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.

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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).

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)].

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.

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.

24 Comments

  1. Iñigo MartinezSolano
    Posted April 5, 2013 at 8:40 am | Permalink

    These are beautiful creatures all right. I’ve been monitoring populations of this species for several years now and observed this behavior often, it’s really amazing. You can see another pic here:

    http://www.vertebradosibericos.org/anfibios/interaccion/plewalin.html

    • Lars
      Posted April 5, 2013 at 1:39 pm | Permalink

      Thanks, Inigo, that shows the correspondence of orange spots with emerging ribs very nicely.

    • Posted April 5, 2013 at 2:35 pm | Permalink

      Thank you for posting this link. A beautiful and informative website. I have bookmarked it.

  2. Posted April 5, 2013 at 8:51 am | Permalink

    Thank you for this wonderful article!

  3. Kevin Alexander
    Posted April 5, 2013 at 9:46 am | Permalink

    Sharp spines plus toxic slime to inject. Nope, don’t want to bite that.

  4. David Duncan
    Posted April 5, 2013 at 9:53 am | Permalink

    I recall a documentary by David Attenborough discussing an amphibian (IIRC) that secreted poison before being eaten by a predator. It didn’t save the first victim but the meal was sufficiently unpleasant that that particular predator never dined on that amphibian again.

    Since that adaption didn’t save the first victim perhaps it is an example of kin selection.

  5. E.A. Blair
    Posted April 5, 2013 at 9:56 am | Permalink

    These are adaptations which probably benefit the species rather than the individual (since it’s doubtful that the deterrent value becomes apparent until the salamander is already a mouthful).

    There’s a lesson for humans there. Individual advantage is not more important than the well-being of the community.

    • Gregory Kusnick
      Posted April 5, 2013 at 11:22 am | Permalink

      Natural selection doesn’t care about benefit to the species or the well-being of the community; it cares about propagating the genes that cause that behavior. As David Duncan suggests in #4, the most likely explanation is kin selection.

      • aspidoscelis
        Posted April 5, 2013 at 11:32 am | Permalink

        Kin selection is just a way of shoehorning higher-level selection into an individuals-only perspective.

        So, yeah, it’s probably kin selection. Just kindly avert your eyes to any beneficial effects on individuals who don’t happen to be close kin. 🙂

        • whyevolutionistrue
          Posted April 5, 2013 at 11:35 am | Permalink

          If it didn’t benefit the individual who first had the genes for the trait (and I suspect it did, since spines deter your being killed), then it would have to benefit relatives to get off the ground. So no, kin selection is not just a way to shoehorn higher-level selection into an individuals-only perspective. Inclusive fitness of individuals is what in fact WHAT IS MAXIMIZED by natural selection.

          And if other individuals benefit, so what? That’s the effect of having a kin-selected trait fixed in a population.

          • aspidoscelis
            Posted April 6, 2013 at 12:02 am | Permalink

            “If it didn’t benefit the individual who first had the genes for the trait (and I suspect it did, since spines deter your being killed), then it would have to benefit relatives to get off the ground.”

            OK… then show me a group of -unrelated- newts!

            • John Scanlon, FCD
              Posted April 16, 2013 at 6:14 am | Permalink

              Well, relatively related then. 🙂

              The point is what happens close to the origin of a mutation, when only close relatives share it. OTOH a gene that operates for the ‘good of the species’, regardless of degrees of kinship, can never increase by selection.

  6. Marcoli
    Posted April 5, 2013 at 10:18 am | Permalink

    Weirdness is relative. Bipedality and having an enormous brain capable of abstract thought is very weird.

  7. Butames
    Posted April 5, 2013 at 11:24 am | Permalink

    Could the poterusion of the ribs through the skin also be used in mating displays? Don’t the males of some species go through the pretense of defensive/aggressive behavior in efforts attract mates?

    Also, one of the first things that came to mind as I read this article was, we have found a link between Wolverine and his amphebious evolutionary ancestors.

  8. Notagod
    Posted April 5, 2013 at 11:26 am | Permalink

    I know, I’ll create a salamander that uses its ribs as defensive weapons. That will show ’em that I can make women from ribs, for sure.

    – One or more of the christian gods. Defying the other christian gods that want to be known by “faith” alone.

  9. Wayne Van Devender
    Posted April 5, 2013 at 2:14 pm | Permalink

    It gets even better! Check out the papers by Brodie referenced in this one to read about Echinotriton andersoni, which has even more specialized ribs that are really long and do a break-away action after passing through a poison glad into the potential predator. (I have a shot of one used by Nussbaum in that work and a couple of a very well fed Cincinatti Zoo specimen at: http://classdat.appstate.edu/AAS/Bio/vandevenderr/Echinotriton/ There is another paper about Salamandra salamandra spraying their toxins at predators.

  10. Torbjörn Larsson, OM
    Posted April 5, 2013 at 2:40 pm | Permalink

    I believe (or tell myself) that I’ve heard of this one. A bit more “Wolverine” than squirting blood from the eye as deterrent, indeed.

    [I hope they find a relative with the same trait. There seems to be a dearth of comic characters in species naming.]

    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.)

    For a moment there I read that as downing an adaptation. It would be messy to take care of a sperm whale spermaceti organ!

    So, ‘drink chess’ for biologists!? In chess circles it is sometimes used as a leveler of skills (you down one if you win), here it would enhance differences. Suits!

    • Torbjörn Larsson, OM
      Posted April 5, 2013 at 2:42 pm | Permalink

      I forgot to add that there was a lot of detail on the trait, and interesting detail at that. Thanks!

  11. Marella
    Posted April 5, 2013 at 3:32 pm | Permalink

    X-rays and computed tomography show that the spines, which normally point backwards

    I think you meant “ribs” instead of “spines” here.

    while injecting your own species’ toxin into yourself has no obvious effects.

    And here you meant “itself” instead of “yourself” I think.

    I have heard of this before, the Wolverine of salamanders, very weird, but then salamanders are weird.

  12. M Janello
    Posted April 5, 2013 at 4:42 pm | Permalink

    The evolution drinking game reminded me of the thought experiments done in the 1970s by Richard Alexander about what conditions would be necessary for eusocial mammals to evolve. At that time eusociality had only been observed in insects.

    Another biologist heard his list of criteria for habitat and so on and told him to go study naked mole rats, which fit all those criteria but had not been thought of as actually eusocial (that is, limited reproduction by the queen only). (I think that they were merely poorly studied at that point, not that eusociality had been ruled out. Also, I imagine that prior to routine genetic testing, figuring out which rodents in burrows in remote areas in East Africa were related to each other was probably not very easy to figure out!)

    Mole rats are truly eusocial, it turns out (as well as having some very strange other traits such as living for 30 years when the normal rodent life span is 2).

    http://ncse.com/rncse/17/4/predictive-power-evolutionary-biology-discovery-eusociality-

  13. M Janello
    Posted April 5, 2013 at 4:44 pm | Permalink

    Here’s my evolution drinking game prediction:

    In areas where predators have evolved to avoid rib-piercing salamanders, other salamanders will evolve orange side spots to mimic rib piecing spines but not actually have them.

  14. Mary Canada
    Posted April 5, 2013 at 6:51 pm | Permalink

    Tomography images are amazing.

  15. marksolock
    Posted April 6, 2013 at 4:22 pm | Permalink

    Reblogged this on Mark Solock Blog.

  16. lisa
    Posted April 22, 2013 at 5:23 pm | Permalink

    A few months ago I saw yet another documentary (I do that a lot) about a site in Germany that for reasons unknown with a pit of very bizarre evolution “fails” including the fossil of a bipedal crocodid with hoofed feet. I can’t decide if it’s a good or bad thing that that one didn’t make it.


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