Imagine you’re a small animal that has been pinned down by a predator. You are essentially doomed.
|Let go, you lazy beast!|
Unless of course, you can shed a relatively large body part that thrashes and wiggles about to distract the predator—knowing that in a few months you will have replaced the body part entirely!
|Come and get me!|
And when you caught the fella, all you got away with was this:
|Ah! Duped again!|
Blue-tailed skinks (and many other lizards with conspicuously colored tails) are common to multiple continents, so scientists wanted to investigate the possible evolutionary benefit of this. Bateman et. al 2014 created clay lizard models with blue and brown tails, and left them out in the environment to see if the models were attacked differently by avian predators.
|Y axis is % of days damage was recorded on the models. Blue-tailed models were attacked more overall, and attacks were targeted on their tails.|
They found that blue-tailed models more frequently attacked on tails than the brown models. However, the blue-tailed models were more frequently attacked overall—BUT since the attacks were tail-directed, the attacks would not be fatal to a real lizard, which could simply autotomize its tail and get away. The brown modeled lizards sustained more fatal, head-directed injuries, which suggests that the flashy blue tail either prevents predators from zeroing in on the head or is somehow more appealing. This risky-decoy defense tactic has been fascinating scientists (and apparently predators!) for decades.
|Maybe I can't regenerate like I used to...|
The jury is still out on how the tail is severed off, and what conditions must occur to cause muscle contractions that can sever the nerves, bone, and corresponding connecting tissue with such a clean break. Lizards have cool adaptations to help facilitate autotomy and prevent trauma that many other animals would experience after tearing off a body part!Lizard tails have fracture planes—which are essentially segments where the tail can break off. In fact, legless lizards can break their tail into so many pieces due to fracture planes they are known as "glass lizards"! The planes are between vertebrae, and are accompanied by fibrous fracture planes and also separate the myomeres, or muscle segments. The arteries in the tail are squeezed shut by specialized sphincters to prevent blood loss, so the lizards can make a clean getaway!
|Everything below the arrow is a tail full of fracture planes.|
|A typical caudal segment, showing the "tears" in muscle fibers.|
The predator *theoretically* becomes distracted by the twitching tail, which can jump and thrash about for several minutes. The two general types of tail movement are ballistic (jumping) and rhythmic (swinging) movements.
An example of ballistic tail movement:
Higham et. al (2014) hypothesize that different muscle fiber types in tail are responsible for each twitch type. There is a superficial layer of slow-twitch, oxidative muscle fiber in the tail, which may be responsible for the rhythmic swinging movement of a tail, as this motion tends to outlast ballistic behavior.
An example of rhythmic tail movement:
The fast twitch, less oxidative muscle fibers compose the majority of the tail. These fibers are likely responsible for the short-lived but ballistic movements seen in some autotomized tails.
|The arrow points to lightly stained slow twitch, oxidative fibers (rhythmic movement). The darker stained areas are fast-twitch fibers (ballistic movement).|
While the predator celebrates its victory in catching a twitchy meal, the lizard escapes and will eventually regenerate its lost body part.
Not all lizards are capable of regenerating to the same capacity, but in those that do, the process begins with re-epithelialization of the wound site, followed closely by new epidermal growth to replace the skin and scales. Underneath, a blastema forms, which is an accumulation of mesenchymal-like tissue. This means the blastema is full of undifferentiated cells that are able to develop into different tissue types to facilitate regrowth of arteries and veins, muscles, and cartilage. The new tail is also innervated by growth of ganglia from the original spinal cord, and an ependymal tube forms in place of a true spinal cord in the regenerated tail. It is thought that this tube is formed from remnant cells from the neural tube of the developing embryo, but this is uncertain.
Here you can watch a time-lapse of the regeneration process, set to music:
|Original tails reach a higher maximum velocity while twitching!|
|Regenerated tails are composed of cartilage, not bone, so fracture planes are not regenerated.|
So next time you try to catch a lizard and end up with a tail-- just think about the evolutionary and physiological implications of twitchy tails we still have to explore!
Bateman, et. al 2014 Fig 1B.
Fig 1 Higham et. al 2013 A typical caudal segment!
Fig 5D, Higham et. al, 2013.
Naya et al 2007 Figure1D
Meyer, V., Preest, M.R., and Lochetto, S.M. 2002. Physiology of original and regenerated lizard tails. Herpetologica 58(1):75-86.
Gilbert, EAB, Payne, SL, and MK Vickaryous. 2013. The anatomy and histology of caudal autotomy and regeneration in lizards. Physiological and Biochemical Zoology 86(6):631-644.
Higham, TE, Lipsett, KR, Syme, DA, and AP Russell. 2013. Controlled chaos: three-dimensional kinematics, fiber histochemistry, and muscle contractile dynacmics of autotomized lizard tails. Physiological and Biochemical Zoology 86(6):611-630.
Higham TE, Russell AP, and PA Zani. 2013. Integrative biology of tail autotomy in lizards. Physiological and Biochemical Zoology 86(6): 603-610.
Bateman PW, Fleming PA, and B Rolek. 2014. Bite me: blue tails as a ‘risky decoy’ defense tactic for lizards. Current Zoology 60(3):333-337
Naya DE, Veloso C, Munoz JLP, and F Bozinovic. 2007. Some vaguely explored (but not trivial) costs of tail autotomy in lizards. Comparative Biochemistry and Physiology, Part A 146:189-193.