Northern Elephant Seals:
picture courtesy of: http://kibak.monterey.edu/~seal
The Second largest of all Phocids (seals), southern elephant seals are the largest
One of the best divers of all marine mammals
Can dive to a maximum of 4900 feet with an average of 1500 feet and has the ability to stay submerged for 115 minutes at a time.
Adult male northern elephant seals range from 15-18 feet and 3600-4500 pounds (sea lion and harbor seal fact card).
Northern elephant seals have many adaptations which allow them to dive deep:
Thick layer of blubber
Minimal air sacs
High Concentration of oxygen in blood
Low heart rate during dive
Blubber:
Their streamlined body and thick layer of blubber contribute to their diving behavior. A study done on northern elephant seals residing at Ano Nuevo State Reserve north of Santa Cruz California, found those with a thicker layer of blubber are much more buoyant than the leaner ones. They also found that buoyancy influences their diving behavior by affecting their rate of descent, lower buoyancy (leaner animal) faster the descent however, their ascent rate was not affected with varying buoyancy. Blubber isn't only advantages for buoyancy, but it also allows provides them with the warmth they need at those depths. It is also a great place for them to keep an energy supply because it is a low metabolism tissue.
Air Sacs:
The ultimate measure of how deep a mammal can dive, is how well their body can get rid of air sacs. The less air sacs a mammal has, the deeper it can dive. The physiology of the northern elephant seal eliminates all air sacs in many different ways:
One way seals get rid of problematic air sacs is by exhaling at the start of their dive.
By exhaling, they reduce the air in their lungs allowing them to be compressed as the depth and pressure increases.
Compressing their lungs minimizes the amount of nitrogen that can enter into the bloodstream preventing harmful nitrogen bubbles (the bends) from forming upon resurfacing.
Studies found that seals collapse their lungs at 20-50m collapsing their whole trachea.
Northern elephant seals like all other deep diving mammals fill their ear holes and sinuses with fluid to get rid of those air sacs.
Filling their ears with fluid is beneficial not only for reducing air sacs, but it also allows these mammals to hear better in water then they do on land.
Oxygen Storage:
Since their lungs collapse so rapidly, northern elephant seals must have the ability to store most of their oxygen supply in their blood and muscles. Only 5% of their oxygen concentration is actually stored in their lungs, while 70% is stored in their blood, and the remaining 25% is kept in their muscles (Hurley). Blood is the best place to store oxygen and seals are the best at it because they have a blood volume of 15-20%, which is greater then humans, walruses, and small, toothed dolphins (odontocetes). They are able to carry so much oxygen in their muscles by an increase in myoglobin. Phocids have approximately three times as much myoglobin then humans.
Metabolism/Heart Rate:
A decrease in their metabolic rate is also an adaptation, which allows them to dive deeply. Decreasing their metabolic rate is advantageous because they use less of their limited oxygen supply. They are able to change the blood flow around so that they only send it to areas that are not anoxic tolerant such as the brain, adrenal glands, and heart. Their heart rate also decreases (bradycardia) from 120 beats per minute to 6 beats per minute (Hurley). Their rapid onset of bradycardia at the start of their dive enables their heart to work less and require less oxygen.
Having the ability to get rid of air spaces, supply most of your oxygen in your blood, slow down your heart rate thus lowering your metabolism, are all necessary for deep diving mammals. Not all will have the same statistics as the northern elephant seal, but they will all have the same abilities. As we will see with the sea otter, they lack the physiology, which would allow them to dive deep.
Southern Sea Otter:
picture courtesy of: http://www.silcom.com/~njhua/otter/otter1.html and http://www.ghs.com/people/jimmy/
The sea otter is an excellent example of a shallow diver. The main reason they are not equipped to dive deep is their fur. Southern sea otters unlike pinnipeds have a thick pillage with approximately 850,000-one million hairs per square inch to keep them warm rather then blubber. They use this dense fur coat to keep them warm by spending 48% of the daylight hours grooming their fur and blowing air into it providing them with a layer of warm air between their skin and the marine environment. This layer of oxygen produces a large air sac, which can not be removed, thus keeping them in shallow waters. Southern sea otter pup fur is actually thicker then the adults and traps so much air that they can not dive under the water at all.
Southern sea otters unlike elephant seals normally forage in water less then 60 feet with a maximum of 330 feet
Can only stay submerged for 4-5 minutes at a time
Can only swim 2-3 miles per hour.
While on the surface grooming themselves they float belly-up alternately pumping their hind flippers up and down. They use their broadly flattened tail and webbed feet to increase their propulsion.
When one compares the southern sea otter and northern elephant seal, it is easy to see why one must remain in shallow water and the other has the ability to dive to remarkable depths. Lacking blubber and relying on a thick layer of air for warmth, the southern sea otter will never be able to be a deep diver. All mammals that rely on the oxygen in their lungs (humans, polar bears, and sea otters) are shallow divers. The northern elephant seal on the other hand is perfectly adapted to deep diving with its thick layer of blubber, ability to collapse and fill air spaces, and lower their heart rate.