Amphibians have evolved a number of defence mechanisms against predators. Some secrete poisonous substances from their skin, some detach their tails and use them as decoys, and some have cryptic coloration; but of all amphibian antipredator mechanisms, the Spanish ribbed newt (Pleurodeles waltl) wins the trophy. In addition to some very noxious skin secretions, this little amphibian possesses a bizarre defence tactic. When faced with a threat the newt projects its ribs outside its body wall, turning them into sharp poisonous spines, giving a whole new meaning to the phrase ‘concealed weapons’. F. Leydig described this phenomenon in 1879 and now, 130 years later, Egon Heiss and his colleagues from Austria set out to describe the mechanisms by which these animals perform this peculiar feat.
To understand how the ribs move during their defence displays and to determine the morphology of the ribs and the corresponding vertebrae, Heiss' team analysed x-rays and computed tomography (CT) scans of the newts before and after their defensive behaviour. Repeatedly touching the newts with a cotton bud evoked the curious self defence behaviour.
The authors observed that when threatened and unable to escape, the newts become immobile, enlarge their body and rotate their long, spear shaped ribs forward (from 27 to 92 deg. relative to the vertebrae axis). This rotating motion lacerates the skin of the flanks at orange warts situated on either side of the body. The orange warts also appear to play a role in the defence strategy, as they make the spiny ribs more conspicuous to potential predators.
Surprisingly, instead of the ribs emerging from permanent pores located on the skin, as the authors had thought, these self-inflicted injuries occur de novo each time the animal adopts the ‘antipredator posture’. Meanwhile, the animal secretes a poisonous milky substance from the surface of its skin, which coats the tips of the ribs as they protrude from the body wall. The combination of sharp, spear-like ribs and the very poisonous secretions turns this newt's ribs into very effective and potentially deadly weapons.
Most amphibians have an extraordinary capacity to heal skin injuries and the Spanish ribbed newt is no exception. But even more remarkable is the fact that despite repeated self-laceration, the authors never observed signs of self-poisoning or infection in the animals. In 1969 Edmund Brodie, Jr and Linda Gibson observed that some species of urodels are immune to their own toxins and P. waltl can be added to that list. It is also likely that antimicrobial substances, similar to those seen by Ermin Schadich in 2009 in the African Clawed frog, are produced in the skin of this newt to protect it from infection. Furthermore, the tip of each rib is also covered with a thick periosteum layer which, the authors believe, may serve as a shield against microbial infections when the ribs protrude from the skin.
Self destructive but effective, this bizarre defence mechanism will put most predators in a very prickly situation.
