But the defensive benefits outweigh any risks from the self-inflicted injuries, and the newts quickly recover, the researchers report in a paper published online August 18 in the Journal of Zoology.
"Newts and amphibians in general are known to have an extraordinary ability to repair their skin," Heiss said.
The outer portion of each rib is surrounded by fibers of collagen, a protein used in humans to help heal skin after burns. This suggests its presence on the newt's ribs may accelerate healing, the researchers say.
In addition, a powerful immune system apparently prevents the newt's wounds from becoming infected.
"It seems that, if there is an injury to the newt, it is mild and does not incur a cost," the Open University's Halliday said.
"I have some [of the newts] in a tank in my home office. If I pick them up, I get a mild prick, but the newt seems quite unaffected."
"Chest Puffing"
The new study also offers clues to how the unusual defensive adaptation may have evolved.
The newt's long ribs are attached to its backbone by a flexible two-headed joint that allows the bones to swing forward.
Other salamanders have a similar joint and can expand their rib cages to a lesser degree when threatened. This increases their apparent body size, which is thought to deter predators from attacking.
Such simple "chest puffing" may be what led to the evolution of the Spanish ribbed newt's switchblade-like rib cage, study author Heiss noted.
"If the ribs evolve a longer size, they stretch the skin to the point of piercing it. The rib tips can than be used as weapons."
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