3 The 'diving response'
3.1 Features of the diving response
All the aquatic mammals' adaptations to life in the water - breathing, moving, staying warm and making sense of the environment - come together in their diving behaviour, and the diving abilities of marine mammals are truly awe-inspiring. The elephant seal, for example [p. 192], makes repetitive, long-duration dives and some 80-95% of its time at sea is spent submerged. Its dives are of about 20 minutes duration on average, and the intervals at the surface are seldom more than about two minutes. This pattern is repeated almost continuously for between two and eight months. Such abilities are especially useful for those seals that often travel long distances under ice from one breathing hole to another - the Arctic-dwelling ringed seal shown in the TV programme at 18.59 is one such example and its Antarctic counterpart, the Weddell seal, is another. The sperm whale routinely swims down to depths in excess of 1000 m to hunt giant squid and can stay submerged for over an hour.
The length of time a mammal can spend under water depends on the amount of oxygen it takes on board before a dive and its ability to slow the rate at which it uses that oxygen once the dive is under way. It was not until the 1930s and 1940s that biologists began to understand the diving abilities of marine mammals. Work on captive seals making simulated dives in the laboratory - mainly Weddell seals and elephant seals - identified a number of physiological mechanisms that became known as the 'diving response', detailed below. Subsequent research involving monitoring animals in their natural environment has shown that the full diving response is necessary only in extremis, when the animal is pushed close to its physiological limits.
The animal stops breathing (technically known as 'apnoea'.
The heart rate slows very markedly - a condition termed bradycardia. Many seals can reduce the rate to about four beats per minute.
The blood pumped by the heart is diverted away from peripheral tissues and muscles to the oxygen-dependent heart, brain and other essential organs. In essence, the animal turns itself into a 'heart-lung-brain' machine.
The overall metabolic rate of the diving animal falls as some tolerant tissues - the gut and kidneys, for example - are starved of oxygen. The metabolic rate also falls as the temperature of peripheral tissues - the flippers or flukes, for example - moves closer to that of the surrounding water. (In general, the warmer a tissue the greater its metabolic rate and, hence, the greater the demand for oxygen.) The countercurrent heat exchanger ensures that the blood supply to these peripheral tissues is maintained even as tissue temperature falls (Figure 2b).
The animal swims in a slow and sustained manner, gliding along to conserve oxygen.
These adaptations allow the animal to make efficient use of the oxygen carried on the dive. Two other adaptations allow the animals to increase the amount of oxygen carried, providing what some biologists have called a 'physiological scuba tank'.
As DA explains [p. 194], oxygen is stored by two proteins in the body, haemoglobin in the red blood cells and myoglobin in the muscles, and diving mammals have exceptional quantities of both substances.
In some diving mammals, muscles in the spleen contract to squeeze out more oxygen-holding red blood cells when the animal dives. The spleen is also larger, allowing it to hold and release more red blood cells.
Perhaps surprisingly, the main elements of the diving response are not confined to aquatic mammals. A similar response is found in all mammals and most other air-breathing vertebrates, including reptiles and birds. The observation that the diving response seems to be 'hard wired' into all mammals has led to some debate about its wider purpose. It may be that it has a role in other situations in which the body has to cope with reduced levels of oxygen.
Can you think of any circumstances in which the ability to withstand oxygen starvation might be of use to a terrestrial animal?
There are a number of possible situations, including vigorous exercise or oxygen starvation during birth.
Of course, we are also mammals and we have our own diving response. The PDF reading 'Into the abyss', linked below, describes how humans have explored the limits of their mammalian diving skills to develop a new sport: freediving. Freedivers swim as deep as they can without the aid of scuba tanks. A normal, healthy human being can endure apnoea for one to two minutes and dive to a depth of 10-20 m. But in June 2000, Loïc Leferme dived to a depth of 152 m, and he and his colleagues regularly stay under the water for seven minutes or more. Freedivers can also reduce their heart rates to an amazing six beats per minute, which is not far away from the minimum of a diving seal.
Click 'View document' to open Reading 1: 'Into the abyss'