National Geographic

What Do Lizard Tails Have In Common With Toilet Paper?

Toilet paper comes with lines of small cuts between the individual sheets, so it is easy to tear one off at pre-determined places.  A gecko’s tail works in the same way.

Geckos, skinks, and many other small lizards are known for their ability to amputate their own tails when threatened by predators. The tails don’t break off at random places. Instead, they have sets of “score lines”, where the tissue on either side is loosely stuck together and can be easily separated.  The gecko’s tail effectively comes pre-severed along several easy-to-tear lines.

But shedding a tail is more complex than it might seem. It’s not that a biting predator just pulls it off. The lizard helps the process along by contracting its muscles, which is why it takes more force to break the tail of an unconscious or dead lizard. Typically, the animal jettisons the tail just before the place where it was grabbed. After all, a tail is useful for communication, balance, storing fat, and even aerobatics—it’s not a thing to be casually lost, and the lizard benefits by detaching as little as possible.

Scientists have studied tail-shedding, or “caudal autotomy”, for several decades (there’s a good review here), focusing on when, why and how it happens. It’s the last one that interested Kristian Sanggaard from Aarhus University, who wanted to understand how the tail’s microscopic structures helped it to break off. To do that, he studied the Tokay gecko from south-east Asia, one of the largest of the 1,500 gecko species.

Here’s a slice through one of the gecko’s tail segments, stained with different dyes to highlight the various tissues. You can see the scales in dark blue running along the top and bottom, muscle fibres in red, and a huge core of white fat.

Section through a gecko's tail, showing clear "score lines" between segments

The segments are immediately obvious, with clear lines running through the fat and muscle. These divisions become less clear near the scales, but Sanggaard noticed dense clusters of collagen fibres at the points where the segments separate. You can see these in the image below (and the insets in the image above)—they’re the even patches blue in the midst of more marbled areas. The yellow arrows show the score line where the two segments break away from one another.

Along this line are dark blue dots. These are cells, and Sanggaard likens them to a zipper. It’s possible that when the lizard wants to shed its tail, the cells secrete substances that weaken the collagen, allowing the tissue to split apart more easily.

Left: yellow arrows show a ready-made score line in a gecko's tail. Right: a line of cells forming a zipper in the tail

Gecko tail segment showing muscle wedges

The tips of the broken tail segments end in wedge-shaped ‘fingers’ of white muscle. In an intact tail, these wedges fit into grooves within the preceding segment, like the finger jointsyou see on furniture.

Sanggaard looked at them under a powerful electron microscope, and saw that the muscle fibres end in mushroom-shaped tips. When the tail is intact, these fibres have flat heads that meet one another and stick together. When it’s time to detach the tail, Sanggaard thinks that the muscles contract and the ends expand into the rounded mushroom shapes. This reduces the adhesive forces between them, and allows the segments to disconnect.

An MRI scan of an unbroken tail confirmed his suspicions. There are clean gaps between adjoining segments with no structures running through them. This means that they’re held together by sticky forces, rather than by any physical anchors. In this way, the gecko gets the best of both worlds – a tail that holds together under normal circumstances, but that can be easily broken off at pre-determined points when its life is in danger.

Muscle fibres at the end of gecko tail segements end in mushroom shapes

That’s not the end of its defence, though. The severed tail will dance, writhe and wriggle for up to half an hour, probably to distract the predator’s attention from the escaping lizard or to put it off entirely. And if the tail isn’t eaten, the lizard will often return to it later to gulp it down itself. After all, why waste so much valuable fat?

Reference: Sanggaard, Danielsen, Wogensen, Vinding, Rydtoft, Mortensen, Karring, Nielsen, Wang, Thogersen & Enghild. 2012. Unique Structural Features Facilitate Lizard Tail Autotomy. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0051803

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There are 7 Comments. Add Yours.

  1. Michelle de Villiers
    December 21, 2012

    I really enjoy the humorous imagery in your writing!

    [Thanks! - Ed]

  2. Elizabeth Moon
    December 21, 2012

    Are a gecko’s blood vessels so small that yanking some of them apart doesn’t cause serious blood loss? Or do the lizards secrete a clotting factor even as they’re preparing to release the tail at its “zipper?”

  3. Paul Knoepfler
    December 21, 2012

    Since the tail can partially grow back, I wonder if there is some sort of special machinery at the break points (e.g. stem cells) that kicks into gear after tail loss?
    Paul

  4. Cath
    December 22, 2012

    I don’t have anything to say other than that I love your writing. It was the fascinating things that animals can do that first got me interested in biology, to the point I majored in it in college, although all my jobs have been largely chemistry-based. I love reading pieces that would have interested me just as much as a child, but now I can appreciate them because I understand the science behind it, and you make it incredibly easy for me to explain even to my non-science friends. Thank you very much. :)

    [Thanks Cath - Ed]

  5. Brooks
    December 23, 2012

    Ed, am i missing something, or is there no bone present in the original tail? As far as i’m aware, anoles begin with a bony tail, and regenerate a tail with a cartilaginous rod. To your knowledge, are geckos similar?

  6. Ed Yong
    December 23, 2012

    Brooks, there are vertebrae there. You can see a little piece in the middle of the cross-section. You only can’t see more bone because of the place where the tail has been sliced.

  7. Dee
    December 27, 2012

    This might seem like a stupid question but what are the plain blue areas? If the dark blue is scales, red is muscles, and white is fat? Are the plain blue spots collagen or just simple “tissue”? I’m speaking of the blue areas where you find the darker blue dots in the middle (the darker blue dots being the area where the cells “secrete substances that weaken the collagen, allowing the tissue to split apart more easily”).

    [Not a stupid question at all. It's mainly collagen and other connective tissues. - Ed]

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