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I’ve lost count of the number of times people have asked me if turtles (or tortoises, for that matter) outgrow their shells and need new ones.
This is actually a great question, especially for anyone who has ever been lucky enough to watch hermit crabs battling it out for a bit of prime “found” real estate.
Truly, I’m not sure why I never wondered about this myself, although maybe it was because I grew up keeping turtles and watching both them and their shells get progressively bigger without the insides ever outgrowing the outsides.
But it is still totally a fair question.
A turtle’s shell, box turtle or otherwise, is actually a type of skeleton that is a lot like a person’s spine. As such, the shell – both the upper part (carapace) and the lower part (plastron) – is part of the turtle’s whole body, not an extra “addition” tacked on after the fact for protection or shelter or something like that.
The vertebrae of the turtle’s spine are partially fused to the carapace, specifically the middle section of vertebrae. The top section of vertebrae are free to allow the turtle to move its head and neck. The bottom section of vertebrae are similarly un-fused to allow the turtle to move its tail freely.
The carapace handles keeping all the vertebrae connected and talking to one another, while the plastron is responsible for providing a point of contact for the clavicles (shoulders) and rib cage.
Then the exterior portion of the shell is covered in protective shingles called “scutes.” The scutes are kind of like human fingernails, only stronger and thicker. They provide protection to the shell itself.
The most important thing to remember here is that the shell is a living, breathing part of the turtle or tortoise – just like any other organ or tissue. The shell is sustained with oxygen, blood and nerves. If bitten, cut, compressed or scraped in whole or in part, the turtle’s shell may crack, split, bleed, rot or worse.
A turtle’s shell will continue to grow with its occupant all throughout the turtle’s life. At some point, both turtle and shell will stop growing in an “I’m trying to get bigger” sort of way and reach the turtle’s full adult shape and size.
But even then, the turtle’s shell is always shoring itself up and replenishing its keratin shield, just like a person’s skin is always replenishing and renewing itself.