Why did hospitals have these frogs in their supply rooms? - Carly Anne York
From the 1930s well into the 1960s,
pregnancy testing required a slippery
piece of equipment:
a female African clawed frog.
For decades, hospitals and research labs
around the world
had a trusted supply of these
handy amphibians,
employing their help
in testing for pregnancy
and in numerous other
scientific endeavors.
So, what makes these phenomenal frogs
so special?
The remarkable features
of the African clawed frog
have fascinated researchers
since the 19th century.
We often think of frogs as hopping
around to snatch up bugs
with their lengthy tongues.
But this water-dwelling species
is almost exclusively aquatic,
having adapted primarily for swimming
rather than crawling or jumping.
Weirder still, African clawed frogs
have no tongues at all.
Instead, they suck food directly
into their mouths
or use their hands to catch larger animals
which they tear apart
with powerful hind claws.
To help get close to their prey,
their smooth skin can quickly change color
to blend into their surroundings.
This camouflage ability was particularly
interesting to a trio of biologists
in South Africa during the 1920s.
Hillel Shapiro, Harry Zwarenstein,
and their teacher, Lancelot Hogben,
were researching what role
the pituitary gland,
a small region of the frog’s brain,
might play in the
color changing mechanism.
Removing the gland altogether impaired
the frogs’ ability to camouflage.
But when Hogben injected them with
a pituitary extract derived from an ox,
he found an even more surprising result—
the frogs began laying eggs.
Normally, African clawed frogs
only release eggs
when a male frog is
nearby to fertilize them.
But this ox hormone triggered their
ovulation without the presence of a male.
And this process gave
the researchers an idea.
They knew there was a chemical similarity
between the ox hormone
and a hormone found in the urine
of pregnant people—
a compound we know today as the
chorionic gonadotropin hormone, or hCG.
In pregnancy tests of the day,
animals like mice and rabbits were
injected with human urine
as a way of testing
for the presence of hCG.
However, this required multiple
days of injections,
as well as killing and dissecting
the animals
to see whether the hormone was present.
Because of these hurdles, the test was
reserved for specific use cases,
leaving most people waiting for visible
signs to determine if they were pregnant.
But these frogs changed everything.
They responded to hCG in roughly 9 hours
with no need for dissection.
And since their large ovaries
constantly generate eggs,
they could participate in numerous tests
over their 15 to 30 year lifespan.
Better still, these frogs were abundant
in their native habitat,
making them easy to find,
catch, and export.
Before long, tens of thousands
of African clawed frogs
were being shipped to hospitals
all over the world,
making reliable pregnancy tests
widely available for the first time.
In the 1970s, the frogs would be
superseded by chemical tests
that detected hCG directly.
But by then their widespread presence
in laboratories
had made African clawed frogs
the star of numerous studies.
Their rapid development and ability
to produce eggs in any season
made them invaluable models
for developmental biology.
African clawed frogs have even been
part of biological studies
aboard the Space Shuttle Endeavour.
However,
frogs can be slippery subjects.
More than a few of these amphibians have
escaped their laboratory enclosures,
and in many places,
their ferocious appetite allowed them
to outcompete native amphibians.
To make matters worse,
they often carry a deadly
fungus called chytrid,
which can cause an infection
that lethally disrupts
the delicate functioning
of amphibian skin.
In the 20th century, this fungal infection
has devastated amphibian populations
around the globe,
causing the extinction
of multiple frog species.
Because of all these concerns,
the African clawed frog is now classified
as an invasive species,
and illegal in many regions.
So in the end, like other pregnancy tests,
the use of these frogs
in scientific research
has had both
positive and negative results.
