Secrets of the world’s extreme divers

This article was originally published on February 24, 2011.

Near the edge of Antarctica live some of the world’s best divers. They are black and white and zip through the ocean like bullets. They can stay underwater for more than 20 minutes without surfacing for air.

Emperor penguins are among the animals living in the Antarctic and elsewhere that are extreme divers. Some of these animals can hold their breath even longer, and continue to perform strenuous work even after the oxygen levels in their bodies are low enough to render a human diver unconscious.

“These animals do things that it’s still impossible for us to do except with really major mechanical help” — like small submarines, says Jerry Kooyman, a marine biologist at the Scripps Institution of Oceanography in La Jolla, Calif.

Researchers are probing how emperor penguins and other animals perform breath-defying feats. And the answers might eventually prove useful in medicine. When a person has a stroke or heart attack, the cells in the body don’t get enough oxygen (the person has a low oxygen level). If scientists could figure out what allows some animals to not only survive but also thrive in such tough conditions, that knowledge could show how to help human patients handle low oxygen levels better.

Underwater supermarket

If they were forced to stay at the surface, emperor penguins would simply go hungry. Such animals must dive to reach their food, which lies deep below the ocean’s surface.

To think about how that feels, imagine arriving hungry outside the doors of a supermarket. “I give you a basket, and I say you can go in and put anything in this basket that you want,” says Randall Davis, a marine biologist at Texas A&M University at Galveston. “But as soon as you go through those doors, you have to hold your breath.” And you can’t take another until you come back out into the parking lot.

Humans can’t last long on one breath. We wouldn’t get much food. Without enough oxygen, we quickly go unconscious. But diving animals have ways of making one breath really last.

As most birds and mammals inhale, oxygen enters the lungs and then moves on into the blood. A kind of molecule in the blood called hemoglobin sticks to oxygen, ferrying it — like a boat carrying a passenger — to different parts of the body. Another molecule called myoglobin stores oxygen in muscle. Oxygen is important because it helps cells get energy from food. This chemical reaction makes water, carbon dioxide and ATP, a molecule that contains energy and powers the body’s activity. The carbon dioxide leaves the body when the animals exhale.

Diving animals have higher concentrations of hemoglobin and myoglobin in their bodies than humans do, so these animals can store more oxygen. For example, emperor penguins, which are birds, have hemoglobin levels that are about 25 percent higher than a human’s, and myoglobin levels that are about 16 to 20 times higher. In a sense, “they have an internal scuba tank,” says Daniel Costa, a marine biologist at the University of California, Santa Cruz. Some diving animals also have more blood per kilogram of body weight, which increases the oxygen load they can carry.

The penguin corral

As a kid, Jessica Meir was interested in biology and space travel. When she grew up, she helped manage experiments at NASA to test what happens to the human body in outer space. Lately, she’s turned her attentions to studying how birds and marine mammals cope with similarly extreme environments. An extreme environment is one that is harsher than a normal environment. For example, it might be very hot, very cold or have very little oxygen.

Lots of animals dive, but Meir was interested in “animals that really pushed it the furthest,” she says. So Meir joined researchers at the Scripps Institution of Oceanography in studying emperor penguins.

The scientists set up a field camp in Antarctica with wooden huts for people to live in. Called Penguin Ranch, it sat on a sheet of ice covering a huge expanse of ocean. Emperor penguins were brought to the ranch by helicopter or snowmobile and then set loose in a corral with fences on the ice around holes in the ice. In the wild, penguins usually dive off the edge of the ice into the ocean. At the ranch, they dove through the holes and swam under the ice. But they could still dive freely and look for food in the ocean, as they normally do.

Meir and her colleagues attached heart-rate recorders to the penguins and let them dive. The recorders showed how quickly the animals’ hearts had been beating during a dive. And the heart rates slowed a lot from when the penguins had been resting on land. At its slowest, one penguin’s heart beat only three times per minute! By contrast, 20 to 30 beats a minute would be considered very slow for a human.

A low heart rate means blood flow has slowed; when blood moves throughout the body more slowly, an animal’s supply of oxygen (carried by the hemoglobin molecules) lasts longer. The penguins’ heart rates got back up to speed by the time they returned to the surface. And right after a dive, their hearts beat much faster than normal so the animals could load up on oxygen and get rid of carbon dioxide.

Extra help under the hood

Meir’s team has also worked with elephant seals in California. At sea, these animals are constantly diving. Scientists recorded one elephant seal on a sustained dive that kept it underwater for two solid hours!

Meir and her colleagues wondered how the seals conserved oxygen for so long. With Costa’s team, the scientists attached oxygen monitors to the seals and let the animals dive in the ocean. Even after using up almost all of the oxygen in their blood, some seals were still active.

Humans would pass out if they had such low oxygen levels, says Lars Folkow, an animal physiologist at the University of Tromsø in Norway. Our brains need oxygen to keep working. To investigate why the same low levels don’t make marine mammals such as hooded seals pass out, Folkow’s team has been studying hooded seals that dive off the coast of Greenland.

These seals have a hard life. “It’s cold and wet and dark and pretty miserable,” observes Arnoldus Blix, an Arctic biologist also at the University of Tromsø. “They are really tough animals, that’s for sure.”

Blix and Folkow’s team looked at neurons, cells that are in the brain. The team compared neurons from hooded seals to neurons from adult mice. When the researchers decreased how much oxygen the neurons received, seal neurons remained active longer than did mouse neurons. Seal neurons “endured the lack of oxygen much better,” says Folkow.

The scientists think a molecule called neuroglobin might be important. Similar to hemoglobin and myoglobin, neuroglobin holds on to oxygen. Scientists don’t know neuroglobin’s exact function, but the molecule might help move oxygen into cells, says Folkow. In hooded seals, most neuroglobin resides in a different type of brain cell than it does in mice and rats. But the researchers aren’t yet sure why that would necessarily help seal brains remain active during diving.

Really high fliers

Now Meir has turned to yet another animal that copes with low oxygen supplies: bar-headed geese. These birds fly over the Himalayas. People have observed some bar-headed geese cruising over the Himalayas’ Mount Everest which, at about 8,800 meters or 29,000 feet, is considered the world’s highest peak. Clearly, there isn’t much oxygen up there.

Meir keeps a flock of bar-headed geese in her lab. She was the first thing the geese saw upon hatching, so the birds are very attached to her. When she rides a bike or scooter, the geese fly after her. “They think I’m their mom,” says Meir, who is now working at the University of British Columbia in Vancouver, Canada.

Meir is going to test how the geese respond to low oxygen. She will put them in a tunnel with wind blowing through it. The wind tunnel is like a treadmill because the birds have to fly to stay in place. Each bird will wear a mask with a tube for its oxygen supply. Throughout each bird’s flight, Meir will control the amount of oxygen going into the tube. As the bird flies, Meir will monitor oxygen levels in the animal’s blood and exhaled breath, and will also monitor the animal’s heart rate.

Mysterious bubbles

There remains plenty more that scientists don’t understand about diving animals. For example, those that dive very deep are under huge amounts of pressure from the weight of the water above. Humans would go into convulsions under this much pressure, but the animals don’t seem to mind.

Scientists also aren’t sure how diving animals deal with a gas called nitrogen. When humans dive deep using scuba gear and later swim too quickly back to the surface, bubbles of nitrogen can form in their tissues, the groups of cells that make up different parts of the body. These bubbles are not only painful but also dangerous because they can damage tissue or block blood flow. Recently, whales found dead have been shown to have such bubbles in their organs. But scientists aren’t sure exactly what’s causing the bubbles, how often diving animals get them or how harmful the bubbles really are. For example, it’s possible that some animals get bubbles but aren’t bothered by them, says Costa. Maybe the bubbles cause problems only when the animals push themselves too hard during their dives.

Costa and his colleagues are planning to try and answer these questions by studying elephant seals. The team will glue devices to the animals that record pictures of the seals’ insides during each dive. Then the researchers will scan the images for bubbles. They will also measure how much nitrogen is in the seals’ tissues.

One thing is for sure: These animals are way better at diving than we are. “They do everything that we do, except do it better,” says Costa.