[alix.leclerc]

Marine mammals
Marine mammals, warm blooded vertebrates like humans and all other mammals, are specially adapted for a life predominantly in the ocean and have characteristics that allow them to remain submerged for great lengths of time at astounding depths. There are four main groups of marine mammals. Firstly the Cetaceans, consisting of whales, dolphins and porpoises of which there are over seventy species and who spend their entire life in the oceans, sometimes in family groups (pods) and who are known for their seemingly playful behaviour. Secondly the taxonomic order of Pinnipeds, carnivorous marine mammals which use flippers for movement, such being seals, walruses and sea lions. These spend most of their lives in water although they venture onto dry land to breed and bear young, sunbathe and moult. Another group is Sirenia, who like Cetaceans spend their whole lives in water and consist of Manatees and Dugongs. A final group consists of mammals that have "returned relatively recently to the sea, or are in the process of doing so" (Other marine mammals [online] Available at http://www.afsc.noaa.gov/NMML/education/others.htm [Cited 11 July 2007]) such being sea otters (carnivora) and polar bears. Although land based mammals this group feed predominately in the ocean. However while focusing on freediving capabilities this last group will be omitted.

Freediving mechanisms in marine mammals
The freediving characteristics in any mammal that help them dive are all related to what is known as the mammalian dive reflex. The diving reflex is apparent in all mammals and begins as soon as the face is immerged in cold water where sensors detect a drop in temperature; it is a natural body response that primarily helps the body conserve oxygen although at depths it also prevents damage due to high pressures. The mammalian dive reflex puts the body into oxygen saving mode, allowing the body to utilise the oxygen available with more proficiency and so maximise the length of time spend underwater and the activity that can be sustained without needing to breathe for a length of time.
Primarily the diving reflex consists of three mechanisms: bradycardia, peripheral vasoconstriction and blood shift. As well as this there are certain other factors that contribute to the reflex such as the Bohr Effect and contraction of the spleen resulting in increased haemoglobin levels and, whilst not necessarily a primary diving reflex, laryngospasms help to prevent drowning, especially in humans. Individual species have specific capacity for storing oxygen in their bodies; marine mammals store up to 90% of the oxygen for use in a prolonged dive in their haemoglobin and myglobin rather than in the lungs which actually have a small residual volume.
Firstly, bradycardia is the natural slowing of the heart rate and varies greatly between species and even differs in individual organisms. In Pinnipeds, the heart rate may drop to five beats per minute while a few cetaceans can have a decrease of up to 5% of the predive heart rate, sirenia and most whales show a drop of up to 50% of the predive heart rate, similar to that of trained human freedivers. Secondly, peripheral vasoconstriction whereby blood vessels in the outer extremities slowly constrict reducing or stopping the blood flow to outer extremities such as the arms and legs and causes a blood shunt to the more important areas of the body, these being the brain and vital organs. At the same time this results in hypertensive blood pressure and a peak in bradycardia. Thirdly, at extreme depths the lung volume of a mammal is greatly reduced, far beyond that of normal residual volume and there is a great risk of the lungs and chest collapsing. If this were to occur the alveoli in the lungs would stick together therefore making the passage of oxygen to the blood impossible and the animal would die. To compensate for the decreased volume of air in the lungs thoracic filling occurs and there is a shift of blood plasma to the lungs, preventing chest collapse, by filling the alveoli; In effect the entire lung is filled with liquid which cannot be compressed. When returning to the surface this plasma returns to the blood.
During deep dives especially, marine mammals also experience contractions of the spleen; there is an elevation of red blood cells and the vital oxygen carrying protein haemoglobin as the spleen shrinks and 'squeezes' the most amount of blood into the circulatory system. This adaptation means that the Weddell seals experience an increase of up to 65% red blood cell concentration. On top of this, a natural property of haemoglobin described as the Bohr Effect, allows oxygen to be transported with more ease when there is a high concentration of carbon dioxide. The Bohr Effect states that in the presence of high carbon dioxide concentration in the blood and lungs, oxygen binds to haemoglobin with less affinity, and so is more easily picked up and deposited to the muscles that need it most. (The Bohr effect [online] Available at http://en.wikipedia.org/wiki/Bohr_effect [Cited 11 July 2007]) Finally, in the case of near drowning, a laryngospasm is triggered when water hits the back of the throat. Water in the airways of all mammals causes the muscles of the larynx to constrict and close the airway, preventing water from entering the lungs and often relaxes some point after unconsciousness. In humans, this reflex can often prevent drowning since there are a few minutes to get the victim to the surface.
Because of these responses, species such as the Weddell seal can dive to depths of 700m and stay submerged for 80 minutes. Most Pinnipeds can remain submerged for up to twenty minutes while some whales can dive to extreme depths of beyond 900m and remain submerged for a few hours, although most cetaceans dive to lesser depths of around 270m for times of about 15 minutes. When diving marine mammals store the majority on their oxygen in myglobin in the muscles, which appear almost black when exposed to air due to the very high concentration of this protein.


Freediving mechanisms in human freedivers
Although we cannot achieve such astounding depths as the Weddell seal, the diving reflex is as apparent in humans as in marine mammals although often needing time and practice to be fully developed. Furthermore, many experiments have been conducted into the strength of the diving reflex in humans and the physiological effects that freediving has on the body. Whilst it would be difficult for me to implement such experiments due to inaccessibility to sufficient time, participants and equipment, and that such experiments often require some careful consideration of ethics, I have chosen to use secondary data to outline the similarities and differences of the diving reflex in humans and marine mammals. I would for example have liked to conduct an experiment into the effects of diet on bleeding time, blood pressure, heart rate and the effects this had on my own diving experiences and that of others. However, it is clear that this would not be a sufficient sample size and could be ethically unsound.
I will use experiments -where possible- investigating the main five parts of the diving reflex: bradycardia, peripheral vasoconstriction, blood shift, the Bohr Effect and spleenic contractions along with some broad data of the maximum reflex results achieved to conclude the level to which the diving response in human freedivers compares to other marine mammals.