Have you ever smelled Carbon dioxide? Can you sense the odour of Carbon dioxide? But these mice can.
Specialized neurons in the nose of mice help them to smell carbon dioxide at levels just higher than that in normal air. Insects, too, can detect carbon dioxide, but they do it via membrane receptors rather than through any kind of nose.
The neurons that mice use to detect carbon dioxide have been tracked down by Minmin Luo at the National Institute of Biological Sciences, Beijing, and his colleagues. They report, the level of carbon dioxide above which the mice smelled the gas, was just 0.066% — about twice the average level of carbon dioxide in the atmosphere (0.038%), but much less than the concentration in exhaled breath (about 4.5%) or the level considered safe for humans (0.5%).
The team targeted neurons in the mouse nose that were already known to express the carbon dioxide -processing enzyme carbonic anhydrase type II (CAII). These cells, called guanylyl cyclase D cells, glowed in the presence of carbon dioxide, showing when mice were picking up the scent.
Luo’s research shows that, to his surprise, in mammals this isn’t the case — the mice literally smell the gas. “We did not expect it at all,” he says. “Most people don’t think carbon dioxide is an odorant. It is used as an irritant, not an olfactory cue.”
When mice were exposed to more and more carbon dioxide, their behaviour changed: given a choice between areas of high and low carbon dioxide concentration, the mice avoided anything higher than 0.2% carbon dioxide.
This could mean that as climate change causes atmospheric carbon dioxide levels to rise (predicted to be 0.05-0.1% by 2100), changes in mouse behaviour might be spotted. “There will be some behavioural effect,” says Luo; but what that effect will be is not known.
Luo’s colleague Peter Mombaerts from Rockefeller University, New York says that if the CO2 increases are gradual, the mice might be able to adapt. "We do that too," he explains, likening it to the way the initial stench encountered on getting into a New York taxi tends to gradually fade. The alternative is a scenario in which mice get more fearful or aggressive as carbon dioxide levels rise, he says.
Despite their sensitivity, mice aren’t going to become modern-day canaries for elevated carbon dioxide. For that to work, mice would have to be genetically engineered so that they don’t respond to any other smells, says Mombaerts. Not a very practical solution — particularly when mechanical sensors already exis
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