Bottlenose dolphins can control their heart rate to avoid the bends

Dolphins seem to adjust their heart rate while diving to avoid decompression sickness, also known as bending, which is caused by sudden changes in pressure.

Human divers must avoid surfacing too quickly, as the drop in pressure can force nitrogen bubbles into their airways and cause joint pain or even paralysis.

Marine mammals like dolphins were thought not to have this problem, says Andreas Fahlman of the Oceanographic Foundation in Valencia, Spain, but researchers recently reassessed this idea.

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To test it, Fahlman and his colleagues trained captured bottlenose dolphins to make short or long dives on command. They measured the animals’ heart rate using electrocardiography and found that their heart slowed down just before diving underwater.

When preparing for a long dive, the dolphins reduced their heart rate faster and to a lower rate than they would on a shorter dive. This saves more oxygen and reduces decompression sickness by limiting nitrogen uptake.

Fahlman says this is likely a conscious response rather than an automatic response: the dolphins control their heart rate by emptying part of their lungs to allow blood or air to flow into areas under pressure. “They control how much blood goes to and where in the lungs to avoid nitrogen uptake,” he says. “You can basically step on the gas pedal and leave it when you want.”

Stress from sounds like sonar or machines used for oil exploration can interfere with that conscious control of heart rate, says Fahlman, potentially increasing the chances of a dolphin getting the turns. If we learn more about the physiology of dolphins, we may be able to find ways to alleviate these problems, he says.