaired; in this case the myoglobin would be unable to bind with oxygen properly.
To overcome this problem, marine mammals have developed a modified version of myoglobin – with an overall positive charge on the molecule. This results in repulsion between molecules, preventing self-association.
While the adaptation offers a clear advantage to marine mammals, there are risks involved. There is no problem while the myoglobin is contained in the muscle; however muscle damage from fighting or predators’ attacks may cause it to enter the bloodstream. Myoglobin is toxic to the renal tubular epithelium, so when the blood passes through the kidneys, it can lead to renal failure.
Despite the risk to the kidneys, the outcome of this adaptation has evidently contributed to the evolutionary success of marine mammals; sperm whales can hold their breath for 90 minutes and common seals for 30 minutes. Seals are even able to sleep underwater, taking refuge from land-based predators – a vital part of their survival.