Sea lions frequently dive down to the depths and ocean pressures where large amounts of nitrogen would be forced into the blood. As air-breathing mammals, they must also return regularly to the sea surface to breathe. In humans, this rapid change of pressures would result in decompression sickness. However, sea lions do not experience this problem. In 2012, scientists unlocked the sea lions' secret for avoiding decompression sickness.
What causes decompression sickness?
Nitrogen makes up 78 percent of Earth's atmosphere by volume. It is not a normal part of physiological processes. Under normal atmospheric pressures, a person breathes out as much nitrogen as he or she breathes in. It does not accumulate anywhere in the body.
When a person breathes in air at deep-water pressures, some of the nitrogen which is being breathed in is forced to dissolve into the bloodstream. When that pressure is eased too quickly, the dissolved nitrogen comes out of solution as bubbles in the bloodstream. Those bubbles can migrate to any part of the body, where they can cause symptoms ranging from joint pain to sensory disruptions to death. These symptoms are known as decompression sickness.
Humans usually prevent decompression sickness by pausing for extended periods at different depths when returning to the surface. Decompression tables are used to calculate the depth, length and frequency of each decompression interval. Limited recreational dives may not require decompression at all.
Like humans, sea lions and other marine mammals breathe surface air, complete with its high ratio of nitrogen. Unlike humans, they can dive deep and change depth and pressures rapidly without developing decompression sickness. Sea lions can reach a depth of 1,640 feet in less than 10 minutes, the total amount of time they can hold their breath. This kind of flexibility is essential for ocean survival.
How sea lions avoid decompression sickness
When they reach an average depth of roughly 731 feet, sea lions collapse their lungs to prevent surface air from entering their bloodstream. After that, they depend on air from their upper airways to provide enough oxygen until they reach the water's surface again.
This was discovered in September 2012, when Birgitte McDonald and Paul Ponganis attached loggers to a California sea lion. This gave them a record of oxygen pressure in the sea lion's main artery, as well as the time and depths to which the sea lion dived. Altogether, the loggers recorded data from 48 dives, with each dive lasting roughly six minutes.
The loggers proved that there was a sudden plunge in the sea lion's oxygen pressure each time it reached a depth barrier which averaged roughly 731 feet. This is consistent with the deliberate lung collapse observed in some other marine mammals. At this point, no new air could reach the bloodstream from the lungs, which also limited the amount of nitrogen which could reach the bloodstream.
At an average of roughly 802 feet, the oxygen pressure in the main artery increased again. At this point, the sea lion is drawing oxygen from reserves in its upper airways back into the alveoli.
Changes between the dives
In the study, the depth barrier was deeper and the original inhalation was larger if the dive was deeper. How this works is not yet known. The deeper dive may happen because an original inhalation was larger by chance. Alternatively, the sea lion may plan for a deeper dive, which would give insight into its cognitive skills.
Similar mechanisms are known to be used by some other marine mammals. Others, such as the emperor penguin, protect themselves from decompression in completely different ways.
However, none of those methods are perfect. Autopsies of dead marine mammals which had been stranded sometimes show symptoms similar to those of decompression sickness. Based on some modeling studies, some marine mammals may have permanently elevated nitrogen concentrations in their blood. In that case, they could be at risk for decompression sickness if they experience sudden changes to their dive behavior, such as stranding.