Answer: Frogs

Frogs aren’t exactly the most convenient pregnancy test in the world, but they are quite accurate. A series of experiments in the 1930s revealed that female frogs and toads reacted to injections of human urine with human chorionic gonadotropin hormone present (the same hormone detected by modern pregnancy dip-stick style chemical tests and blood tests) by immediately producing eggs within the next 24 hours.

This discovery didn’t languish on the shelf as a novel science experiment destined to be included in a future trivia column, but was put to heavy use. Prior to the use of frogs as pregnancy detectors, doctors used rabbits and other small animals. The problem with the mammal-based system, however, was that you could only detect pregnancy by dissecting the animal and examining its organs. This was costly and inconvenient which made widespread testing impractical. The frog-based method was cheap, easy to maintain, and the frogs could be used repeatedly for the testing procedures.

Eventually the frogs were replaced by the modern methods we’re familiar with (at home and in-lab dip sticks to check the urine and simple blood tests). This replacement process is exactly why you can find large populations of the most commonly used frog (the African clawed frog) in urban centers around the world; many of the hundreds of hospitals and clinics that used the frogs simply released them into the wild when their services were no longer needed.