Why do we refer to people as guinea pigs when discussing the subjects of an informal experiment? Surely mice, rabbits and rats are much more common experimental subjects. Indeed, it's rare that you'll hear of real guinea pigs being used experimentally.
Guinea pigs might not be the most common experimental subjects today, but we have to look at how common they were when the term was first coined, which was many decades ago, as Etymonline points out:
Wikipedia seems to be backing that up:
Looking further, I found this essay on the History of the Guinea Pig:
Emil Adolf von Behring actually won the (first ever) Nobel Prize in Physiology or Medicine in 1901 for developing a serum therapy against diphtheria. He used guinea pigs in his experiments, though later the serum was extracted from sheep and then horses.
I do not know much about biology, but Wikipedia states that guinea pigs have been often used in scientific experiments since the 17th century or before. It also explains that now they are not used as often as before:
Linguistically speaking, it was because the metaphoric meaning for "rat" (despicable person) was already taken, while "guinea pig" was wide open for having a metaphoric meaning attached to it.
(And of course it was also because they were used in experimentation. But so were many other species.)
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