Canary in the coal mine

Sentinel species are organisms, often animals, used to detect risks to humans by providing advance warning of a danger. The terms primarily apply in the context of environmental hazards rather than those from other sources. Some animals can act as sentinels because they may be more susceptible or have greater exposure to a particular hazard than humans in the same environment. People have long observed animals for signs of impending hazards or evidence of environmental threats. Plants and other living organisms have also been used for these purposes.

There are countless examples of environmental effects on animals that later manifested in humans. The classic example is the "canary in the coal mine". The idea of placing a canary or other warm blooded animal in a mine to detect carbon monoxide was first proposed by John Scott Haldane, in 1913 or later. Well into the 20th century, coal miners brought canaries into coal mines as an early-warning signal for toxic gases, primarily carbon monoxide. The birds, being more sensitive, would become sick before the miners, who would then have a chance to escape or put on protective respirators.

In Minamata Bay, Japan, cats developed "dancing cat fever" before humans were affected due to eating mercury-contaminated fish. Dogs were recognized as early as 1939 to be more susceptible to tonsil cancer if they were kept in crowded urban environments. Studies similarly found higher disease rates in animals exposed to tobacco smoke.Yushō disease was similarly discovered when poultry began dying at alarming rates due to polychlorinated biphenyl poisoning, although not before approximately 14,000 people were affected.

Animal sentinels must have measurable responses to the hazard in question, whether that is due to the animal's death, disappearance, or some other determinable aspect. Many of these species are ideally unendangered and easy to handle. It is important that the species' range overlap with the range being studied. Often the ideal species is determined by the characteristics of the hazard.

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