Can Box Turtles Close Their Shells? The Hinged Plastron

How does the box turtle’s hinge work?

A turtle’s shell has two parts: the domed upper shell, called the carapace, and the flat lower shell, called the plastron. In most turtles the plastron is joined rigidly to the carapace by a bony bridge on each side. In box turtles there is no rigid bridge. The two shells are connected by tough, flexible tissue instead, so the plastron can move.

The plastron itself is divided into a front lobe and a back lobe by a hinge of connective tissue between two of its bones. When the turtle is threatened, it first draws its head, legs and tail inside the shell. Then muscles pull both lobes upward, like two doors folding shut, until their edges press against the rim of the carapace. In a healthy adult eastern box turtle the seal is tight enough to leave almost no gap anywhere around the shell.

The domed shape helps. A high carapace leaves room inside for the folded legs and head, and the round shape is hard for a predator’s jaws to grip.

How good is the box as protection?

For an adult box turtle, the closed shell is an excellent defense. Foxes, coyotes, skunks and many other predators simply cannot get a grip on the smooth, closed box and give up. This is one reason adult box turtles survive so well and can live for many decades.

The box is not perfect, though:

• Young turtles are vulnerable. Hatchlings have soft shells and a hinge that does not seal tightly. For their first few years they rely more on hiding than on closing, and many are eaten by raccoons, snakes, birds and other predators.
• Strong chewers can still do damage. Raccoons and dogs sometimes gnaw the edges of the shell or a leg that is not fully tucked in.
• It does not stop cars, mowers or fire. Closing up is useless against traffic, lawn mowers, farm machinery or wildfires, which are among the main causes of death for adults today.
• A very large or heavy turtle may not close fully. In some individuals the plastron does not quite fit the carapace, leaving a small gap.

Closing the shell also has costs. While shut, the turtle cannot see, move or feed, so box turtles usually open up within minutes once danger seems to have passed.

What other turtles can close their shells?

Box turtles are not the only turtles with hinges. Hinged shells have evolved several times in different turtle families:

The African hinge-back tortoises are a striking case. Their hinge is in the carapace, not the plastron, so the back of the upper shell can drop down to protect the hind legs and tail.

Which turtles cannot hide at all?

At the other end of the scale, some turtles cannot even pull their head inside the shell. Sea turtles have streamlined shells built for swimming and cannot retract their head or flippers. Snapping turtles have a small, cross-shaped plastron that leaves the legs and neck exposed, which is why they defend themselves by biting instead. Side-necked turtles of South America, Africa and Australia fold their neck sideways under the rim of the shell rather than pulling it straight back.

Myths about box turtle shells

Myth: a closed box turtle is dead or sick. No. Closing up is a normal reaction to being approached or picked up. Given time and space, the turtle will open and walk away.

Myth: a box turtle can come out of its shell. No turtle can. The shell is part of the skeleton, fused to the ribs and backbone, as explained in Can a turtle live without its shell?

Myth: box turtles are tortoises. Despite living on land, box turtles belong to the pond turtle family. Their hinged plastron is one of the features that sets them apart; see where box turtles live for their woodland habitats.

How to tell if a turtle is a box turtle

If you find a small land turtle in the eastern United States with a high, domed shell, a hinged plastron and orange or yellow markings, it is very likely an eastern box turtle. Look, take a photo and leave it where it is: box turtles live in small home ranges and do best when they are not moved.