Elephant seals are some of the world’s most elite divers, plunging to depths of 800 meters and holding their breaths underwater for nearly half an hour at a time. They also have as much carbon monoxide in their blood as a two-pack-a-day smoker, a new study shows. The poison even appears to offer a protective effect for them. 

 

Carbon monoxide (CO) is known as the “silent killer” because it’s a colorless, odorless gas that binds to hemoglobin, especially in the blood of smokers and firefighters. It clogs up the oxygen-carrying protein, prevents oxygen transport to tissues, and eventually causes light-headedness or even suffocation. Our bodies do naturally produce CO when heme-proteins are broken down; about 1 percent of the hemoglobin in non-smokers is bound by CO. Previous work has shown that low concentrations can be beneficial: reducing inflammatory responses and cell death caused by heart attacks and strokes. 

 

The northern elephant seal (Mirounga angustirostris) has the largest blood volumes of any mammals. To conserve oxygen for vital organs like the heart, brain, and lungs during long dives, their bodies can shut off blood flow to nonessential areas of their body, without damaging the tissues. “These animals are constantly holding their breath,” Michael Tift of Scripps Institution of Oceanography at University of California, San Diego, tells Nature. “But they don’t have any injuries.”

 

Tift and colleagues (pictured below) wanted to see how much CO elephant seals carry. So the team mildly sedated 24 elephant seals ranging from pups to the easily recognizable adult males at Año Nuevo State Park in California and collected blood samples. Then they used a blood gas analyzer to measure the proportion of hemoglobin incapacitated by CO -- called carboxyhemoglobin (COHb) -- relative to oxygen and carbon dioxide. 

 

The animals' COHb levels were 10.4 percent of their total hemoglobin concentration. These are the values found in people who smoke up to 40 cigarettes a day, Tift says. And COHb levels increased as the animals aged. While high, their levels didn't lead to carbon monoxide poisoning; the gas becomes deadly when it incapacitates more than 50 percent of hemoglobin stores, Tift explains to LiveScience.

 

So why that high? There’s only one way for an animal to clear CO from its body: Exhale it. Elephant seals probably accumulate so much COHb because they hold their breath for about 75 percent of their lives. “If they are at sea, they are constantly diving; if they are on land, they are going into sleep apneas [breath holds]. Since they are producing this stuff, they may be producing it at a similar rate to humans, but they may not be getting rid of it at the same rate,” Tift says. Alternatively, he suggests that the seals may simply produce more CO than other species because they break down heme-proteins faster, turning over more of the molecules.

 

While these COHb levels might reduce the seals' dive limit, the constant presence of elevated CO in the blood may protect them against damaging inflammation incurred when the blood rushes back into tissues after their extreme breath-holds. 

 

In humans, blood flow is interrupted during organ transplants, strokes, and heart attacks; when blood suddenly flows back into the tissues, chemical reactions can lead to inflammation and cell death. These are called ischemia-reperfusion injuries, and they don’t seem to happen to seals. “These results are helping us find answers for the rates at which you can expose organs and tissues to this gas,” Tift says in a news release. “The elephant seal is giving us the big picture of which concentrations of carbon monoxide might be the most beneficial.” 

 

The work was presented at Experimental Biology 2014 in San Diego last month and published in the Journal of Experimental Biology last week. 

 

[Inside JEB, Scripps via Nature, Science]