Sarah Bedolfe
Marine mammals are well adapted to a deep-diving lifestyle, but they aren’t immune to decompression sickness.

Focusing on science, technology, engineering, and math (STEM), as they pertain to the ocean.

In a previous post, we discussed how humans can SCUBA dive underwater – and even live there, with the help of bases such as Aquarius – without suffering from the bends. Decompression sickness, known as the bends, occurs after the nitrogen a diver breathes enters the blood stream while at depth; if the diver ascends to the surface too quickly and doesn’t allow time to exhale the gas, the nitrogen bubbles up in the blood stream.

Typically this isn’t a concern for people who freedive – diving without an air tank, holding their breath. Since they don’t get extra air at depth and they can’t spend very long underwater, the body doesn’t absorb enough nitrogen to cause such problems, but doing many repeated dives can be more dangerous. Symptoms of decompression sickness have been found in commercial freedivers, for example pearl harvesters, who make long, deep, and frequent dives.

For a long time, scientists were unsure if marine mammals could get the bends during long and repetitive deep dives – or if they don’t, how they avoided it. We know now that marine mammals typically don’t get the bends, but that they can under certain conditions. 

Photo by Joyce (headharbourlight) via Flickr, Creative Commons License. 

Diving response is a reflex that all vertebrates have, including humans, but it is most strongly developed in diving animals. They automatically slow their heart rate and send blood flow away from the muscles and to the most important organs such as the brain to conserve oxygen. They also have high levels of hemoglobin and myoglobin, which store oxygen, in their blood.

Their main defense against the bends is that their lung structure can collapse under high pressure. This forces the air away from the alveoli and into the upper airways where the gas can’t enter the bloodstream, and that’s what keeps the blood from absorbing too much nitrogen at depth. This also preserves a reservoir of oxygen that becomes available again during the trip back to the surface.

While it has been known for a long time that marine mammals can collapse their lungs, a study published just last month was the first to measure it. Researchers were able to place a data logger on a female California sea lion, which would keep track of the partial pressure of oxygen in its main artery. A decrease in the oxygen pressure indicates lung collapse (because the air leftover in the lungs would be cut off from the blood stream), so they could see how its lung collapse corresponded to dive patterns.

Photo by John Norton via Flickr, Creative Commons License. 

The sea lion in the study did 48 dives of about six minutes each, and collapsed its lungs each time around 225 m (731 ft) deep. The sea lion could keep going to about 300 m (994 ft) before heading back to the surface. Oxygen pressure increased again around 247 m (802) feet, showing that it was drawing air back from the upper airways into the alveoli. The researchers also found that the sea lion inhaled more air and collapsed its lungs at a deeper depth if it was going on a deeper dive.

What are the conditions, then, under which a marine mammal can get the bends? A 2011 study that found symptoms of bubble formation in the bodies of beached whales, dolphins, and seals showed that their defenses against decompression sickness don’t always work.  The authors suggest that conditions, such as cold water, could force the whales to balance other needs (eg. circulation for warmth) with the need to dive safely.

Photo by Willy Volk via Flickr, Creative Commons License. 

There is another important factor: the symptoms of the bends were often found in the carcasses of animals that were exposed to extremely loud noise, such as naval sonar testing. Sonar has been linked to marine mammal deaths for a long time, though scientists don’t know yet exactly how or why. It may disorient them or force them to surface too quickly.

While there is a lot that we still don’t know about how marine mammals dive, we do know that even these animals who are so well adapted to a deep-diving lifestyle, aren’t immune to decompression sickness – and human activities may even be causing it.


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