I thought this title was kind of inappropriately appropriate for Mother’s Day weekend. Just wrong enough to make you squirm a little bit, but not quite offensive enough to be frightening. Ah, what a satisfying line to walk.
Today’s post is actually about a little lizard. This little guy, in the photo, is a young Five-lined Skink photographed at Freedom Park not quite two weeks ago. You can tell that it’s a juvenile lizard because in this particular species that blue tail is only found in young lizards. Now up to this point, I’ve talked a lot about acoustic communication, especially bird song. That blue tail, though, is communicating, too. The question is, what?! One hypothesis that has been presented to explain this sort of signal is that these lizards are drawing attention to their tail because it is the most expendable part of their body. Lizards, you see, have a trait called caudal autotomy. Caudal autotomy means that there are fracture planes within certain vertebrae of the tail, so that when the tail is grabbed, that vertebra will snap easily, severing the tail from the rest of the animal (and the predator only ends up with a tail while the actual lizard escapes). One hypothesis that has been presented and that continues to pop up here and there is that the bright color is meant to draw attention to the tail so that the predator attacks that (and only gets the tail end) instead of attacking the head or body.
There is something rather unsatisfying about that explanation, though. What is it? Yes? You’ve got it! If it’s meant to attract predators, then why is it only found in young. Adult skinks face the same predators and yet they have generally lost that blue color. This predator attraction hypothesis becomes, well, less attractive. There are certainly better ways to avoid being eaten than a blue tail. Already, though, we’re talking about the tail color as a signal. It conveys information to any others that see it. The question to resolve, though, is who is being signaled?
One research team evaluated this very question in this very species.
They also noticed the weakness of the decoy theory of predator attraction for the reason mentioned above, as well as a couple of others. If the blue tail is not meant to signal predators, they hypothesized, maybe it’s meant to say something to other skinks! Well, if you were a young skink, what would you want to say. And, as a follow-up, why might you want to stop saying it as you got older?
One possibility is that young lizards benefit from being identified as young. What are young lizards good for (or, rather, what aren’t they ready for)? Well, the first thing that might come to mind is reproduction. Juveniles with the blue tails are not yet reproductive. Adult males who are reproductive are very aggressive in search of and in defense of females.
Cooper and Vitt show that large males will typically win encounters with smaller males, and since size is related to age, young males often avoid fights altogether. The Clark and Hall idea becomes more appealing, then. Maybe these young lizards are trying to avoid being attacked by roving male skinks by using the blue tail to say, “Leave me alone, I’m not ready yet!” It is certainly a possibility.
In at least some species of skink, however, adult males will attack just about anyone. The colored tail doesn’t necessarily stop them from predating young skinks. Cooper and Vitt came out with a follow-up study, too. In it they examined the decoy hypothesis more closely. Since blue tails don’t necessarily dissuade males, maybe they do aid against predators. They tested this idea on two species of skinks and found that having a bright tail didn’t provide any benefit against snake predators. Maybe it would against some birds (but they didn’t test that). Interestingly, though, they did find that adult female skinks may be less likely to predate a young lizard with a blue tail than any other color.
‘Hold on,’ you say, ‘This is getting confusing.’ Indeed! Welcome to science. It’s fun! Well, let’s start with the question of tail color function. What does a blue tail do? Well, it may serve to decrease mortality from predators. The follow-up question, though, is which predators? It doesn’t appear to have any affect on snakes. It may or may not have an affect on birds (untested). It probably plays very little role in deterring damage and predation from adult males, though, this is where the Cooper-Vitt and Clark-Hall papers disagree. It’s possible that since they looked at different species, adult males of the two related species actually have different responses. The last possible predator class to signal is the adult females. At least in one of the species tested, predation by adult females may be inhibited by the blue tail color. ‘Please mom, don’t eat me!’
It’s an interesting way to avoid all sorts of odd family entanglements. Even more interesting, though, is the way these studies continually challenge how we think about the kinds of signals that these animals are using. On top of all of that cool stuff is the back and forth in these papers: ‘hey, that’s silly!’
‘oh yeah? your ideas are silly!’
‘oh yeah? your ideas about my ideas are silly’
‘hey! we’re both wrong!’
That’s the real beauty of the process of science. We keep challenging how we think about and what we know about the world. All of this from the little blue tail of a tiny little skink.