To Harm Yourself is to Live

Pleurodeles waltl on land.

Iberian ribbed newt on land.

At a glance, the Iberian ribbed newt or Spanish ribbed newt, Pleurodeles waltl, looks pretty unremarkable and seemingly harmless. However, don’t get yourself fooled by this slender-bodied, long-tailed amphibian, with dark gray dorsal and lighter gray ventral, belonging to the family of Salamandridae. When faced with a threat, P. waltl is capable of not only releasing a poisonous secretion onto its skin surface, but also displaying a “bizarre defensive mechanism” against predator. As described by Matt Walker in his BBC article, “the Spanish ribbed newt pushes out its ribs until they pierce through its body, exposing a row of bones that act like poisonous barbs.”

Defensive behaviour of Iberian ribbed newt.

Defensive behaviour of Iberian ribbed newt. Left: The newt's sharply pointed rib tips stick outside its body (indicated by arrows). Right: The arrows point to a poisonous, milky substance secreted onto the skin surface.

F. Leydig, a German zoologist and comparative anatomist, noted the Iberian ribbed newt’s ability to expose its rib bones for the first time in 1879, but little was known about the exact mechanism of this antipredator adaptation. In 2009, a group of Austrian scientists, Heiss et al., managed to shed some light on how the defense mechanism of Iberian ribbed newt really works, using photo- and X-ray imaging along with computed tomography techniques.

Heiss et al. simulated a predator threat by touching the newts repeatedly using a cotton bud, inducing the self-defense behaviour. Comparison between X-ray images before and after the simulated threat shows that the animals rotated their ribs anteriorly, as shown in the pictures below. The forward movement of the ribs increases the body size, causing the sharply pointed rib tips to lacerate the skin and protrude outside the body like exposed spines. Each newt is able to swing its bones forward because its long ribs are attached to the backbone by a flexible two-headed joint. It was revealed in the same study that the ribs are not projected through permanent pores located on the skin, but instead the self-inflicted injuries occur de novo each time. Meanwhile, to add into the already complex defense mechanism, the newts also actively secrete poisonous, milky substance onto their body surfaces, coating the tips of the ribs as they are projected out of the body, turning them into highly effective and potentially deadly weapons.

Radiograph showing anterior rotation of ribs from before (a) to after (b) a mildly threatening stimulus of the same animal. (c,d) Schematic drawings of 8 vertebrae (numbers 4-11) with corresponding ribs, pointing to the differences in rib angle relative to the sagittal body axis before (c) and after (d) stimulus.

Radiograph showing anterior rotation of ribs from before (a) to after (b) a mildly threatening stimulus of the same animal. (c,d) Schematic drawings of 8 vertebrae (numbers 4-11) with corresponding ribs, pointing to the differences in rib angle relative to the sagittal body axis before (c) and after (d) stimulus.

What is surprising is that the Iberian ribbed newts are not injured in the process. The combination of their extraordinary capacity to heal skin injuries, the presence of antimicrobacterial peptides released from specialized cutaneous glands (Schadich, 2009) as well as the immunity against their own toxin, similar to some other species of urodels studied by Edmund Brodie, Jr. and Linda Gibson (1969), is believed to serve as a shield against infections and self-poisoning.

Besides P. waltl, there are other species in the Salamandridae family using this type of defense mechanism. For example, Echinotriton andersoni, a species belonging to a sister genus of the Pleurodeles, also use the ribs as “concealed weapons” to defend against predators. Detailed study on E. andersoni can be found in the paper by Brodie, Nussbaum and DiGiovanni (1984).

References:

“Pleurodeles waltl,” by J. Gallego. Flickr. URL: http://www.flickr.com/photos/javig/4465950862/ (accessed on 8 April 2010).

“Bizarre newt uses ribs as weapons,” by Matt Walker. BBC Earth News, 21 August 2009. URL: http://news.bbc.co.uk/earth/hi/earth_news/newsid_8212000/8212623.stm (accessed on 8 April 2010).

“Newt Cuts Itself to Use Ribs as “Concealed Weapons”,” by Scott Norris. National Geographic News, 28 August 2009. URL: http://news.nationalgeographic.com/news/2009/08/090828-spanish-newt-ribs-spines-wolverine.html (accessed on 8 April 2010).

Brodie, E.D. Jr. & L.S. Gibson, 1969. Defensive Behavior and Skin Glans of the Northwestern Salamander, Ambystoma gracile. Herpetologica, 25(3): 187-194.

Brodie, E.D. Jr., R.A. Nussbaum & M. DiGiovanni, 1984. Antipredator Adaptations of Asian Salamanders (Salamandridae). Herpetologica, 40(1): 56-68.

Heiss, E. et al., 2009. Hurt yourself to hurt your enemy: new insights on the function of the bizarre antipredator mechanism in the salamandrid Pleurodeles waltl. Journal of Zoology, 280(2): 156-162.

Schadich, E., 2009. Skin peptides activity against opportunistic bacterial pathogens of the African clawed frog (Xenopus laevis) and three Litoria frogs. Journal of Herpetology, 43(2):173-183.

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