Wanted: African Clawed Frog
Famous for: Being a living pregnancy test
Crime: Accidentally decimating the world’s amphibian population
The earliest reliable pregnancy test, developed in 1928, involved injecting mice with a woman’s urine several times over a number of days. The mice would then be killed and their ovaries examined; if the ovaries were enlarged, the woman was pregnant. But this method was rather cruel to the mice and, more importantly to the scientists at least, it was also very costly because new animals had to be used every time.
Then, in the 1930s, scientists managed to refine this technique so that the animal wasn’t killed in the process. This time frogs were used. Frogs usually keep their eggs on the outside of their bodies, which means they didn’t have to be cut open and killed. They were reusable. Using this updated technique, a woman’s urine was injected into the back of a female frog and the frog would lay eggs within twelve hours if she was pregnant.
The frog most commonly used as a biological pregnancy test was the African clawed frog, an unusual-looking flattened frog with no eyelids, tongue, teeth or ears, although as its name suggests it possesses (uniquely among amphibians) small claws. For a few decades, injecting urine into frogs was the only truly reliable way of knowing whether you were pregnant or not, and so during this period there was a very high demand for this species. The African clawed frog was imported from its native range in Africa to many other places around the world in large numbers, since it was much easier than farming them.
In the 1960s, though, newer and easier pregnancy tests were developed and so the African clawed frog became redundant. Laboratories that held large numbers of this species but no more use for them simply released them into the wild. They flourished in their new environments and are now considered pests in many countries around the world.
But the African clawed frogs did more than merely annoy the human population – they also had a massively detrimental effect on other amphibians. Recently, it has become known that a type of chytrid fungus has been decimating amphibian populations worldwide. It lives on the skin of amphibians and, since they often partially or even wholly breathe through their skin, the amphibians find themselves struggling to exchange essential gases. The fungus also produces lethal toxins and prevents the frogs from taking in water through their skin, resulting in dehydration.
Though the fungus has probably existed for some time, it is thought that its recent global spread is linked to the trade in the African clawed frog. The African clawed frog can carry the fungus on its skin but suffer no ill-effects from it. When it is released into foreign lands en masse, it can quickly transmit the chytrid fungus to other, more susceptible amphibians – usually with catastrophic results. Around a third of all frog species are currently threatened due to the chytrid fungus and many species have already disappeared completely. It is predicted that virtually all amphibian species will be considered endangered within a hundred years if this crisis is not averted.