New research on the Spanish ribbed newt (Pleurodeles waltl) has shown exactly how this remarkable animal uses its ribs as a defence. When attacked, the newt pushes out its ribs until they literally pierce through its body, exposing a row of sharp bones. Meanwhile, a poisonous milky substance is secreted onto the body surface, turning the bones into lethal barbs. And despite all this, the newt appears to suffer no harm. BBC
See also the original abstract in the Journal of Zoology with the remarkable title Hurt yourself to hurt your enemy