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Why can amphibians, fish and even some reptiles regenerate limbs, while birds and mammals can’t? Researchers think they might have found a clue on the tip of the alligator’s tail.

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Go to to learn how you can  take your STEM skills to the next level. [♪ INTRO]. Regeneration is the ability of some  animals to regrow lost body parts.

It’s famously found in lots of  invertebrates, like sea stars and flatworms. And though vertebrates can regenerate  too, how much varies from group to group. Some fish and amphibians can  regenerate nearly perfect limbs, and many lizards can regrow semi-functional tails.

But other groups, like snakes, birds, and mammals, aren’t known to regenerate tails or limbs at all. And scientists are really interested  in working out why those groups have lost their ability to regenerate  over the course of evolution — because it might tell us more  about those groups in general. A 2020 study broke this topic wide open by  describing tail regeneration in alligators.

We don’t typically think of  crocodilians as regenerating animals. But there have been reports as far back as  the early twentieth century of alligators, caimans, and crocodiles  regrowing parts of their tails. However, the 2020 study is really the  first to explore the tissues involved.

Specifically, these scientists examined  three juvenile American alligators who had lost and regenerated the tips of their tails,  an average of fifteen centimeters per tail. These were not perfect recreations. The  scales and color patterns were different from the original tails, and while the  regrown tips had all-new blood vessels and nerves inside, they did not have bones.

A normal tail contains a row of vertebrae, but these tips were supported  by a rod of cartilage instead. So far, these are all patterns that also hold  true in lizards, another group famous for regrowing their tails. But there was one  big feature that the gators were missing.

Regenerated lizard tails also regrow  skeletal muscle. But the gators did not. Instead, their regrown tail tissue was a  lot like scar tissue in a healed wound.

This makes the gator tails a little  less complete than regrown lizard tails. It probably means the gators  couldn’t bend these tail tips, but could still use them as  a stiff paddle for swimming. Now you might think this doesn’t  sound all that surprising.

After all, gators and lizards are both reptiles. But crocodilians’ closest living  relatives are not lizards. They’re birds.

And birds cannot regenerate. This is where the gator study  could be really informative. The gators aren’t quite as good  at regeneration as lizards, but they are better at it than birds,  so they might be a sort of middle stage that can help us understand  how regeneration can be lost.

This contrast runs deep into the past, too. Like, there is one known fossil of  an ancient croc cousin with evidence of a regrown tail. But there’s  no known fossil evidence of limb regeneration in dinosaurs, the  lineage that gave rise to birds.

So if regeneration is a shared  feature across groups of reptiles, then somewhere along the line, birds’  ancestors seem to have lost this ability. And so did the ancestors of mammals. Regeneration of appendages is  common not only in reptiles, but also in fish and amphibians, so  it’s most likely an ancestral ability that existed early in vertebrate evolution.

It’s more likely that birds  and mammals lost the ability, than that all these other groups picked it up. The truth is, we don’t know for sure  why some groups have lost regeneration. Part of the answer is that regeneration  is a trade-off.

It can be very useful, but it takes a lot of energy and  resources away from other body processes. Some scientists have suggested that the  loss of regeneration might be related to the evolution of warm-bloodedness, or the  development of the specialized immune systems of mammals and birds, but  there are no solid answers yet. We’ll need more studies to know for sure, but this could tell us about what  makes birds and mammals different.

Or at least, why our bodies heal so differently. It’s wild to think that regrown alligator  tails can teach us about evolution. But if you want to learn more about how  our world fits together, there’s Brilliant.

Like their new Statistics I course, which  is all about how we use limited datasets to learn about the whole. We  can never know everything, so statistics is the math of  making really good guesses. Brilliant has loads of other courses, as  well as short challenges posted every day.

If you’re interested in learning more, you can get 20% off an annual premium  subscription at [♪ OUTRO].