When biologist Kenro Kusumi opened the strange package in his mail, he found a pickle jar containing ethanol and a seemingly deformed alligator tail.
At his Arizona State University lab, Kusumi studies, among other things, how lizards regenerate their tails. So he’s seen his fair share of unusual animal appendages—but the one mailed to him in October 2017 stood out. The tail was discolored, its tip slightly forked and its scales abnormally small.
It looked like it had regrown after being severed, which intrigued Kusumi. The ability to regenerate tails has been documented in a number of reptiles, including geckos and iguanas. But it had never been reported in American alligators, which can grow 13 feet long and rely on their tails for balance and to propel themselves through water.
Analysis by Kusumi and colleagues determined that the tail had, in fact, regrown. The team also was able to study regrown tails from three other alligators. Their research, described in a paper published in November in Scientific Reports, found that the young gators can regrow their tails up to nine inches.
“We were excited. We knew we had something really cool here,” says biologist and co-author Jeanne Wilson-Rawls.
Alligators are now the largest animals known to regrow limbs. The discovery could help scientists understand how this ability evolved and functions—and possibly benefit research into regeneration-based medicine in humans.
A tale of two tails
All animals are capable, at some level, of repairing wounds through regeneration—but there’s a spectrum. Mammals, for example, can regenerate small amounts of skin, blood vessels, and minor nerves, but cannot replace limbs. Other animals, such as the axolotl salamander, can not only regenerate bone and organ tissue, they can replace lost limbs with near-identical precision.
Within reptiles, several species can regrow their tails, but these replacements aren’t always as good as the originals. For example, when a green anole jettisons its tails to evade a predator, it regenerates one that’s reinforced with cartilage instead of bone. Regrowing bone takes a lot more time and energy than regrowing cartilage, which is why it may be more evolutionarily favorable.
Kusumi’s team examined a total of four regenerated American alligator tails harvested from nuisance alligators that were euthanized by the Louisiana Department of Wildlife and Fisheries (which sent him the original “mysterious” package). All came from young animals.
To examine the tails’ anatomy, researchers used an x-ray machine, magnetic resonance imaging, and good old-fashioned dissection. They found that on the spectrum of regenerative ability, alligators fall somewhere between lizards and mammals.
“We saw a lot of similarities between regenerated alligator tails and lizard tails, including the presence of a cartilaginous structure, the scale patterning, and the [mismatched] coloration. We also saw the regrowth of peripheral nerves and blood vessels,” says Cindy Xu, the study’s lead author and a recent Ph.D. graduate of Arizona State who now studies tendon regeneration and repair at Massachusetts General Hospital.
“But what really surprised us about the alligator was that there was no regrowth of skeletal muscle,” Xu says. Skeletal muscle facilitates movement of the body through contraction and relaxation. The lack of this muscle was unexpected, she says, because lizards and even some mammals have the ability to regenerate this type of muscle.
While it’s unclear exactly why these alligator tails lacked skeletal muscle, the researchers suspect that it has to do with conserving energy.
“The regrowth of tissue is very energetically expensive,” Xu says. “If you’re putting all of your energy into regrowing a structure perfectly, you’re deviating energy away from other more essential processes such as developmental growth.”
Although Kusumi and his colleagues were the first to confirm the regenerative abilities of American alligators, experts such as Adam Rosenblatt, a biologist at the University of North Florida, have long suspected that young alligators could regrow their tails.
“I’ve seen what they’re talking about, in terms of tail regrowth, among alligators and other related species in the wild,” says Rosenblatt. “Alligators are really resilient. They are built to withstand damage at an immune system level and at a physical level.”
Although alligators are built tougher than most, as youngsters they are vulnerable, Rosenblatt says: “They’re on the menu for pretty much every predator that's out there,” including birds, raccoons, and even other gators, he says—and even the partial loss of a tail could jeopardize their ability to hunt and evade predators.
For an alligator, having the ability to regrow a tail that’s been lost “has obvious fitness benefits,” Rosenblatt says. (Related: How 3D printing helped Mr. Stubbs, the tailless alligator.)
Though only young gators have been seen regrowing tails, it’s possible adults can as well, Kusumi says: “Our collaborators have not seen a full-grown adult with a big regrown tail, but it doesn't mean that they're not out there.”
The next regeneration
The discovery that a reptile as large as a young alligator can regrow its tail has raised new questions about the prevalence of this trait among the 27 species of crocodiles, alligators, caimans, and gharials, Rosenblatt says.
“I would be curious to know how many of them have this ability,” he says. Determining that could also help scientists better understand the evolution of this phenomenon in birds and dinosaurs.
“The ancestor of both dinosaurs and alligators have this trait, but birds don't, so the question is, when did it get lost?” Kusumi says. “Did dinosaurs have the ability to regenerate their tails? It's still an open question.”
The discovery of tail regeneration in alligators could also help scientists studying how humans might regenerate tissue or lost body parts.
“Up until now, the [animals] used to study regeneration were relatively small compared to humans,” says Xu. “Since the alligator is large in size, it could help us understand how large body size and mass may have an influence on regenerative capacity.”