Scientists study how marine mammals survive at great depths

Whales, dolphins and seals have evolved to hunt prey deep in the ocean, an environment that would otherwise prove deadly to animals that depend on breathing air to live. Until recently, scientists believed marine mammals' known physical adaptations protected them from the effects of such punishing depths.

But scientists were baffled by computer models that showed that, even with the known adaptations, 50 percent of animals studied still should have experienced the bends. Researchers concluded there must be some else going on.

A new study by the Woods Hole Oceanographic Institution and the Oceanographic Foundation of the City of Arts and Sciences in Valencia, Spain, may provide an answer. The study, funded by the U.S. Navy, found that deep-diving marine mammals use a physical adaptation — the collapse of one portion of the lungs — to block the flow of nitrogen into the blood and prevents the animals from getting the bends, the crippling release of nitrogen gas that can occurs when surfacing from dives deeper than 130 feet.

“If you get the conditions right, you can get a nice exchange of oxygen and carbon dioxide but block the nitrogen,” said Michael Moore, a WHOI senior scientist who specializes in the analysis of marine mammal mortalities. Moore is a co-author of the study, which was published April 25 in the journal "Proceedings of the Royal Society B."

The mammals have other physical adaptations that help them survive the depths when they exhaust available oxygen in their lungs, including a high amount of proteins in blood and muscle that bind oxygen and a higher ratio of red to white blood cells. Their ribs and lungs can collapse under pressure without breaking and their airways are hardened bunkers that remain partially open to power the signals they use to locate one another and their prey.

The idea whales don't get the bends was dashed in 2002 when 14 deep-diving beaked whales beached and died on the Canary Islands. A necropsy found nitrogen gas bubbles in their tissues. Even though scientists weren’t sure exactly how it happened, the blame fell on a new type of sonar being used by the U.S. Navy taking part in a NATO military exercise.

Tests show lung changes

Under pressure, nitrogen gas in the lungs enters the bloodstream through alveoli, tiny sacks hanging like fruit from the branches of the respiratory tree. Each alveolus is like a border crossing, where the inhaled oxygen dissolves into the blood through the net of capillaries that surround them. It is how oxygen moves from the air into bodies for the chemical reactions that produce energy.

Nitrogen gas, however, is inert. It does not react with other chemicals and is generally expelled from the lungs along with carbon dioxide. But under extreme depths with no respiration occurring, and given enough time, nitrogen gas is forced into the bloodstream and enters body tissues.

If a diver surfaces too quickly, the nitrogen does not get a chance to reenter the lungs and be expelled through respiration. The sudden drop in water pressure as a body nears the surface allows the gas to expand explosively — think of carbon dioxide rushing out of a newly opened bottle of soda — tearing apart the tissue, obstructing arteries and damaging the central nervous system.

For the study, Moore designed a hyperbaric chamber linked to a CT scanner to compare how the bodies of a dead dolphin, grey seal and a domesticated pig changed under simulated pressures at depth. What researchers observed was the collapse of the bottom portions of the lung and the alveoli there, where the blood supply was robust.

While the upper portion of the lungs remained functional, it is an area of the lung where the blood supply is diminished and the transfer of gases from lung to blood the lowest. High air-to-blood ratios favor the exchange of oxygen and carbon dioxide but reduce, or even reverse, the exchange of nitrogen, the study concluded.

The study also proposed that the partial lung collapse was voluntary, controlled by a sphincter muscle that determined the flow of air and blood to the upper portion of the lung.

Noise can disrupt the process

But it is a very fine balance, Moore said. The introduction of a sudden noise, like the blast of sonar, can upset that balance with dramatic consequences.

Sonar is an alien sound in what is not a silent world, said Aran Mooney, a WHOI scientist specializing in the sensory biology of marine animals. Diving down into the ocean is more like entering a forest — some parts of the forest have lots of life, and there’s a lot of chattering going on as whales can hear their neighbors vocalizing with clicks, whistles and other sounds to locate one another or their prey.

“They generally know that someone else is around,” Mooney said.

Mooney said whales and other marine mammals have become somewhat accustomed to most human noise, like ship traffic, although it has been shown to raise stress levels. It is like the adjustment one makes when living next to a busy highway.

“But sonar is a little different; it’s abrupt, startling,” Mooney said. It is believed whales may associate it with the approach of a pack of killer whales, their worst enemy in the natural world.

Whales and other deep-diving marine mammals are already operating on the edge of what they can physically endure while hunting at depths. Scientists believe marine mammals prepare mentally and physically for a dive. Moore said studies on California sea lions revealed that their pre-dive preparations demonstrated they knew how deep they were going, so a change in their dive plans is a big deal.

“The theory is that (the dive profile) is finely tuned,” Mooney said.

Being 'in the zone'

Sobonna Ong, a freediver from Alexandria, Virginia, understands that concept. Diving without the encumbrance and noise of scuba tanks is a meditative experience quite different from his job as an information security specialist supporting federal agencies in Washington. To achieve the depth he wants takes a balancing act of mind and body — and preparation.

Prior to a dive, Ong clears his mind of distractions. Depending on ocean temperature and how far down he wants to go, he will sometimes do a pre-dive breathing exercise that packs air into his lungs, but generally he takes a big gulp of air and dives following a vertical line marked off in 5-meter increments.

He works hard for only the first 30 feet or so, kicking to overcome his body’s tendency to float. Then he enters the “sink” phase of the dive, where his body is neutrally buoyant and the 2 to 3 pounds of weights he carries around his neck or on a dive belt pull him effortlessly down in silence.

His heartbeat slows. He feels an overwhelming calmness.

“Then you know you’re in the zone,” said Ong, who free dives without scuba gear to 130 feet.

The slowed heartbeat, the constriction of blood vessels and capillaries that concentrates blood around vital organs and the brain, is part of what is known as the mammalian dive reflex. It occurs in everything from lab rats to whales.

Ong and his fellow freedivers, some of whom go down over 700 feet on a single breath, do not stay down long enough for nitrogen to invade their tissues, and don’t breathe the compressed gas that scuba divers do that helps force nitrogen into the blood. But a disruption of a finely tuned dive plan can still prove fatal.

“If you’re startled at depth, it makes your heart rate jump and you automatically consume more oxygen. The Zen-like state is broken and the brain becomes more active,” Ong said.

The same thing happens to a marine mammal.

“An acoustic stress makes the animal frightened, changes the heart rate and chemical levels such as adrenaline that change the dilation of blood vessels,” Moore said.

Increased blood flow means more nitrogen passing into the bloodstream from the lungs. That could explain dolphin and whale deaths around Navy testing, the study concluded.

“Certainly from this study it is pretty obvious these animals are very delicately balanced,” Moore said. “Any stress like seismic testing (for oil and gas reserves) or sonar especially runs the risk of unbalancing that dive.”

A spokesperson for the U.S. Navy, which funded the research, wrote in an email that the agency is trying to balance protection of environmental resources and marine animals with the need to test technology and train personnel. Funding research at major marine science institutions like WHOI, Scripps Institution of Oceanography, Cornell University and other private and academic institutions informs the protections they employ. It also works with the National Marine Fisheries Service and the U.S. Fish and Wildlife Service to develop protocols and tools to minimize effects.

— Follow Doug Fraser on Twitter:@dougfrasercct.