The title makes it sound like a pharmaceutical ad. “Ask your doctor about caudal autotomy today.” I should follow it up with a list of possible side effects, too. But let’s start this way…what is it? Caudal autotomy (which does just roll off the tongue. Say it out loud. Really, stop reading and say it. Feels good, doesn’t it?) is a trait that we see in a lot of lizards and salamanders. Many of these animals have fracture planes in the vertebrae in their tails. If something (a predator or a child) grabs the tail, the tail will pop off and the animal can run away without getting eaten.
I bring this topic up because on my last run through Freedom Park, I came across a little Five-lined Skink. Juveniles of this species have bright blue tail tips. Adults, however, lose all that color. The one that I saw didn’t have any color at all. In fact, it didn’t have a tail. It had been autotomized (say it again!). Why are juveniles the only ones with the colored tails?
We can answer that question from two angles. First of all, why should any lizard have a blue tail? What is the function of that color pattern? Bright colors stand out, so it is definitely a way of advertising. In most of these lizards, that color pattern is a way to draw a predator’s attention. Some bird sees a lizard moving swiftly through the grass and zooms in on the most visible part- the bright blue tail. If the tail can then just pop off, then the bird gets the tail to eat (and so never learns to avoid hunting for bright blue tails) and the lizard gets to run off. But why is it that adult Five-lined Skinks don’t have blue in their tails? To answer that question, we need to know what they use their tails for.
Tails are not just attractive structures, they also function as good places to store extra nutrients. Adults may store a lot of nutrients in their tails, so the loss of the tail may actually affect their survival in a way that juveniles are not affected.
Vitt, L.J. and W.E. Cooper, Jr. 1986. Tail loss, tail color, and predator escape in Eumeces (Lacertilia: Scincidae): age-specific differences in costs and benefits. Canadian Journal of Zoology 64: 583-592.
Here’s where things get tricky. In some species of lizard, females who have lost their tails have less energy available to use in reproduction. They produce smaller eggs with fewer nutrients than females who still have their tails.
In other species, the amount of energy invested in reproduction doesn’t change. Females without tails produce the same number of eggs as females with tails. The difference, though, is that females without tails are less likely to survive to reproduce- not as a function of being predated, but because of the amount of available energy they have to make it through the seasons. If they can make it to reproduce, they still give it their all.
In each of these cases, though, juveniles don’t apparently have to pay the same sorts of costs that adults do because they don’t have the same sorts of decisions to make. They can get away with the blue tails as a relatively low cost way to avoid predation. And you know, when you really think about it, juvenile humans get away with a lot of fashion choices that just don’t work for adults. I don’t think the reasoning is quite the same since we, unfortunately, don’t have the autotomy option.