A frog as a pregnancy test

The model animal I work with is the tadpole of Xenopus laevis, the South African Clawed frog. It has been a model system for developmental biology for a long time, so let’s look at the development:

Credit: BBC iWonder

Though most developmental biologists work with developmental stages that are even smaller and younger than what is shown above.

When I started working with these tadpoles, I wanted to learn about their ecology: where/how do they normally live? It turns out, there is little to no literature about this (please let me know if you know any!), despite the ‘model animal’ status that it’s had for at least 80 years. There is one thing though, that I came across in my search, that I found very strange and that I want to share here: Female Xenopus laevis have been used for pregnancy testing for quite a while during the last century. This is how it works (for instance described in this paper by Polack from 1949): you collect urine from the woman who might be pregnant, and then you inject that urine into a female frog. If she (the frog!) spawns within 2 to 12 hours, that tells you that there were gonadotropic hormones in the urine, meaning the women is pregnant.

Now, this seems all very reasonable for times when there were no other and easier pregnancy tests around. However, what I have always wondered: Who came up with the idea to inject human urine into a frog? I read around a bit, and of course there is some context – this did not just come out of the blue. Before frogs, pregnancy tests were based on rabbits or mice, also by injecting female human urine. However, those animals had to be killed and dissected to look at their ovaries, so an animal that indicates the presence of the hormone in question without killing it was much preferable. This is basically how Xenopus spread around the world – and probably it spread into the wild after new tests made the use of animals obsolete. And now they are pests and displace the native amphibians because they are more resistant to chytridiomycosis fungus… but that is a story for another blog post.

Sources/extra reading: A Slate article by Daniel Engber, an encyclopaedia britannica blog, and a report on what it was like to work as a ‘urine injector’ in the 1950s.

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