Amphibians
Clawed Frog
Pipidae · also called platanna, tongueless frog
A clawed frog barely looks like a frog. It is flat, slippery, and never leaves the water, it has no tongue, its eyes point straight up, and its hind feet carry small black claws. And yet one species, the African clawed frog, is among the most important laboratory animals in the history of biology — and one of the more troublesome amphibian invaders.
Overview
Clawed frogs are the family Pipidae, around 30 species, split between sub-Saharan Africa (the clawed frogs, genus Xenopus) and northern South America (the Surinam toads and their kin). All are fully aquatic, flattened, tongueless frogs of still and slow fresh water, and all are carnivorous scavengers that shovel food into the mouth with their hands.
Claws and no tongue
The inner toes of each hind foot end in true keratin claws — rare in a frog — used to shred food held in the forelimbs and to rake at rivals and predators. With no tongue, the frog lunges and grabs, then uses suction to draw small prey in. As a permanent water-dweller it also keeps a fish-like lateral line to sense movement in murky water.
The pregnancy-test frog
From the 1930s to the 1960s, injecting a female African clawed frog with a pregnant woman’s urine made it lay eggs within a day — a cheap, reusable test that shipped clawed frogs around the world by the thousand.
A laboratory workhorse — and an escapee
The large, hardy eggs of Xenopus helped establish developmental biology, and a clawed frog was the first vertebrate cloned from a specialised cell, in 1962. Escaped lab, pet, and test animals now live wild on several continents, where they strip wetlands of native tadpoles and are prime suspects in spreading the amphibian chytrid fungus.
Status
Clawed frogs as a family are Not Evaluated. The African clawed frog is abundant and expanding, but several African and South American pipids are of concern as the wetlands they need are drained and polluted.
Clawed Frog: common questions
Why does a clawed frog have claws?
The claws are small, hard, dark caps of keratin on the inner toes of the hind feet, and they are true claws, not just pointed toe tips, which is rare among the roughly 7,000 frog species. A fully aquatic, tongueless frog cannot catch prey the way a typical frog does, so the clawed frog instead grabs food in its front feet and holds it while raking backward with the clawed hind feet to tear off pieces. The same claws are used in defence, kicking and scratching at a predator or a competing frog, and they can cut skin. The frog's scientific name, Xenopus, means strange foot.
How was the African clawed frog used as a pregnancy test?
In 1930 researchers found that injecting a female African clawed frog with the urine of a pregnant woman caused the frog to lay eggs within about 8 to 12 hours, because the pregnancy hormone hCG triggers ovulation in the frog. This Hogben test was cheaper, faster, and more reliable than the earlier method using mice or rabbits, and crucially the frog survived and could be reused after a rest. Hospitals kept tanks of clawed frogs for decades, and huge numbers were exported from South Africa. The test fell out of use in the 1960s once chemical immunoassays were developed, but by then clawed frogs were established in labs worldwide.
Why are clawed frogs important in science?
Their eggs and embryos are large, tough, produced in great numbers on demand, and develop outside the body where they can be watched and manipulated, which makes them ideal for studying how a fertilised egg turns into a body. Work on clawed frogs established many basics of developmental biology, including how the nervous system is induced and how the body axis is set up, and a clawed frog was the first vertebrate cloned from a differentiated cell, in John Gurdon's 1962 experiment that later shared a Nobel Prize. The western clawed frog was chosen for genome sequencing because, unlike the African species, it has a simple two-set chromosome complement.
Are clawed frogs a threat as an invasive species?
In several regions, yes. Populations descended from escaped or released laboratory, pet, and pregnancy-test animals now live wild in parts of the United States, Chile, Britain, France, Portugal, Japan, and elsewhere. Clawed frogs are hardy, tolerate poor water, breed prolifically, and are voracious generalist feeders, so where they establish they can strip a wetland of native tadpoles, invertebrates, and small fish and out-compete native amphibians. They are also frequent carriers of the chytrid fungus Batrachochytrium dendrobatidis, usually without falling ill themselves, and the worldwide trade in clawed frogs is one of the leading suspects for how that devastating disease was spread between continents.