How the Scorpion Lost Its Tail (And Its Anus)

Many lizards can break off their tails when they’re in danger. Many spiders can do the same to their legs. This is called autotomy. The body parts have in-built lines of weakness that can easily tear, and the wounds tend to rapidly close and heal. Usually, the limbs grow back. The animal temporarily loses a bit of itself, which is better than losing its life.

But these trade-offs aren’t always so straightforward.

Camilo Mattoni from the National University of Cordoba in Argentina has discovered that a group of rare South American scorpions—the Ananteris genus—can also break off their tails. He has seen six species do so after he tried to grab them from behind with forceps; they wriggle furiously for a few seconds and then sever their tails after the second or third segments. In one case, the detached tail “writhed intensely, as if attempting to sting”.

As in lizards, these amputations are voluntary. If Mattoni anaesthetised the scorpions, or grabbed them by other body parts, the tail would never break.

The amputations are also common. Mattoni observed tailless individuals of eight more species, and he estimates that between 5 and 8 percent of wild Ananteris scorpions are wandering about with stumps instead of stings. They clearly survive the process—but they’re on borrowed time.

Here’s the problem. A scorpion’s anus isn’t where you think it probably would be. Instead, it’s at the end of the tail.

The gut extends all the way through the tail and opens up at the back of the fifth segment, just before the bit with the sting. So, when a scorpion performs autotomy, it leaves the final bits of its digestive tract writhing on the ground. And since the tail never grows back, that scorpion can never defecate again. Mattoni could actually see their abdomens swelling up thanks to the build-up of poo. In some cases, the increasing pressure forced another segment of the tail to break off, providing temporary relief.

To make matters worse, the scorpions lose the ability to defend themselves or to catch big prey. They still reflexively try to whack their targets with the stumps but their efforts are useless without the deadly sting. They are reduced to catching small prey with their pincers.

Despite these problems, the tailless scorpions can survive for around eight months. That’s plenty of time in which to find mates and reproduce. This probably explains why young scorpions never perform autotomy. They wouldn’t get the chance to reproduce before they died, so they’re better off trying other defences.

Females are also much less likely to self-amputate than males. Again, this makes sense. Females live longer, so they have more time to lose. They also have embryos to nourish, and they need their stings to catch larger prey. The males have no such needs and they can impregnate a lot of females in eight months. Better to do that, even if it means an abridged life without an anus.

Reference: Mattoni, García-Hernández, Botero-Trujillo, Ochoa, Ojanguren-Affilastro, Pintoda-Rocha, et al. (2015) Scorpion Sheds ‘Tail’ to Escape: Consequences and Implications of Autotomy in Scorpions (Buthidae: Ananteris). PLoS ONE http://dx.doi.org/10.1371/journal.pone.0116639