Lizards. Creatures that make you think twice about whether it is mentally tolerable to chase them away with an apparatus of any kind, or pick them up with you hands, due to the fact that most (but not all) lizards have the ability to detach and leave their tails behind, wriggling and silently mocking whatever or whoever approached it and was deemed threatening. Especially if they happen to reside on a high plane and thus their detached tail drops from overhead.
This shocking ability, or more precisely, anti-predation, defense mechanism is known medically as caudal autotomy; originating from the Greek words meaning self-amputation.
When the lizard is approached by a predator that it deems dangerous and life threatening it may drop it’s tail. However just being close to the predator or witnessing scary, aggressive displays by the predator is often not enough for them to choose to do this. Significant pressure must be applied to the tail, creating situations where the lizard is trapped by force and unable to escape this grip. Where upon this, the tail is detached by will so the lizard can gain it’s freedom to rapidly scurry away to safety. Since there are bundles and bundles of sensitive neurons and complex motor pathways in the tail that do not die immediately when cut off from the rest of the body, the tail can twist, turn, move and remain wriggling for up to 30 minutes after detachment. Without any information from the brain or blood supply!
The rationale is that hopefully the predator would be too confused, distracted and freaked out by this ‘appendage that seems to have gained a life of it’s own’ to chase after the lizard. Besides this, tails are often a good storage of fats for lizards; which makes ‘hibernation’ so much less worrisome when the lizard knows that it already has a sufficient energy supply to last it through it’s dormancy. Hence, at times predators may also chose to eat this tail that is closer in range rather than spending time and effort to chase after the lizard- a hunt uncertain of success.
This phenomenon however does not happen as frequently and randomly as people think.
Depending on the genus and species of lizards, their tails can either rapidly grow back after being dropped several times, or not grow back at all. The former types of lizards include green iguanas and bearded dragons, while crested geckos are examples to the latter.
Nevertheless everything comes with a price however small it is. For the lizards, this being that each time they lose a tail, the new one is not 100% the same as their original tail.
While initially the tail would be made from bony skeleton, new tails are composed of a rod of cartilage—more symbolic of a ‘make do replacement tail’. Regrowth also takes longer than expected, reaching up to weeks or even months for full recovery. As implied, each regeneration takes a toil on the lizard, and eventually if caudal autotomy was performed too many times the lizard may lose it’s regenerative ability forever. Each time a tail is lost, new tails may be shorter, weaker or generally slightly more defective than the last, leading to potential adverse long term effects such as mobility disability, reduced growth and increased difficulty to fit in with the lizard social communities.
Scientists have made new breakthroughs upon which they discovered that the mechanism to this tail detachment is “not a biochemical one but rather a mechanical process”.
Lizard tails consist of several weak spots along their vertebrate called fracture plates, making them connect very loosely to the rest of the body, always in preparation for an emergency autotomy. It is believed that besides loose muscular interactions the tail is held by friction and adhesion forces alone. When particular points on the tail detect excessive pressure and strain, specialized muscle fibers at this junction reduce their interaction with the fracture plates and contracts to pull away. “This is known as a reflex muscle spasm”. Since there are many fracture plates at intervals from each other, a lizard can decide how long a portion of their tail they should detach. One advantage to this loose hold is that only minimal blood would be spilt when the tail is ‘cut off’, especially since both bone and cartilage are avascular (not supplied with blood vessels).
Recently other studies have found that this adaptation originated to predominantly counter one particular type of predator: venomous predators such as vipers. This is demonstrated in the result findings that lizard species found in habitats where there are only few venomous predators gradually lose this ability altogether, or at least lose the speed and number of times this can be performed.
It makes sense now that the main function and benefit to severing a tail is not primarily to escape a predator’s hold, because lizards are small, agile and quite difficult to get a grip on already the chances of this occurring is quite low. The main purpose is when a venomous predator, whose lethal venom can travel through the bloodstream to reach every cell within minutes, makes contact or worse bites the tail, the lizard can lose their tail to cut off the venom from the blood flowing along the rest of it’s body and thus protect it’s vital organs from failure and systems from being damaged—a much more efficient, effective and logical explanation for this unique behavior.
Therefore, as conclusion to today’s article, it goes without saying that there are a multitude of beneficial outcomes to research into this superb ability, including but not limited to regenerative medicine and treatment of neuromuscular or musculoskeletal disorders like spinal cord injuries, thus studies in this field of medicine should be promoted and there is still much to learn from lizards, reptiles or just insects and animals in general.
Written by Sun Feng Jun