(CN) — Dolphins are masters of the ocean. Strong, tenacious and keenly intelligent, these ocean-dwelling mammals actively cooperate with other species, from killer whales to humans, and are one of only two known saltwater species to use tools.
But an enduring mystery surrounding their diving skills has kept scientists guessing for decades: How do these graceful creatures dive to great depths and rapidly ascend without developing decompression sickness?
Commonly called “the bends,” decompression sickness occurs when nitrogen builds up between a diver’s joints, causing crippling pain, paralysis and even death in untreated cases. Human divers follow strict protocols to avoid the bends, pausing frequently as they ascend to mitigate pressure changes and allow their bodies to slowly reabsorb the nitrogen stored in their bloodstream.
Now scientists have discovered how dolphins successfully dive without developing this potentially deadly condition.
Dolphins actively slow their heart rate before diving, adjusting it based on how long they plan to dive, according to a study published Monday in Frontiers in Physiology. The key involves conserving oxygen and counteracting pressure during the dive.
Researchers worked with three male bottlenose dolphins trained to hold their breath for different periods of time.
“We trained the dolphins for a long breath-hold, a short one, and one where they could do whatever they want,” explains Dr. Andreas Fahlman of Fundación Oceanogràfic in Valencia, Spain. “When asked to hold their breath, their heart rates lowered before or immediately as they began the breath-hold. We also observed that the dolphins reduced their heart rates faster and further when preparing for the long breath-hold, compared to the other holds.”
Relying on the strong bond of trust between dolphin and trainer, the dolphins in the experiment were asked through hand signals to roll belly up and hold their breath underwater. Electrocardiogram (ECG) sensors attached to their undersides measured heart rate during the exercise. At the appointed time, the dolphins turned right side up and breathed into a custom-made spirometry, a device held over the blow hole to measure the volume of each breath and the oxygen and carbon dioxide of the exhaled air.
“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,” Fahlman said. “This allows them to conserve oxygen during their dives and may also be key to avoiding diving-related problems such as decompression sickness.”
Until now, the practical challenges of measuring a dolphin's heart rate and breathing have prevented scientists from studying changes in their physiology during dives.
But it takes more than specialized equipment to study dolphins this way. Equally important is cooperation from the ocean mammals themselves, who can reach lengths of 10 feet or more and weigh as much as 600 pounds.
“The close relationship between the trainers and animals is hugely important when training dolphins to participate in scientific studies,” said Andy Jabas, dolphin care specialist at Siegfried & Roy's Secret Garden and Dolphin Habitat in Las Vegas, where these dolphins live. “This bond of trust enabled us to have a safe environment for the dolphins to become familiar with the specialized equipment and to learn to perform the breath-holds in a fun and stimulating training environment. The dolphins all participated willingly in the study and were able to leave at any time.”
Once again, the ability of dolphins to cooperate with other species has long-reach implications. In this case, researchers think the dolphins use techniques employed by other marine mammals to dive safely.
Scientists say this new understanding is crucial to mitigate the health impacts of human-caused sound disturbance below the ocean’s surface, including underwater blasts during oil exploration.
“If this ability to regulate heart rate is important to avoid decompression sickness, and sudden exposure to an unusual sound causes this mechanism to fail, we should avoid sudden loud disturbances and instead slowly increase the noise level over time to cause minimal stress,” Fahlman said. “In other words, our research may provide very simple mitigation methods to allow humans and animals to safely share the ocean.”
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