Sentinel species are animals, used to detect risks to humans by providing advance warning of a danger. In Japan they use cats to test for mercury-contaminated fish; honey bees are super susceptible to air pollution; and, due to its diet of insects, a sick bat is a very good indicator of pesticide contamination.

But, the classic example of this genre, as evidenced by its over use as metaphor, is that of the “canary in the coal mine”.

The idea of using canaries in coal mines to detect carbon monoxide and other toxic gases before they hurt humans, is credited to Scottish physiologist John Scott Haldane (1860-1936).

Canaries, like other birds, are good early detectors of carbon monoxide because they’re vulnerable to airborne poisons. Because they need such immense quantities of oxygen to enable them to fly, their anatomy allows them to get a dose of oxygen when they inhale and another when they exhale, by holding air in extra sacs. Also, a canary's heartbeat is up to 100 times faster than a human being’s, so the carbon monoxide is attracted quicker into the haemoglobin in the bloodstream.

When affected by the noxious gas, a canary would stop chirping and become agitated, and of course the longer it was in that situation, the more agitated it would become until finally dropping off its perch. This action would give the miners enough time to retreat to fresh air before succumbing themselves. [As an aside, the idea to use mice was once considered but dismissed. A dying mouse curled up in the corner of a cage (that would be their reaction) being way less obvious than a bright yellow bird falling off a perch.]

In 1986 canaries in coal mines were thankfully replaced by electronic detectors. Phew.

Although ending the use of the birds to detect deadly gas was more humane, miners’ feelings were actually mixed. They had become so ingrained in the culture. Miners were treating them as pets.

Perhaps the most interesting thing we found when doing the research for this post was an extraordinary cage designed to resuscitate the unlucky canary after exposure to unfriendly gases. Closing the door, and turning on the oxygen, if done quickly enough, would have the bird chirping again.
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Canary in the Coal Mine exists in printed form as chapter 10 of RR#1 … available to order HERE
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References

Wikipedia: Sentinel species | (Text) CC BY-SA
Smithsonian Magazine, 16 December 2016 “The Story of the Real Canary in the Coal Mine”

Images

1. Miner with Canary. Hollinger Mine, Ontario. Photo Credit: George S. McCaa
2. Domestic Canary. Serinus canaria forma domestica
3. Avian Lungs. Photo Credit: C. Abraczinskas via Wikimedia Commons
4. Philip Healey and Tommy Rainbird testing for gas with a canary, 1950s
5. Cage for Reviving Canary with Oxygen Cylinder by Siebe Gorman & Co. Ltd, London
6. Resuscitation Cage for Mine Canaries
7. In Memory of Little Joe. Died November 3rd 1875, aged 3 Years. Photo Credit: British Antique Dealers Association