Amphibians
African Clawed Frog
Xenopus laevis · also called African clawed frog, platanna, Xenopus
The African clawed frog is an unglamorous animal with an outsized place in science. It is a flat, blotchy, fully aquatic frog with no tongue and no teeth that eats by shoving food into its mouth with its hands — and for decades it was the world’s pregnancy test, and it remains one of biology’s essential lab animals.
Overview
African clawed frogs are native to still and slow fresh water — ponds, dams, ditches, and sluggish rivers — across sub-Saharan Africa, and are especially common in the south. They are entirely aquatic, coming out of the water only to move between pools when forced to, and they can survive drought by burrowing into mud. They are nocturnal, tolerant of stagnant and polluted water, and will eat essentially any animal matter they can subdue or scavenge.
Built for the water
The frog has no tongue and no jaw teeth, so it cannot hunt the way a typical frog does. Instead it uses its muscular clawed hind feet (the “claws” are three hardened toe-tips) to tear food apart and its hand-like front feet to stuff pieces into its mouth, and it can inhale prey with a sudden gape. A lateral-line system along its body, like a fish’s, senses the water movements of nearby prey in the dark.
The pregnancy test
In 1930, Lancelot Hogben discovered that a female Xenopus injected with a pregnant woman’s urine would lay eggs within about 12 hours, because the hormone hCG triggers ovulation in the frog. The Hogben test was cheap and repeatable, and from the 1930s to the 1960s hospitals worldwide kept tanks of clawed frogs for it. The global shipping of frogs for this purpose helped seed today’s invasive populations.
A laboratory workhorse
Xenopus females lay thousands of large eggs on demand, and the embryos develop outside the body and are big enough to handle. Much of what is known about early development, the cell cycle, and cell biology was discovered in this frog, and a related clawed frog was central to the first cloning of a vertebrate in the late 1950s.
An amphibian problem child
Escaped and released clawed frogs have established on four continents. They eat native tadpoles and fish, breed fast, and can walk overland between waters. They also carry the chytrid fungus without being harmed by it, and the worldwide Xenopus trade is considered a likely route for spreading the disease that has wiped out amphibians elsewhere.
Status
The African clawed frog is Least Concern — abundant at home and, unfortunately, abroad.
African Clawed Frog: common questions
How was a frog used as a pregnancy test?
In 1930 the British scientist Lancelot Hogben found that injecting a female African clawed frog with the urine of a pregnant woman caused the frog to lay eggs within 8 to 12 hours, because the pregnancy hormone hCG in the urine triggers ovulation in Xenopus. This "Hogben test" was cheaper, faster, and reusable compared with the earlier method of injecting mice or rabbits and dissecting them. From the 1930s into the 1960s, hospitals around the world kept tanks of clawed frogs for this purpose, and the large-scale export of the frogs for testing is one route by which they became invasive. Chemical pregnancy tests eventually replaced them.
Why is Xenopus so important in laboratories?
Several reasons. The females can be induced to lay thousands of large, sturdy eggs on demand year-round, and the embryos develop outside the body and are big enough to manipulate by hand, making them ideal for studying how a fertilised egg turns into an animal. Much of what is known about early vertebrate development, cell division, and the cell cycle was worked out in Xenopus. A related species, Xenopus tropicalis, has a smaller genome and is used for genetics. Clawed frogs also flew on space missions and were central to early cloning experiments.
Are African clawed frogs a problem outside Africa?
Yes. Wherever they have escaped from labs, pet collections, or pregnancy-testing programmes, they tend to establish and spread, because they tolerate a wide range of conditions, eat almost anything, breed prolifically, and can walk overland between water bodies. They prey on native tadpoles, insects, and fish and compete with native amphibians. They are also a carrier of the chytrid fungus Batrachochytrium dendrobatidis without being harmed by it, and the global trade in Xenopus is considered a likely vector for spreading that disease, which has devastated amphibians on several continents.
How does a tongueless frog eat?
The African clawed frog is built for underwater feeding. It has no tongue to flick out and no true jaw teeth. Instead it uses its powerful, webbed, clawed hind feet to kick and shred food, and its slender front feet to cram pieces into its mouth. It can also feed by suction, suddenly opening its mouth to inhale a mouthful of water and whatever is in it. Sensory organs along its flanks (a lateral line, like a fish) let it detect the ripples made by moving prey in murky water or darkness.