Salamander pokes its ribs through its skin to defend itself

As the old saying goes, “Natural selection is smarter than you are,” and I continue to be amazed at the things it’s come up with. In fact, sometimes I think that biologists should invent a drinking game in which we imagine adaptations and then try to find out if they’ve actually evolved. (To ensure reality, you have to down one if you lose.) Here’s a bizarre adaptation that I heard about the other day but found hard to believe. I looked it up and, sure enough, it’s true.

There’s a species of salamander whose members actually defend themselves by poking their ribs through their skin to produce predator-deterring spines. It thus injures itself in the cause of survival.

This was reported three years ago on the BBC Earth News, and I tracked down the original report, a 2010 paper in the Journal of Zoology by Heiss et al. (reference below). The paper reports morphological and X-ray studies of the “Iberian ribbed newt,” Pleurodeles waltl, found in Spain, Portugal, and Morocco.

First, let’s see a male of the species (photo bu Javier Fuentes, taken from here); note the orange spots, which is where the ribs will poke through.

The authors examined five male and four females adult salamanders.  To simulate a predator attack, the animals were repeatedly touched with a “cotton bud” (I think that’s a Q-Tip to Americans).  Salamanders would then adopt a defensive posture involving either flattening or arching the body.  Their ribs would then protrude, stretching the skin to make predator-deterring spines, and sometimes those ribs would actually pierce the skin (they went through the orange spots shown above).  The beasts would also, like many salamanders, secrete a milky, viscous secretion on their neck, trunk (top and sides) and tail.  Both behaviors are shown in the photos below:

Figure 1.  Defensive behaviour of Pleurodeles waltl. (a) When threatened, P. waltl releases a viscous milky and poisonous secretion onto the skin surface. Arrows indicate regions of high secretory activity. (b) In addition to poison secretion, P. waltl holds an immobile posture, stretching the skin of the lateral orange warts to the point of penetration by the sharply pointed rib tips (indicated on the left side by arrows).

Although the orange spots may highlight the presence of spines to the predators, they don’t contain any pores or openings through which the ribs can protrude. Dissection of salamanders killed (:-( ) while defending themselves show that the ribs do indeed poke through the skin, making holes in the body.  X-rays and computed tomography show that the spines, which normally point backwards, can rotate forward as much as 50° forward to create the “spines”:

Figure 2.   Radiographs showing anterior rotation of ribs from before (a) to after (b) a mildly threatening stimulus of the same animal. (c, d) Schematic drawings of eight 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. Both drawings are based on the above radiographs [(c) on (a); (d) on (b)].

Computed tomography images, below, show the forward curvature of the ribs at the tips, which makes for better spines, and their pointy-ness, also a deterrent. The authors note that the proximal (outer) third of each rib is surrounded by a sheath of connective tissue, speculating that this can perhaps speed up closure of the wound after the ribs pierce the skin.

Figure 3.   Computed tomography images showing bony structures of the trunk. For orientation, the crossing arrows on each image indicate the directions: a, anterior; l, left; p, posterior; r, right. (a) Dorsoposterior view of the vertebral column with corresponding ribs. Note that proximal three-fourths of ribs run posteriorly and downwards, whereas the distal fourth is flexed upwards. The asterisk indicates the neural spine of the vertebrae (v). ri, rib. (b) Ventral view showing a slight curving of ribs in the horizontal axis (indicated by the arrowheads). ri, rib; v, vertebra. (c) Ribs connected to corresponding vertebra by a well-developed two-headed joint consisting of: diapophysis (di) and tuberculum (tu) dorsally, and parapophysis (pa) and capitulum (ca) ventrally. ri, rib; v, vertebra.

One question is how salamanders that secrete a presumably toxic slime at the same time they make holes in their skin can avoid poisoning themselves. After all, the toxins could enter the body through the holes. Well, like other salamanders, these are almost certainly immune to their own toxins. And this immunity is likely an evolved one, for, as the authors note, when you inject even small amounts of skin secretions from one species into members of a different species, it is fatal, while injecting your own species’ toxin into yourself has no obvious effects.

Now we’re not sure that the ribby “spines” (or secretions) really do deter predators in this species, but secreted slime has been shown to be a powerful deterrent in other species.  And the fact that the erection of ribs occurs only during a simulated predation event strongly suggests that it’s an antipredator device. Showing this definitively would be difficult, as you’d have to somehow have two identical salamanders, one of which doesn’t stick out the ribs when attacked.

h/t: S.

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Heiss, E., N. Natchev, D. Salaberger, M. Gumpenberger, A. Rabanser, and J. Weisgram. 2010. Hurt yourself to hurt your enemy: new insights on the function of the bizarre antipredator mechanism in the salamandrid Pleurodeles waltl. Journal of Zoology 280:156-162.