Freaky Frogs

African Clawed Frog

African clawed frog
Image Source: Brian Gratwicke

From the 1930s through to the 1960s, a small aquatic frog was used as a biological pregnancy test. This may sound suspiciously like one of those medieval folk remedies – in the same vein of shaving the rear end of a live chicken and holding it under your armpit to cure the plague – but this frog really could tell if you were pregnant or not.

In truth, frogs weren’t the first animals to be used as a reliable indicator of pregnancy. One technique, developed in the late 1920s, involved injecting sexually immature female mice with a woman’s urine several times over a number of days. The mice would then be killed and their ovaries examined. Quite simply, if their ovaries were enlarged, the woman was pregnant. Of course, this method was rather cruel to the mice and, more importantly to the scientists at least, it was also quite costly because new animals had to be used every time.

But a few years later, the technique was refined 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 (though there are exceptions; see here), which means they didn’t have to be cut open and killed for the scientists to examine them. In other words, they were reusable. Using this updated, faster and simpler method, a woman’s urine was injected into the back of a female frog, which would lay eggs within 12 hours if the woman in question was pregnant.

The frog most commonly used as a living pregnancy test was the African clawed frog. This unusual-looking amphibian is lacking quite a few body parts – it has no eyelids, tongue, teeth or ears – although, as its name suggests, it does possess (uniquely among frogs) small claws, located on its hind feet, which it uses to tear large items of food into more manageable chunks [1]. This frog is not a fussy eater; it will try to consume pretty much anything that will fit into its mouth, including its own tadpoles. However, its lack of tongue means it must grab food using its front feet and shovel it into its mouth, rather like the Surinam toad does.

For a few decades, injecting urine into frogs was the only truly reliable way of knowing whether you were pregnant or not, and so, understandably, there was a very demand for this species. Hundreds of thousands of frogs were exported from their native range in Africa to hospitals and testing labs all around the world, destined to be infused with urine.

But although African clawed frogs revolutionised pregnancy testing, they were eventually superseded by more modern techniques. In the 1960s, newer chemical pregnancy tests were developed that searched directly for the hormone that the amphibians were reacting to – human chorionic gonadotrophin, or hCG, which is released after a human egg is fertilised – making the frogs largely redundant in this respect. However, because they were long-lived, resistant to disease, and could lay eggs on command simply by injecting them with hCG, these frogs were used in many other lines of research – to study how cells work and how embryos develop, for example. The African clawed frog therefore became one of the most intensively studied organisms on the planet, and, with live colonies spread across five continents, the world’s most widely distributed amphibian.

Albino African clawed frog
Albino African clawed frogs are very common and are often sold as pets or for laboratories. This species was the first vertebrate to be cloned, in 1962.
Image Source: kuribo

Even so, laboratories in some places had a greatly reduced need for this species, and they sometimes simply released their colonies of clawed frogs into the wild, outside their native range. Capable of thriving in a variety of conditions and environments, and being able to eat pretty much anything, these adaptable amphibians dispersed to new habitats and flourished. They are now considered pests in many countries around the world.

But the African clawed frogs did more than merely annoy us – they also had a massively detrimental effect on other frogs. In the 1980s, herpetologists started noticing that entire populations of amphibians, in many different parts of the world, were disappearing, often quite suddenly. What made this situation even more mystifying was that frogs were vanishing not only from populated and disturbed areas but also from remote, relatively pristine places, too.

The culprit of this catastrophic decline was eventually identified as a type of waterborne chytrid fungus called Batrachochytrium dendrobatidis. We have already seen on this blog that amphibians have thin, permeable skin that they use to partially or even wholly breathe through (some salamanders and at least one small species of frog, the Bornean flat-headed frog, are completely lungless). Batrachochytrium infects and degrades the skin of amphibians, causing a disease that prevents the absorption of oxygen and essential electrolytes, often triggering a death spiral that ends in cardiac arrest.

In all likeliness, the fungus has existed for a long time, but its recent global spread is thought to be linked to the international trade of the African clawed frog. Indeed, recent research has linked their arrival in several locations with the appearance of the chytrid fungus and the disappearance of native frogs. This species appears to have an immunity to the fungus and is able to resist its suffocating, skin-eating effects, but when it is released into foreign lands en masse, it can quickly transmit the disease to other, more susceptible amphibians.

Chytrid has caused an amphibian apocalypse. Around a third of all frog species (including Darwin’s frog and the ‘scrotum frog’ of Lake Titicaca) are currently threatened due to the fungus, while many others, such as the gastric-brooding frogs, have already disappeared completely. In our final Freaky Frogs article (for now at least), we’ll be looking further at how frog populations are being decimated by the chytrid fungus, climate change and other threats, and what is being done to try and save them.

[1] The hairy frog may appear to have claws, but they are actually made of bone that the amphibian forces through the skin of its toe pads to defend itself.


  • Jason Woodcock

    With a background in conservation and animal behaviour studies, Jason's passion lies in the natural world. He adores all things nature and enjoys nothing more than spotting rare and interesting species out in the wild. He has also worked in a zoo and knows plenty about keeping the animals inside our homes healthy and happy, too.

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