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Cornered by a dangerous predator, a gecko can self-amputate its still contracted tail, creating a fleeting moment of distraction – an opportunity for the lizard to escape with its life.
Small reptiles such as geckos and skinks are well known for this remarkable ability to sacrifice themselves and then quickly regrow their tails. Now, to the surprise of scientists, it turns out that even much larger alligators can regrow theirs. But only when they are young.
Juvenile American Alligators (Alligator mississippiensis) can regrow up to 18% of their total body length. This is approximately 23cm or 9 inches in length.
The really interesting thing is that this regrowth seems to happen through a mechanism that we have never seen before.
By analyzing and dissecting tail regrowth, researchers at Arizona State University (ASU) found that alligators do it very differently from other animals we know that can regenerate their appendages.
Regrown tails are visibly identified by scale coloration, dense scale pattern, and lack of dorsal shields. pic.twitter.com/adFdXs6rIY
– Cindy Xu (@xucindy) November 19, 2020
When it comes to regrowth of body parts, amphibious axolotls are the champions of regeneration among terrestrial animals with internal skeletons.
If injured, they can reform a segmented skeleton, complete with muscles that differ along their height, distinguishing from top to bottom.
Regrown lizard tails do not have a segmented skeleton, but lizards reform their muscles, although they have the same uniform appearance, with no variation in the structure of the top compared to the bottom.
This may be because appendage regeneration is physiologically expensive and in smaller lizards it has been shown to reduce the overall growth rate.
Alligators, apparently, don’t even bother with regrowing muscles.
“Clearly there is a high cost to produce new muscle,” said ASU animal physiologist Jeanne Wilson-Rawls.
The team believes that even an extra bit of muscle-free tail should give these dangerous predators an edge in their waterlogged homes.
Unlike lizards, they cannot self-amputate: their tail loss is usually the result of trauma inflicted by territorial aggression or cannibalism from larger individuals.
Damage from human interactions, such as damage to the motor blade, was also recorded.
The anatomical difference between original and regenerated tail. (Arizona State University)
Connective tissue alligators replace skeletal muscle with is more akin to wound repair you would see in tuatara or mammalian wound healing, the team explains.
“The regrown alligator tail is supported by an unsegmented cartilage tube rather than bone … it lacked skeletal muscle and featured scar-like connective tissue populated with nerves and blood vessels,” explained ASU cell biologist and lead author by Cindy Xu research Twitter.
“The tails regrown from young American alligators show both regenerative and wound healing characteristics.”
But the regrowth of cartilage, blood vessels, nerves, and scales is similar to what is seen in lizards.
“Future comparative studies will be important to understand why regenerative capacity varies between different groups of reptiles and animals,” Xu said.
It may also take much longer to regrow the missing pieces. While skinks can do this in as little as six months, a related crocodile, the black caiman (Melanosuchus niger) takes up to 18 months to reform the tail.
Alligators are an ancient lineage of reptiles, which shared a common ancestor with birds around 245 million years ago, when non-avian dinosaurs ruled the Earth.
There is fossil evidence of an ancient Jurassic period crocodile that also had a regenerated tail.
This “raises the question of when during evolution this ability was lost. Are there fossils out there of dinosaurs whose lineage has led to modern birds with regrown tails?” ASU biomedical scientist Kenro Kusumi asks questions.
“So far we have not found any evidence of this in the published literature.”
The team notes that so far they have only been able to observe the end product of tail regenerations in alligators.
Since it is an endangered species, further studies on how this process works could be challenging, but it could provide some useful information.
“If we understand how different animals are able to repair and regenerate tissues, this knowledge can then be harnessed to develop medical therapies,” said Rebecca Fisher, ASU anatomist.
This research was published in Scientific reports.
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