Here today, gone tomorrow: tail loss in reptiles

While working in the field, I’m often asked by volunteers about the interesting little feat of autotomy; that is, the ability for a lizard to drop its tail and re-grow it… which is a subject that has been of interest to physiologists for some time. Fair enough, I say. I mean, imagine being in the pub, you’re being hounded by an inebriated would-be lover, and then *pop!* there goes your arm, freeing you from said would-be lover to enjoy the rest of your night in peace. And that’s ok, because you’ll grow it back in a few weeks time. Mind you, lizards don’t actually do this in response to unwanted attention by conspecifics…  but the thought of it is rather amusing.

So because this is a big topic, in this blog post I’ll be focusing just on the subject of when, why, and how lizards drop their tails. The next post in this series will be dedicated to the subject of how they grow them back.

One question I’m often asked around this topic is whether geckos can drop their tails[1]. Most people here in New Zealand know that skinks can do it, but geckos…? So, let’s see…

Which reptiles can drop their tails?

All three of our reptiles in New Zealand can drop their tails; that is skinks, geckos and tuatara. Yes, even tuatara! Snakes, many (but not all) lizards, and amphisbaenids can do it as well. Crocodilians (crocodiles, alligators and caimans) cannot voluntarily pop off their tails. The major difference between snakes and lizards is that while lizards can at least partially re-grow their tails after dropping them, snakes cannot.

In lizards, the likelihood of tail dropping varies; many iguanas have fragile tails, but agamids, monitors and chameleons do not easily lose their tails. Lizards with very tough tails often cannot successfully regrow a complete tail. We don’t really know what the evolutionary basis for this variation between taxonomic groups is; but we do know that tail loss is a primitive trait that has been lost, and sometimes regained, in some lizard groups. There is also variation between species in the likelihood of losing a tail, caused both by differences in tail structure between taxa and also due to external factors like predation pressure, social interactions, or other environmental features. In the field, we have noticed that some species are more likely to drop their tails than others, after capture; notoriously bad for this are copper skinks (Oligosoma aeneum: Scincidae) and Pacific geckos (Dactylocnemus pacificus: Diplodactylidae). However, according to an article by Kelly Hare and Kim Millar, the rates of tail losses and regenerations in New Zealand skinks and geckos that we see in the nature isn’t a good indicator of their innate propensity to drop their tails. Rather, it is more likely to be due to the frequency of predation, types of social interactions or some other environmental factor.

Handle with care! The copper skink (Oligosoma aeneum) is notorious for dropping its tail when captured. Photo by Trent Bell.

 

Why lose your tail?

Autotomy (meaning “self-severing” in Greek) is a response to predation – the lizard chooses to drop its tail to distract the predator to buy it time to escape. In fact, some interesting traits have sprung up in lizards to attract a predator to their tail. For example, we have seen what we think is a defensive posture in the Nevis skink (Oligosoma toka: Scincidae), where the animal, when threatened by us (because, really, we must look like King Kong to them), will curl its tail around so that it lies over its body and head. This behaviour has also been seen by Mandy Tocher in the Big Bay form of cryptic skinks (Oligosoma inconspiccum). McCann’s skinks (Oligosoma maccani) will sometimes run away with their tail held up into the air, perpendicular to the body. Many lizards when caught will thrash their tails wildly (“no stupid, grab this end, it’s tasty!”), and still other lizards (for example, the red-tailed Top End Firetail Skink from Australia (Morethia storri: Scincidae), and many Eumeces and Plestiodon skinks from North America) have brightly coloured tails to attract a predator to their latter end. Young Plestiodon fasciatus skinks forage within leaf litter, but this presents a problem because they can no longer see potential predators. As a nifty way of overcoming this problem, when these skinks dive into leaf litter to search for prey, they will thrash their bright blue tail around for a bit, presumably to tell their bird and snake predators “Hey! Peck here! (You don’t know it, but this bit of me is dispensable).”

Another nifty tail defense strategy (if you’re a big strong lizard)

Some lizards actually use tail whipping as a defense in itself: listen to this (abbreviated) account of ferocious tail whipping from eminent reptile biologist Laurie Vitt! “I will never forget the first time I attempted to capture a large Iguana iguana [that’s an iguana, if anyone was confused?] in the field… I decided that I should be able to shake the lizard from the tree and, if I were quick, catch it as it fell. The technique seemed straightforward at the time. When I began shaking the tree, however, the iguana only held on more tightly. I shook and shook, and just as I was about to give up, the iguana made a long jump. I scrambled to get under it and managed to grab it about midbody. I was not wearing a t-shirt. In the few seconds that I held on to the meter-plus iguana, its tail whipped violently over my shoulders and under my arms, smacking me on the back like a bull whip. With its front claws, it scratched both of my arms with several quick motions. I released my hold immediately and the iguana charged off across the ground, disappearing into the thorn forest… As I walked back to my field lab, arms covered with blood and long welts rising on my back, I realized how well prepared these putatively harmless herbivorous lizards were for encounters with large predators… [from Pianka & Vitt, 2006]”. Well folks, we have been duly warned.

How does tail dropping work?

To be classified as autotomy, the tail loss must be spontaneous and not necessarily due to pulling on it, involving a break between vertebrae, and to be able to grow back. Many lizards have special “fracture planes”, which are weak places between vertebrae and the surrounding tissue that allow for easy breakage and minimal bleeding. Others have squeezing muscles in the tail that allow it to pop off (for example, the western banded gecko Coleonyx vareigatus from SW USA and Mexico), and/or a means to quickly close off blood vessels to reduce bleeding, often by contraction of sphincter muscles around the artery in the tail (the caudal artery). Once the planes have separated and the tail drops, it will thrash about madly, and in some cases emit squeaking noises, all to fool the predator. Tails are often used by lizards to store fat, therefore the tail has plenty of energy to keep thrashing; they can keep moving for up to 5 minutes in some species! Since the tail is no longer connected to the lizard, metabolically speaking, it is powered by anaerobic respiration and it is the action of nerves that keep it wriggling. This isn’t such a loss for the predator, as it gets to eat the fatty tail. In fact, some predators actually have developed a tail-harvesting behaviour for species where the lizard prey is too large to subdue, but the tail is just the right size to make a tasty morsel.

A Durrell’s night gecko (Nactus durelli) that had recently dropped its tail. If you look closely at the tail end, you can see how the muscles have contracted around the broken part – this mechanism allows lizards to stop any bleeding after dropping their tail.

There also exists another strategy of tail-dropping, that is pseudoautotomy which is seen in many snakes. This is traditionally classed as frequent tail breakage that occurs due to a predator pulling on the tail, separating the tail by fracturing a vertebra (rather than breaking between vertebrae, as for autotomy), and the tail does not regenerate.  There are two types of this: “specialized”, which means that the animal has a specific morphology that facilitates breaking, or “non-specialized” which is basically just having a long fragile tail that is easily broken. Although some species don’t bother to fit in with this type of classification, and use elements of both. The Eastern Ribbon snake of Nova Scotia, Canada (Thamnophis sauritus: Colubridae) is one species that will pseudoautotomise in response to a predator gripping the tail, and occasionally in response to being caught by a researcher (Todd and Wassersug report three tail drops from 477 captures, so not a lot). When gripped hard by the tail, eastern ribbon snakes will quickly roll sideways and drop the tail. However, eastern ribbon snakes separate the tail between vertebrae, rather than by vertebral fracturing, which shows that the distinction between autotomy and pseudoautotomy isn’t always clearly defined by the place of tail separation. Rather there is an argument for using whether the tail regenerates or not as the point of discrimination between these two tail-dropping types.

Problems with tail dropping as a predator defense strategy

Tail dropping does come at an expense for the lizard, as they have lost their valuable fat storage as well as all the other functions that a tail has: including predator defense, attracting mates, helping the lizard to climb (gecko tails in particular are prehensile) or for balance. There is always a trade-off between the importance of having a tail and losing your life. However, it isn’t always bad news, as some lizards (particularly Ctenotus skinks from Australia and Scincella and Eumeces from North America) can overcome the problems of lost energy reserves resulting from tail drop: they come back later to the their tail (or anothers’!) and eat it.

I couldn’t end this post without commenting on forked tails in lizards! It’s something that people often remark on. I couldn’t find a whole lot of scholarly information on this (though that may be due to time limits), but it seems that this occurs when a tail is incompletely severed, causing the tail to regenerate as if the original tail was completely dropped.

To conclude, we can see that tail loss is quite a common strategy for predator defense in a variety of lizards, tuatara and snakes. Limb dropping and regrowth is also used by crocodiles and alligators for mending fight injuries, and is also possible in some lizards. Although ‘voluntary’ tail or limb dropping is by no means the only means of predator defense that a lizard is armed with, as in the case of the bad-ass tail-beating iguana! Tail dropping is a rather complex process that can be achieved in a variety of different ways, generally split into autotomy and pseudoautotomy, principally depending on whether the tail can regrow or not.  While it is a rather useful means for a lizard to escape from a predator, it is not without its costs to the lizard. So, next time you see a lizard, please handle with care.

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P.S. Because really these posts are a form of “productive procrastination” for me, I’m necessarily time-constrained by how much I can research and write for each blog post. So, if anyone feels there has been an important omission, or an inaccuracy please comment on this post. More than happy to write an erratum to make sure this blog stays accurate and informative!

References

Bateman, P.W. and Felming P. A. (2010) To cut a long tail short: a review of lizard caudal autotomy studies carried out over the last 20 years. Journal of Zoology 277:1-14.

Hare KM & Miller K. A. (2010) Frequency of tail loss does not reflect innate predisposition in temperate New Zealand lizards. Naturwissenschaften 97: 197-2003.

Pianka and Vitt (2003) Lizards: windows to the evolution of diversity. University of California Press, Berkeley and Los Angeles, California.

Tocher, M. (1999) Big Bay skink (Oligosoma sp.): taxonomy, distribution and habitat requirements. Conservation Advisory Science Notes No. 228. Department of Conservation, Wellington, New Zealand.

Todd, J. and Wassersug, R. (2010) Caudal pseudoautotomy in the Eastern Ribbon Snake Thamnophis sauritius. Amphibia-Reptilia 31: 213-215.

Personal observations of: Trent Bell, Sarah Herbert and Dr. Geoff Patterson.

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[1] We only have three types of reptile in New Zealand: geckos, skinks, and tuatara so naturally I don’t get many questions about, say, the whiptail (teiid) lizards.