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Clawed Frogs: Nature’s Pregnancy Test

on August 26, 2014

Mating and breeding are fundamental aspects of surviving as a species and although many animals have breeding seasons, humans as a species, do not. This means that women don’t have specific times during the year in which they are more fertile and willing to engage in mating in order to get pregnant. Instead, we go through monthly fertile cycles between the first menstruation (menarche) and menopause. This is good on one hand because it is not necessary to wait for a specific month or months of the year in order to get pregnant but also bad because, as it can occur at any given time, women might fail to notice they are pregnant and not take basic pre-natal care to ensure the survival and best development of the baby.


Old school pregnancy tests

For many reasons it quickly became important to find out if a woman was pregnant or not as soon as possible. Was the queen finally going to provide an heir? Was the future virgin bride not so virgin after all? Was the husband that had just come back from the sea really the father? Being able to tell, with a certain accuracy, if a woman was pregnant or not would solve many problems but we still have no idea how our (very ancient) ancestors were doing it (if they were at all) since no information was ever found. We are aware however that the first known pregnancy test started to be used in the ancient Egyptian and Greek cultures with the use of urine and grains. They would introduce, on one side, urine of the lady that wanted to find out if she was pregnant, and on the other side (as the control subject) the urine of a priest, on bags of barley and wheat. Both bags would be observed for how long germination would occur. If a lady was with child her grains would sprout a lot faster than the priest (we could easily infer he would not be pregnant).

Throughout history many other “urine analysis” techniques were used to find out if women were pregnant, some of them more scientific than others; but it wasn’t until the last century that animals started to be used in the procedure.

With the discovery that the hCG (human chorionic gonadotropin) hormone was produced by the placenta during the first trimester of pregnancy, scientists were able to design a better and more reliable way to test for pregnancy.

This discovery, in the 1930’s, boosted the development of pregnancy test worldwide with very big implications. For the first time in known history, scientists were using live animals such as mice and even rabbits for their testing. Infantile females (non-sexually mature) were injected with the urine of the woman to be tested and later on killed and dissected in order to look for the presence of ovulation in the animal. The ovulation would have been triggered by the hCG presence in the injected urine of pregnant women and would pose as a positive result.

This method, however, meant that for every tested woman we would end up with a dead mouse or rabbit. Luckily for humanity shortly after, still in the 1930’s, a gentleman named Lancelot Hogben found out that he could use a specific genus of frogs, the Xenopus, to get the same results but without having to kill the animal.


Xenopus: 1930’s-1950’s pregnancy test

Clawed frogs (Xenopus) were subjected to the same method; the urine of the woman was injected in its dorsal lymph sac and if the frog produced eggs in the first 12 to 24 hours the woman was with child. The big difference between mice and rabbits and these frog species is that amphibians, unlike mammals, have external fertilization so the new eggs could be easily observed without having to kill and dissect the animal.

By the 1940’s, this test, named the “Hogben Test” in honour of its discoverer, was already widely used in hospitals.

Many facts helped this method achieve worldwide recognition:

  • It was extremely accurate
  • It was very easy to perform
  • Results were achieved in 12-24 hours
  • The frogs were easily bred
  • The frogs were conveniently kept in aquariums
  • Amphibians in general have large eggs which can be easily examined and manipulated
  • Eggs are released outside the animal (they use external fertilization)
  • Clawed frogs reacted to the hCG hormone released by pregnant women
  • Since the frogs didn’t have to be killed and dissected in order to have their eggs examined, they could be used multiple times
African Clawed Frog (Xenopus laevis)

African Clawed Frog (Xenopus laevis)
Photo Credit


Pregnancy tests post-Xenopus

Even though there were many advantages to using this African species in laboratories all over the world, there were a few cons that ultimately made the test obsolete.

  • The clawed frogs (Xenopus) had to be imported from Africa in large numbers
  • Live animals need to be housed, fed and taken care of
  • One disease outbreak in a lab with frogs would be enough to close it down


Eventually science evolved enough that by the 1960’s the Hogben test became obsolete and today we can buy a small, cheap and easy to use pregnancy test that will give a result within minutes and without going through several technicians, scientists and doctors. The same result can be achieved in private and in minutes.

Unfortunately, all the Xenopus testing for decades came with a price. In 2006 researchers found out that this genus might have been the carriers of the (in)famous chytrid fungus, a deadly amphibian fungus that caused the mass extinction and population decline of almost 200 amphibian species all around the world. African clawed frogs are known to be one of the only species of amphibians in the world to be not only immune but also a carrier of this deadly fungus and several decades ago we shipped them all over the world..

There is no treatment for this fungus and the only way to control this mass extinction is through quarantine.A staggering 30% of known amphibian species are currently threatened with extinction  and about 500 amphibian species are so threatened at the moment that no human effort will be fast enough in order to stop its extinction. Projects like the Amphibian Ark are doing their best to help the remaining species we have surviving these dark times. If you can, visit their “How can I help?” page. There are many things all of us can do to help out, either by donating time, money or skills.

Small gestures can make a huge impact on these small creatures.


Até à próxima.


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