Quirks and Quarks

Diving a kilometre down and not getting 'the bends' — how dolphins do it

Dolphins have remarkable control of their physiology that allows them to keep nitrogen from entering their bloodstream and forming damaging bubbles as they decompress.
Two new studies reveal how a group of deep diving bottlenose dolphins in Bermuda are able to beat 'the bends.' (Roland Weihrauch/AFP/Getty Images)

Beating the bends

If you've scuba dived before, then you've definitely heard about decompression sickness or "the bends."

When divers ascend too quickly from deep waters, dissolved nitrogen in the blood forms bubbles which can cause excruciating pain in the muscles, paralysis, and in some cases even death. It's a very scary risk for deepwater human divers as well as other diving species.

But two new studies reveal how a group of deep diving bottlenose dolphins in Bermuda have figured out how to beat "the bends".  Dr. Andreas Fahlman, its lead author, is the director of the Oceanographic Foundation's Department of Research in Valencia, Spain.

The Olympic athletes of dolphins

Fahlman explained that within the same species of bottlenose dolphins, there is an enormous variation in diving abilities. The Bermudian population frequently dives to depths of up to 400 meters and as deep as 1000 meters on occasion without suffering from decompression sickness while the shallow diving population in Florida only dives as deep as three meters. Fahlman calls the Florida population the "couch potatoes,"  whereas the Bermudian group are the Olympic athletes of the species. They even have bigger muscles, as athletes often do.

There's been evidence that they can select the heart rate, and basically the volume of blood that is pumped out from the blood per minute.- Andreas Fahlman

Fahlman wanted to know how the Bermudian population is able to dive so deep without experiencing decompression sickness.

The hypothesis in his first study was that the Bermudian dolphins must have a lower metabolic rate and consume less oxygen per unit of time. But that's not what his team detected when they measured the lung physiology and energy consumption of several deep-diving bottlenose dolphins. What they found instead was a difference in red blood cell count when they compared the Bermudian dolphins with the shallow water Florida population.The Bermudian dolphins had more muscle and more myoglobin in their muscle cells to hold oxygen. Their blood also held more oxygen. "They have a bigger scuba tank," says Fahlman. 

Special abilities

In his second study, the hypothesis was that the Bermudian population must have extra squishy lungs to prevent damage from compression, but that's not what they found. This suggests the dolphins might use other means to avoid the bends. So they came up with a new hypothesis: the dolphin is able to control gas exchange between their lungs and blood to manipulate the amount of nitrogen that enters their bloodstream to avoid diving-related problems.

The interesting thing about dolphins and some other marine mammals is that their lung architecture is different from humans'. Once they dive past the five meter point, the bottom half of each of the two lobes of the Bermudian dolphin's lungs naturally collapses due to pressure and their lung architecture. This is very useful during deep dives because it prevents nitrogen from getting into their bloodstream, as there's no air in the bottom half of the lungs.

People extend their hands toward bottlenose dolphins from off the Southern California coast on January 30, 2012 near Dana Point, California. (David McNew/Getty Images)

These adaptable dolphins are also able to manipulate their body to send blood to different regions of the lungs. "There's been evidence that they can select the heart rate, and basically the volume of blood that is pumped out from the blood per minute," said Fahlman.

As a result, blood only goes to the compressed regions of their lungs, and not the gas-filled regions, thereby preventing gas exchange.

But how do they get oxygen? According to Fahlman, they do it by sending tiny amounts of blood to the gas-filled regions of their lungs to exchange a little bit of oxygen and carbon dioxide. But they're able to make sure that nitrogen doesn't get into the blood during the gas exchange by varying the rate of air flow and blood flow.

When the dolphins manipulate this perfectly they're able to avoid getting nitrogen into their bloodstream, says Fahlman. Fahlman calls them the elite dolphins of the species, similar to elite human athletes who can manipulate their heart rate and physiology to achieve mind-blowing feats.

Conservation implications

The bigger goal of this research is dolphin and marine mammal conservation. Under certain circumstances, this amazing physiological mechanism that deep-diving dolphins and whales have fails.

When humans disturb these animals by using loud sonars and seismic oil exploration, it can stress and frighten them, causing their natural mechanisms for averting decompression sickness to fail — making them emerge too quickly from the watery depths. Fahlman hopes to find better strategies to avoid this by understanding how they dive and their limits for doing so.