Decompression sickness, or the bends, occurs when a scuba diver breathes in nitrogen at depth and ascends too rapidly for the nitrogen to dissipate. The nitrogen bubbles in the blood stream, causing pain or even death. Scientists have long wondered how marine mammals, who often dive to depths of hundreds of feet, avoid decompression sickness. They have since learned that marine mammals rarely suffer from decompression sickness, but that they can in certain circumstances.
A 2012 study tracked a sea lion during 48 dives. During each dive the sea lion collapsed its lungs at around 731 feet deep and continued to about 994 feet deep before resurfacing. The ability to collapse the lungs is part of a diving response which is a reflex common to all vertebrates, including humans. It is more developed in diving sea mammals such as sea lions. This allows them to automatically slow their heart rate, which decreases the need for oxygen.
Early studies found that the heart rate could decrease from 150 beats per minute to only 10 beats per minute. Blood is shunted from the muscles, which use more oxygen, to the brain and other important organs. Sea lions also have more myoglobin and hemoglobin in their blood, which helps store needed oxygen. The ability to collapse their lungs due to their unique lung structure forces air into the upper airways, where it cannot enter the bloodstream. If the air can’t enter the bloodstream, it can’t absorb nitrogen at depth. This also gives the sea lion an air reservoir to use on the trip back to the surface.
A study by the Royal Society study theorized that diving mammals may occasionally push the limits of diving time and depth, allowing a tolerance for increased nitrogen levels. The physiological adaptations to deep diving are a combination of automatic and controlled. It is important to remember that sea lions may have to dive deeper or stay down longer when foraging for food. Likewise, avoiding predators may require a very quick ascent and the extreme pressure changes that go with it.
Signs of decompression sickness, including bubbles, have been found in marine mammals caught in fishing nets where death occurred at around 229 to 328 feet. Scientists could not determine if the nitrogen levels increased from struggling against the net or were a normal finding for that animal at that depth. Ultrasound imaging of live-stranded dolphins found nitrogen bubbles in the kidneys and liver. The animals were tagged, and later found to have survived despite the presence of bubbles.
Interestingly, the study of sea lions and other dead mammals has found at least one cause of death from decompression sickness: extremely loud noise. Military maneuvers involving sonar have resulted in the later finding of carcasses in which death was caused by decompression sickness. It is unclear whether the sonar disoriented the animals or caused them to surface quicker than usual. More studies will be needed to further explore the physiological protections and responses that sea lions have toward decompression sickness. Understanding them can be helpful in applying techniques for human divers to avoid decompression sickness and minimize its side effects.