Can Turtles outgrow their shells?

Box Turtle by Roger Smith, on Flickr

There are some questions in life that seem impossible to answer. What came first the chicken or the egg? (It was the egg.) How did the big bang happen? (It’s complicated, but we don’t know.) What happens after death? (Your body rots.) All reasonable questions with many late hours spent deliberating about the answer. But there is another – similarly perplexing – question that is a real head-scratcher: how do turtles grow? Do they grow inside their shell, does the shell grow as they do or does the shell grow later? Here’s how, terrapin lovers:

The Shell is the ‘house’ for a terrapin – it’s what they live in, their permanent ‘home’. It is also part of their skeleton (not like our homes). But more than that, their shell is their armor that is used to protect them from predators and shield them from weather extremities. It is made up of two parts: the carapace, the upper part that covers the turtles back, and the plastron, which covers the bottom half and the turtle’s belly. There is a bridge which connects the two together at the side of the turtle.

Interesting, yes – but now for the explanation. The carapace and plastron are made from bone (to be precise: about 50-60 rib and back bones in the upper part of the shell and a fusion of clavicle/collar bone and rib bones in the lower). And they are attached to the spine and the turtle’s internal bones. So as the turtle grows, the shell does too – in the same way babies are born and grow into adults – the bones grow as the body does. Although it’s odd to think of bones growing on the outside of the body!

So now for the really clever part! On top of the bone, most shells are covered with ‘scutes’ (or shields). They are overlapping pieces of keratin (a type of protein, the same material found in our fingernails), which helps aid repair when a turtle becomes injured. It’s a bit like a protective veneer, similar to our own body’s defense mechanisms. When a turtle’s shell becomes injured, the protein coating slowly grows across the damaged area (like a scab), acting as a framework for the shell to heal itself.

Moreover, as the turtle gets larger, the scutes on the shell shed or peel away to make way for new, larger ones, thus accommodating for the ‘new’ shell size. Hence a turtle can never ‘outgrow’ its shell.

Answer By Chloe Westley

Image Source: Box Turtle by Roger Smith, on Flickr

Article by Chloe Westley

October 23, 2014

Based in Manchester, UK, Chloe spends most of her time getting up close and personal with a zippy bit of kit called a Raman spectrometer. In between doing some high-brow research as part of a PhD, she follows tennis, cricket and Man United (unfortunately) and loves watching Suits, The Big Bang theory and Breaking Bad (obviously!).


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