In 5.400 B.C. a Stone Age culture known as Ertebølle lived on the coastline of Denmark and Southern Sweden. It is from these people that we have the earliest archaeological evidence of human free-diving. After that there are frequent references to free diving throughout ancient history such sponge divers in ancient Greece and pearl divers off the coast of Japan. The famous Greek philosopher Aristotle wrote of the medical problems found in those who dived for a living providing the first records of “the bends” and other medical conditions associated with diving.
The first properly documented free-dive was by the Greek sponge fisherman Stathis Chatzi. An Italian naval ship, the Regina Margherita lost its anchor off the island of Karpathos. Stathis, using a heavy stone was able to dive to a depth of 88 metres in order to retrieve the anchor. For this he was paid a reward of five pounds and permission to use dynamite while fishing.
Since then freediving has grown into a competitive sport with modern divers pushing the limits of human endurance. Through these extraordinary feats, humans have sought to match the diving prowess of some of the great divers of the natural world. Below are some of the best air-breathing divers that can be found in British coastal waters and the adaptations they have which allow them to be such excellent divers.
Length of dive – Average human: about 2 minutes, World Record holder Stéphane Mifsud: 11 minutes, 35 seconds
Depth of dive – World Record Holder Herbert Nitsch: 124 metres
Humans possess the “mammalian diving reflex”. If you immerse your face in cold water, you will find your pulse rate will begin to slow down. This slowing of the heart is known as bradycardia. In the average human pulse rate will be reduced by up to 30% and with training can be reduced even further, almost to the same level as some semi-aquatic mammals. This adaptation helps preserve oxygen reserves by limiting oxygen consumption.
Another adaptation that will also kick in that isn’t immediately so noticeable is the contracting of blood vessels in your skin, large muscle groups and non-vital organs. Vasoconstriction or Vascular constriction reduces and, in cases where an organ or limb can function entirely anaerobically, stops the blood supply to these areas. This is another oxygen saving adaptation which functions in two ways. Firstly reducing the flow of blood to those areas means the heart does not have to work as hard to pump blood around the body. Secondly, oxygen is preserved for the most vital organs in the bod; the heart and the brain, which require a constant supply of oxygen.
A slower reaction to immersion is a contraction of the spleen. The spleens primary task is the destruction of red blood cells. However, due to the large volumes of blood contained within the spleen it can also provide a boost to red blood cell numbers, which in turn allows for greater oxygen carrying capacity. Trained free divers’ spleens can contract by up to 20% while diving; causing a significant increase in the amount of oxygen that can be stored in their blood.
All of these reflexes depend on a variety of other physiological traits, but can also be trained to extraordinary levels. The current world record holder for Static apnea (holding your breath!) is Stéphane Mifsud who can hold his breath for an amazing 11 minutes and 35 seconds.
When attempting to dive to depth, the mammalian diving reflex can also help. The prioritising of blood to the heart and brain shifts a lot of blood to the chest, which can help resist the increased pressure. Aside from this, while there are a few natural mechanical changes which occur at high pressure (such as arching the diaphragm so the abdomen is compressed more than the chest), humans are rather ill suited to exist at such high pressures! There is also the fact that in water, light passing into the human eye does not refract as much which means the image is not properly focused on the retina, leading to distortion. This prevents most divers from seeing clearly underwater.
Humans are also ill adapted to exist at the low temperatures present at such depths. The thermal conductivity of water is twenty-five times greater than air and as such body heat is lost significant faster. Once again the diving reflex can help, as the restriction of blood to the skin can reduce body-heat lost. It has also been found that Korean divers who frequently spend hours in cold water actually developed a seasonal variation in their metabolism, which in January began to burn so fast that they were unable to eat enough to stop themselves constantly losing weight.
Length of dive: 5-15 minutes, though up to half an hour has been recorded
Depth of dive: 200 metres
Grey seals are frequently found hauled up on rocks and beaches around the British coast. While they appear clumsy on land, this is due to being almost entirely adapted to a life underwater.
Seals possess the mammalian dive reflex, but to a much greater degree than humans. Seal bradycardia is far more developed, with the heartbeat able to slow down to an average of a mere four beats per minute. The heartbeat also becomes arrhythmic, with long pauses between heartbeats, followed by two heartbeats in quick succession.
As well as bradycardia, seals also exhibit tachycardia, (the opposite of bradycardia) while on the surface, increasing the speed of their heartbeat up to 140 beats per minute, which increases the speed of oxygen uptake and recovery on the surface. This decreases the amount of time between dives. Oxygen is not stored in the lungs of seals during long dives. In fact, after reaching a certain depth the lungs collapse in order to protect them from the pressure. Oxygen is instead mainly stored in the muscles and the blood, which have high levels of myoglobin and haemoglobin respectively in order to increase storage efficiency. This storage method allows seals to avoid the bends and reduce their buoyancy to dive more effectively. Seals muscles are also adapted to cope with greater levels of lactic acid and so operate anaerobically longer.
The blubber beneath a seals skin provides insulation even in the coldest temperatures as well as helping to create the seals streamlined shape. This streamlined shape combined with extremely efficient swimming allows a seal to reach speeds of up to 20 knots!
Seals have numerous sensory adaptations to let them hunt effectively underwater. In order to be able to see clearly in dim light conditions seals’ eyes are extremely large. The cornea is strengthened and almost spherical to prevent distortion underwater. The eyes are protected by a third eyelid or nictitating membrane which flushes away salt and sand and constantly produce tears for further protection. A seal has additional whiskers known as vibrissae that are extremely sensitive and can help a seal detect vibrations caused fish movements. A seals sense of smell is useless underwater and therefore they possess muscles which allow them to close their nostrils to prevent irritation while diving.
Length of dive: 8 minutes
Depth of dive: usually about 45 metres, up to 700 metres reported
Dolphins and porpoises are some of the most accomplished or mammalian divers. They can frequently be found swimming together in pods or schools. Their streamlined, torpedo-like shapes allow them to achieve a cruising speed of up to seven miles per hour and bursts of up to twenty-two miles per hour. This smooth shape is created by the thick layer of insulating blubber beneath their skin, which is ten times thicker than any land mammal. The entire top layer of skin is replaced every two to four hours to maintain its moisture and smoothness.
However a dolphin’s skin is not entirely smooth. Upon close examination there are microscopic ridges running all along the body. These microscopic ridges trap water molecules at the surface of the skin. This reduces water resistance as the dolphin swims, because the water molecules allow the dolphin to travel through the water more like another liquid, rather than a solid.
Cetacean’s have the same diving reflexes found in other mammals. Cetaceans do not breathe through their mouths at all as their lungs are attached directly to their blowholes. Cetacean lungs are far more efficient than human lungs, replenishing about 80% of their lung capacity when respiring, as opposed to humans 17%. These lungs collapse under high pressure, assisted by a flexible ribcage, with free-floating ribs. Like seals, dolphins can store large amounts of oxygen in their muscles which allows them to slow their bloodflow to a minimum, as the oxygen is already in the muscle tissue. Dolphins also engage in extreme tachycardia when above the water, increasing their heartrate to 120 beats per in order to quickly recover from dives. In order to deal with these sudden changes in blood pressure, dolphins have a special adaptation known as the retia mirabilia. This is a spongy tissue found underneath the ribcage that controls the flow of blood to the brain when the heartrate is rapid, preventing a surge of blood to the brain. It also releases blood when heartrate is low in order to prevent damage due to lack of blood flow.
An oily secretion protects dolphin’s eyes from saltwater. A dolphin’s eyes are well adapted to see both below and above the water, with corneas and lenses shaped to the refraction of water and muscles that can alter the shape of the eye to see better above water. In intense light a dolphins pupil will constrict down into what looks like two pupils. There are two receiving areas at the back of the eye, unlike other animals which normally have one. These allow a dolphin to have binocular vision above the water and binocular and monocular vision below the water.
Length of dive: 4 seconds
Depth of dive: 20 metres
Gannets are the largest seabird found in the North Atlantic Ocean. Bright white with a wingspan of over two metres they are often easily visible from the shore, hunting in groups. Gannets are absolutely fantastic divers diving from up to 30 metres in the air and hitting the water at up to 100 kilometres per hour, allowing them to occupy a niche between surface feeders and deep divers. Such extreme dives require several adaptations to prevent the bird severely injuring itself on impact with the water!
In order to judge their dives accurately, gannet eyes are located at the front of their face giving them binocular vision, like birds of prey. Just before hitting the water gannets will quickly fold their wings in close to their body in order to ensure maximum streamlining and prevent dislocation of the wings due to the high speed of the dive. They also have a network of air sacs beneath the skin in their chests and faces, which they can inflate in order to cushion important organs and the brain. For this purpose gannets have strong bills, with no external nostrils and exceedingly thick skulls.
Length of dive: 95 seconds
Depth of dive: 80 m
European shags are pursuit-dive foragers; diving beneath the water and chasing fish using powerful synchronised leg movements. They have been recorded as the most efficient foot propelled divers due to their long dives and extremely short recovery times.
Shags are extremely streamlined, with their legs at the back of the body for more efficient swimming and plumage adapted to minimise buoyancy. Unlike other water birds, they do not have glands to produce waterproof oil to waterproof their feathers. This allows their feathers to get wet while they dive which prevents the trapping of air and therefore reduces buoyancy allowing for easier dives.
This of course leads to another set of problems, as wet feathers lead to faster temperature loss. You may assume that because of this, shags would be restricted to warm waters; in fact they are one of the most widely distributed diving bird species and don’t appear to need extra food to compensate for this loss of heat. It is thought that a shags diving strategy is centred on controlling this temperature loss, taking time to warm up after a series of short dives. The familiar wings outstretched pose is thought to allow the wings to dry and the bird to control its temperature. Shags also maintain a small amount of air, just enough to provide some insulation without affecting their diving ability.
The particularly interesting thing about shag diving behaviour is how their diving may change when diving in groups. Shags form large foraging rafts, sometimes composed of up to a thousand birds. While in this formation they begin a “leapfrogging” behaviour, with several birds diving at once, and then flying over the heads of the birds in front before landing and diving again. Why are they doing this? Does it speed up the process of finding prey? Are they using other birds as is indicators of the presence or absence of fish under the water? Do they change the length of depths of their dives when in groups?
The honest answer is that we don’t know, and this is why I am particularly interested in their diving behaviour. Particularly in how the presence of other shags might affect how they dive. I hope to attempt to answer some of these questions.