How the Skink Lost its Tail
If you’ve ever seen a lizard lose its tail, it is an utterly bizarre experience. As a predator lunges towards its victim, the lizard consciously sheds everything south of its back legs, which remains frantically twitching on the floor for up to five minutes. More often than not, the assailant is too distracted by this wriggling worm to bother chasing the lizard, which by now is hot-footing it across the floor to the closest protection. In our facilitating base camp, the skinks have become so accustomed to our presence that they barely move a muscle as you approach. In fact, most mornings they can be found sunning themselves on the warm UPS systems that protect our computers, gazing contentedly at us from their perches. These curious characters led us to wonder – how does it work? And is tail-loss really as convenient as all that?
A lizard’s tail is a nice little fat store. This means that the lucky predator will get himself a calorie-rich treat, while the lizard buys itself some time to escape and hopefully keep its more crucial bits and bobs intact. This unique ‘self-amputation’ defence mechanism is otherwise known as “caudal autonomy”, but not all lizards are lucky enough to have this trait, and some even have the bad fortune of being able to lose their tails but never being able to grow them again. The lucky ones have evolved ‘fracture planes’ that run down the length of their tails. When a lizard contracts the muscles at the base of their vertebra, the tail snaps off at these natural breakage points – severing the spinal cord, blood vessels and skin. Special valves in their arteries constrict to reduce excessive bleeding, resulting in a blood clot that prevents catastrophic damage to the lizard. Meanwhile, a unique type of stem cell migrates to the area to begin the regenerative process. Regrowth can range from weeks to months depending on a variety of physical and environmental factors. But is it really as easy as all that?
If only this guinea fowl could self-amputate too!
Some species of lizards have evolved bright-coloured tails that attract a predator’s attention from their equally delicious, but duller, bodies. Take the western zebra-tailed lizard for example, wh0 waves it around to show off the fancy black and white stripes on its bottom. However, for most lizards, losing their tail is usually the last resort and, if it can be avoided, it should be. A lizard’s tail makes up a big chunk of its body. So, without it, the lizard is left unbalanced, without critical fat deposits, and, perhaps most devastatingly of all, a reduced social standing and attractiveness to the opposite sex. If that weren’t bad enough, growing your tail back is such an energetically expensive process that juveniles stop growing completely while their tails are regenerating and the reproductive processes in adults halts altogether. Some lizards, such as our lovely skinks in camp, are even known to eat their own severed tails for the energy stored up in them, on realising they were shed unnecessarily.
A classic skink assailent!
If you’re not a salamander or King’s skink, despite growing back a new tail, it is hardly improved. While it might look new, it really is more for ‘looks’ than function. The true tail is now replaced by a much less flexible single rod of cartilage which lacks the ‘breakage planes’ discussed above, meaning that every time a lizard pops off its tail, this breakage occurs closer and closer to the body and so they must lose a larger proportion of their body. On top of this, re-grown tails are also often not the same length or colour as the original, with abnormal scales or patterns, and, worst of all, if it doesn’t detach itself neatly, lizards have been known to sprout up to six different tails from the original stump. Bifurcated and trifurcated tails are remarkably common (in fact, scientists estimate that as many as 3% of lizards have extra tails), but perhaps the most extreme example of this was an Argentinian black-and-white tegu that in 2015 grew six tails (a hexa-furcation!) (Pelegrin & Leão, 2016).
The ability to regenerate limbs decreases with an increase in cellular complexity. As evolving animals transitioned from aquatic to terrestrial lifestyles, they became able to generate their own body heat (endothermy) and developed enhanced immune systems that favoured rapid wound healing over regeneration. Today, salamanders can regrow everything from their brain to their hearts, but zebrafish can only regrow their tales, and, at best, neonatal human infants have been shown to regrow their fingertips (Illingworth, 1974). However, studying this unique trait has important implications for humans too. In 2021, for the ‘first time in 250-million years’ (since lizards first evolved!), scientists were able to use stem cells to regenerate a perfect tail in Mourning geckos using advanced gene-editing technology, rather than the cartilaginous stumps described above. (An additional side note to add is that Mourning geckos are an awesome all-female species entirely able to reproduce on their own!). But why, I hear you ask, does this matter to us?
Lizards are the closest relatives to humans that can regenerate a lost appendage, making them the most relevant model for potential improvements of human healing. This means that this technology could one day be applied or manipulated to help regrow everything from severed human limbs and spinal cords to repairing birth defects and treating diseases such as arthritis. The story of how our camp skinks lost their tail is a long but fascinating one. I hope you enjoyed learning about it as much as I enjoyed researching it, and next time you should be so lucky as to see this fascinating phenomenon occur – take a moment to think about what it could mean for improving human well-being.