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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.
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