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.
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.
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.
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