In humans, if we spend time beneath the ocean and then travel to the surface, we can suffer decompression sickness, known as "the bends" - when the nitrogen in compressed air that dissolved into our blood during a dive does not have time to clear and forms bubbles in tissues. 

Why don't sea creatures? Using three trained male bottlenose dolphins (Tursiops truncatus), custom-made equipment to measure the lung function of the animals, and electrocardiogram (ECG) sensors they found that the animals, trained for a long breath-hold, a short one, and one where they could do whatever they want, would have lower heart rates as they began the breath-hold. The dolphins reduced their heart rates faster and further when preparing for the long breath-hold, compared to the other holds.

Examples of instantaneous heart rate (ifH) responses from individual dolphins against time before (time –20 to 0) and up to 40 s (time 0 to 40 s) during a breath-hold. (A) ifH for a LONG and SHORT dive (each preceded by a symbol with a pre-determined dive duration), and a dive without a symbol (NS), where dive duration was determined by the dolphin. (B) ifH variation during a breath-hold for 3 NS dives in dolphin D3 (Table 1). The dive durations, the time from the last breath before and first breath after a breath-hold is indicated in parenthesis.

The results reveal that dolphins, and possibly other marine mammals, may consciously alter their heart rate to suit the length of their planned dive.

"Dolphins have the capacity to vary their reduction in heart rate as much as you and I are able to reduce how fast we breathe," suggests Dr. Andreas Fahlman of Fundación Oceanogràfic, Valencia, Spain. "This allows them to conserve oxygen during their dives, and may also be key to avoiding diving-related problems such as decompression sickness, known as "the bends"."

 Dolphins are not the first known to change heart rates underwater. In 1870, Paul Bert showed that ducks dropped from 100 beats per minutes while breathing on the surface to 14 beats per minute submerged, but the new work may provide an explanation why other creatures don't suffer decompression sickness the way humans do.