The recent heavy rains filled an old tank that runs off from our cowshed. I thought I would scan the vegetation around the edge for any dragonflies that might be attracted, but there was not a single one.
There was a sudden movement and on focussing my binoculars on the spot was amazed to see a frog, with eyes set right on the top side of its head, and full webbing on the feet. It was an oily patternless, not very attractive frog immediately recognisable as a Xenopus (victorianus) or Mwanza Clawed Frog — the most aquatic of all frogs — spending virtually its entire life in water.
Sadly for the genus they are often the victims of dissection by budding biologists, to the point that they are the most used vertebrate for investigative biology and medical research at so many levels. This is the most aquatic of any frog unique in spending nearly all its life in water for which it has numerous special adaptations. However, at some point, it must somehow have arrived overland, as we are quite a way from the nearest dam.
There are so many interesting features about these frogs. Firstly they are African representatives of an extremely small family shared only with South America. The group is called “Tongueless Frogs,” and they are the only frogs to lack this organ. Without its assistance they have to engulf and swallow their food and use the long unwebbed fingers of the front legs to assist in this.
They find their food with acute sense of smell underwater, and an area along the flanks like fishes lateral line, which is sensitive and helps in catching unseen food in the silty water bottoms.
Xenopus have the fullest webbing of any frog showing their essentially aquatic existence, and have strange claws on their three hind toes, which is an extension of the skeleton exiting the body, but having a strange sheath like a claw, which gives this group its common name — Clawed Frogs. With a unique structure, although there are no external ears like all other frogs, they can hear underwater, and the males only call underwater although unlike other frogs they completely lack a vocal sac or vocal chords, and use rapid muscle retractions in their throat to produce a clicking sound.
The females then answer back with one of two different calls, one acceptance and the other rejection! Instead of having the eyes near the top of the head as in other frogs, they are located on top of the head and strangely can only look upwards, meaning it can have the body completely submerged with the eyes above the water but it actually has no moveable eyelids.
Uniquely they can change their colour to match the environment they are in. Individuals that don’t leave, can survive the drying of the ponds by cocooning themselves in mud and waiting for the next wet period. Out of water they are incapable of hopping and can only crawl. In captivity Xenopus have been known to live for 15 years!
In the 1940s and 1950s Xenopus were used as a human pregnancy testing kit! The urine from potentially pregnant women was injected into female frogs. If a woman was pregnant, it would cause the frog itself to ovulate. Once a frog had served its purpose, as there could only be one test on one frog, the animal was released which is why Xenopus are now widespread as feral introductions in the United States! It was Xenopus that was the first ever cloned vertebrate, not Dolly the sheep.
In fact this humble but certainly bizarre frog has been subject to research on human health and genetics for a long time. Had it not have been found to have all the properties it has, by now humans might have become extinct! [Google… Xenopus Wikipedia and prepare to be stunned].
By Brian Finch