Whales and dolphins are mammals, just like us, and just like us they need to breathe air. But their food is underwater, so they need to hunt and feed all without taking a breath. Luckily, whales and dolphins have some pretty nifty tricks up their sleeve! In this article, you can learn a bit about how cetaceans have evolved to survive life in the ocean. There may be some terms you are unfamiliar with, which you can check in the glossary at the bottom of the page.
The longest that a human has ever held their breath for is about 25 minutes – which is pretty impressive! But some species of whale can stay underwater for well over 2 hours while looking for food. There are two main adaptations that allow cetaceans to hold their breath for such remarkably long periods of time: they maximise how much oxygen they store in their bodies, and minimise the speed at which it is used up.
Increasing oxygen stores
Surprisingly, whales and dolphins don’t have very large lungs. This is because they store very little oxygen in their lungs compared to land mammals. Instead, the blood and muscles hold most of their oxygen reserves. This allows for more efficient use during a dive. Cetaceans have higher blood volumes and red cell concentrations, and more muscle fibres with a greater concentration of myoglobin.
Decreasing oxygen use
Cetaceans are able to reduce their use of oxygen in some amazing ways. While underwater, normal activity in the gut is stopped. This means that any prey caught during the dive is not digested until the whale is back at the surface.
In the ocean, the water gets colder the deeper you go. Instead of using precious energy and oxygen trying to keep warm, cetaceans allow their body temperature to drop almost to the point of hypothermia.
A reduction in heart rate – bradycardia – reduces the rate that blood flows through the body. This also reduces the rate at which oxygen in the blood is used up. Some cetaceans are able to dramatically decrease their heart rate – blue whales have been measured as low as 2 beats per minute (BPM) during a dive!
As well as this, blood flow to non-essential organs and tissues is reduced or cut off entirely. This means most of their blood can be diverted to muscles and essential organs. The brain is the only organ which receives the same level of blood flow while underwater and at the surface. This central organ ischemia helps to maintain a low heart rate, as blood being pumped to fewer areas of the body doesn’t require as much pressure.
We all know the feeling of getting a cramp in our side after an intense workout or a long run. Exercise increases our body’s need for oxygen, and we get these painful cramps when we have used up all our available oxygen reserves. Cetaceans modify their behaviour to help avoid increased demand for oxygen due to exercise. For example: rather than actively swimming, deep-diving whales can take advantage of natural changes in buoyancy and instead glide passively for much of their descent. This strategy can save up 60% of the energetic cost of dives!
Avoiding the bends
Retaining enough oxygen to last the dive is only half of the battle, as there are other problems that come with storing atmospheric gas at pressure. These are problems that humans face too; SCUBA divers will be familiar with the dangers of decompression sickness (DCS).
DCS, or ‘the bends’, arises when there is excess nitrogen in the blood while at depth – upon rapid ascent, the nitrogen forms bubbles in the blood. In order to avoid this potentially deadly condition, whales and dolphins must keep the movement of nitrogen from the lungs into the blood to a minimum during a dive.
Cetaceans take a breath in before they dive, meaning that there is air in their lungs and airways for the whole dive. To prevent the nitrogen in this air moving from the lungs to the blood, cetaceans’ lungs flatten while they are at depth. Their respiratory tracts are reinforced with cartilage, making them rigid but flexible. When the lungs collapse air is forced into the cartilaginous airways where it is stored throughout the dive.
Adaptations for oxygen conservation can also help to prevent DCS in diving cetaceans: central organ ischemia and a slow heart rate limit blood flow and the movement of nitrogen into organs such as the liver and kidneys.
Beaked whales, military sonar and the bends
Although marine mammals are well-adapted against DCS, this does not mean they are immune. Multiple mass stranding events of extreme deep-diving beaked whales have been linked to naval sonar activity. These whales consistently suffer injuries consistent with DCS. We still don’t know exactly what happens to cause these stranding events, but it is thought that exposure to sonar triggers abnormal dive behaviour – such as extended periods at depth or a very rapid return to the surface – which then leads to DCS.
Beaked whale diving behaviour is unlike that of other cetaceans, involving very deep and long foraging dives, a relatively slow ascent, and either a long surface interval or an extended series of short and shallow dives. This may make them especially vulnerable to DCS – if any part of the dive sequence is interrupted it could lead to an excess of nitrogen in the body, leading to gas bubble formation and potentially death.
Cetaceans are well adapted to an environment that is extremely hostile to other mammals. But there is still a lot to be learned about the adaptations which allow whales and dolphins to thrive in their underwater home – and how human activities could be interfering with them.
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Adaptation - any aspect of a living creature which has been modified by evolution to make it better suited to survive in its environment
Atmospheric gas - the combination of gases which make up air, primarily nitrogen and oxygen.
Bradycardia - a slow heart rate, opposite is tachycardia
Buoyancy - the amount an object sinks or floats in liquid
Cartilage - a connective tissue similar to but more elastic and flexible than bone, found (among other places) in the nose and ears
Central organ ischemia - restriction of blood flow to non-essential organs such as the gut, kidney and liver which can tolerate periods with no oxygen
Hypothermia - a drop in core body temperature to a dangerous level
Mass stranding event - when a number of whales, that may be alive or already dead, come ashore
Myoglobin - a molecule found in the muscle which stores oxygen
Red cells - cells in the blood which carry oxygen from the lungs to the rest of the body