Matt Walker

Editor, Earth News

The Spanish ribbed newt more than lives up to its name

One amphibian has evolved a bizarre and gruesome defence mechanism to protect itself against predators. When attacked, the Spanish ribbed newt pushes out its ribs until they pierce through its body, exposing a row of bones that act like poisonous barbs. The newt has to force its bones through its skin every time it is attacked, say scientists, who have described the form and function of the barbs in detail. Yet this bizarre behaviour appears not to cause the newt any ill effects. The ability of the Spanish ribbed newt to expose its rib bones was first noticed by a natural historian in 1879. But scientists have now used modern photographic and X-ray imaging techniques to reveal just how the animal does it. The ribs have burst through the skin, ready to sting any attacker And what they discovered is even more gruesome than they imagined. When the newt becomes agitated or perceives a threat, it swings its ribs forward, increasing their angle to the spine by up to 50 degrees. As it does this, the newt keeps the rest of its body still. "The forward movement of the ribs increases the body size and stretches the skin to the point of piercing it," says zoologist Egon Heiss of the University of Vienna in Austria. The tips of the newt's ribs then stick outside its body, like exposed spines. Arrows point to a poisonous secretion which coats the tips of protruding ribs But there is more to the newt's defence, Heiss and his Vienna-based colleagues report in the Journal of Zoology. "When teased or attacked by a predator, [the newt] secretes a poisonous milky substance on to the body surface. The combination of the poisonous secretion and the ribs as 'stinging' tools is highly effective," says Heiss. The impact on any predator can be striking, particularly if they try to bite the newt or pick it up using their mouth. Then the poison is almost injected into the thin skin within the mouth, causing severe pain or possibly death to the attacker. A computer tomograph shows the sharp rib points and vertebral column As well as elucidating the spear-like shape of the ribs, and exactly how the ribs swing forward and protrude, the scientists have demonstrated that the bones must break through the newt's body wall every time the amphibian evokes the defence response. Initially, it was thought that the ribs might passively emerge through pores, rather than be actively driven through the body wall. Surprisingly, the newt, which is related to other newts and salamanders, appears to suffer no major ill effects, despite repeatedly puncturing its own body and exposing its rib bones. "Newts, and amphibians in general, are known to have an extraordinary ability to repair their skin," says Heiss. On the left, the ribs of a relaxed newt point backwards, while on the right an agitated newt brings its ribs forward, ready to strike through the skin "Anyway, if this newt can avoid being eaten in some cases, this surely has a positive influence." It also seems that the newt is immune to its own poison, which is normally confined to glands in the newt's body. When the newt wounds itself by exposing its ribs, the poison can seep into its body tissue, again apparently with no ill effects. Heiss now hopes to investigate which compounds are in the poison.



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