When it comes to extracting oxygen from the air we breathe, we humans are just OK.
Birds are more efficient breathers than we are. So are alligators and, according to a new study, so are monitor lizards.
Humans are what are called tidal breathers. When we breathe in, fresh air moves into our lungs along progressively smaller airways, eventually ending in little sacs called alveoli, where our bloodstream picks up oxygen and deposits carbon dioxide. Then the old air moves out of our lungs along the same path it came in.
But birds, alligators and monitor lizards are unidirectional breathers. After the air moves into their lungs, it begins to follow a system of tubes similar to arteries, capillaries and veins. In this system, the air moves through the air tubes in only one direction.
And, it turns out, their system is more efficient at extracting oxygen from the air than ours is.
Scientists discovered that birds are unidirectional breathers in the first half of the 20th century, after researchers noticed that pigeons breathing the sooty air of train stations showed just one black area on the lung. If the pigeons breathed like we do, scientists would have expected the entire lung to be black.
It isnt that surprising that birds have developed a more efficient breathing system. Scientists hypothesize it may have evolved to help them support their high metabolic rates, or to help them survive when they fly at high altitudes, where oxygen is scarce.
But in 2010, Colleen Farmer, a biologist at the University of Utah, published a study showing that alligators are unidirectional breathers as well. I knew cold-blooded animals spend about 80 percent of their lives holding their breaths, she said, and so I formulated the hypothesis that this breathing would be important for mixing gases in the lungs during a breath-hold.
This month, she published a study in the journal Nature that shows monitor lizards are unidirectional breathers as well. She believes that further studies will show that all lizards and snakes are also unidirectional breathers.
(Unidirectional breathing) appears to be much more common and ancient than anyone thought, she said.