"Dinosaur scarfs crocodilian in brutal archosaur-on-archosaur takedown!"

Probably a bit heavy on the jargon, but otherwise that’d be a good headline for photos snapped by Danny Gilliam at central Florida’s Lake Apopka recently: They show a spear-billed great blue heron coolly dispatching a young American alligator. (Birds – evolved from theropod dinosaurs – and crocodilians are the two living archosaur lineages.)

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Image © Danny Gilliam
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Image © Danny Gilliam

A dramatic sight, but not unusual: Great blues are avid hunters of gator youngsters – hatchlings in particular, but, as Gilliam’s pictures prove, even somewhat bigger juveniles are vulnerable to these tall, fast-striking waders.

(It goes both ways, of course: A full-grown alligator could make a meal of an unwary great blue, though “unwary great blue heron” is a bit of an oxymoron. Gators are also known to stake out below waterbird rookeries to snatch fallen nestlings and, later, clumsy fledglings – though evidence suggests the birds may benefit overall from the gators’ presence, given it keeps nest-raiding raccoons at bay.)

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Image © Danny Gilliam
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Image © Danny Gilliam

It’s a good reminder that, while alligators are apex predators as adults, they’re preyed upon when pint-sized by a wide range of other animals: not just birds, but also fish, turtles, snakes, raccoons, bobcats – and certainly their own kind. Gators don’t hesitate to go the cannibalistic route.

It’s easy enough to imagine a big alligator biting off part of the wriggling tail of a little gator that thus manages to escape, albeit a bit stubbier. Well, new research sheds light on the ability of young alligators to regrow their tails, at least to some extent – a handy trick to have up your (scaly) sleeve in the swamps, marshes, and waterways of the southeastern U.S.

The study, a collaboration between scientists at Arizona State University (ASU) and the Louisiana Department of Wildlife & Fisheries just published in Scientific Reports, examined in detail the structure of several regrown tails from Louisiana alligators. While tail regrowth has been reported before among a number of crocodilian species – the American alligator included – this study marked “the first to characterise the anatomy of regrown tails in an archosaur,” lead author Cindy Xu said in a tweet summarising the findings.

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"Regrown tails are visibly identified by scale coloration, dense scale patterning, and lack of dorsal scutes," Xu explained on Twitter.

A variety of vertebrates boast regenerative powers, including some amphibians – not least the remarkable, fully aquatic axolotl salamander of Mexico, which can rapidly regrow lost limbs, tails, and certain internal tissues. Among amniotes – the vertebrate group encompassing reptiles, birds (which, at the broadest level of classification, fall in the reptile category), and mammals – only non-avian reptiles demonstrate the capacity to regrow major bodily structures. Lizards are the best-known in this regard: Some can even actively detach their tails to escape a predator, a “self-amputation” ability shared with certain salamanders.

The Scientific Reports study showed that juvenile alligators can regrow their tails to up to 18 percent of their total body length. The anatomy of the regrown gator tails shows structural overlap with the regenerated structures of some other cold-blooded critters as well as the healing-and-repair wound response found in mammals.

One intriguing result was the absence in alligator tail regrowth of the skeletal muscle produced in resprouted lizard tails. The regenerated gator tail – anchored by an endoskeleton of cartilage (also found in regrown lizard tails, but in a different position) and strung with blood vessels and nerves – instead boasted fibrous connective tissue rather like that found in a mammalian scar. A similar endoskeleton-and-connective-tissue blueprint defines regrown tails in the tuatara – a primitive, lizard-like reptile endemic to New Zealand – and regrown limbs in Xenopus frogs.

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The anatomical difference between the original and regenerated tail. Image © Arizona State University

“What makes the alligator interesting, apart from its size, is that the regrown tail exhibits signs of both regeneration and wound healing within the same structure,” Xu said in an ASU press release.

Aside from providing a zoomed-in look at tail regrowth in a much heftier reptile than those skittering little self-amputating lizards, the study has some interesting implications.

Some are evolutionary. Crocodilians and birds share a roughly 250-million-year-old common ancestor, but split not long thereafter in the early Triassic. (The croc-line archosaurs, or pseudosuchians, dominated that period, but took a real beating in the end-Triassic mass extinction event; crocodilian ancestors squeaked through when most other pseudosuchians winked out, which apparently opened the door for the bird-line archosaurs – dinosaurs, foremost among them – to really step into the spotlight. There’s your snapshot Mesozoic history for the day.)

There’s fossil evidence for tail regrowth in a marine crocodile from the Jurassic, the Scientific Reports paper notes, which suggests the trait is an ancestral one among crocodilians. Thus far there’s been no indication that birds – or non-avian dinosaurs – show the same regenerative capability.

“Our finding that alligators have retained the cellular machinery to regrow complex tails while birds have lost that ability raises the question of when during evolution this ability was lost,” said ASU professor and co-senior author Kenro Kusumi in the press release. “Are there fossils out there of dinosaurs, whose lineage led to modern birds, with regrown tails? We haven’t found any evidence of that so far in the published literature.”

Delving into alligator tail regrowth at the anatomic and histological levels also has potential bearing for human beings. “If we understand how different animals are able to repair and regenerate tissues,” the press release quotes coauthor Rebecca Fisher, “this knowledge can then be leveraged to develop medical therapies.”

Kusumi is one of the principal investigators in the Kusumi-Dolby Lab at ASU, which, among other research areas, looks at regeneration in American alligators and green anoles – at least partly to explore those potential medical ramifications. “We are working to identify conserved genes and pathways involved in this remarkable regenerative process,” the lab’s website states, “with the ultimate goal of reactivating these processes in humans for medical therapies.”