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Box Turtles
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Box turtles get their name from their ability to completely close their hinged shells. Box turtles (family Emydidae, genus Terrapene) have a well-developed hinge in their lower (belly) shells--the lower shell is called the plastron. Box turtles have a single hinge across the plastron. The center portion of the plastron is connected to the upper shell--called the carapace--by a bone bridge on each side. The hinge crosses the belly in front of the bridge, and allows the turtle to pull in its head and legs and completely close the shell. This behavior is very effective against terrestrial predators such as raccoons and coyotes, and is an important advantage for box turtles because they are a terrestrial turtle species.
Chinese box turtles (family Emydidae, genus Cistoclemmys) also have a single, well-developed hinge across their plastron, and are also able to close their shell completely.
Blanding's Turtles
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Blanding's turtles are able to partially close their shells. Blanding's turtles (family Emydidae, genus Emydoidea blandingii) have a single hinge across their plastron, which allows them to close their shells somewhat. Blanding's turtles are semi-aquatic, spending much of their time in the water, and the ability to close their shell completely would not provide the same advantage as for a terrestrial species.
Mud Turtles
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Mud turtles (family Kinosternidae, genus Kinosternon) have two hinges across the plastron, similar to the arrangement seen in box turtles. However, their small plastron cannot cover their legs completely, and therefore they cannot completely close their shells. Mud turtles are primarily aquatic, so the ability to close their shells partially is only an advantage when they are overwintering or nesting in terrestrial habitats.
Hinge-backed Tortoises
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Hinge-backed tortoises (family Testudinidae, genus Kinixys) live up to their name. Unlike other turtles, which have hinges on the plastron, turtles in this genus have hinged carapaces (upper shells). This hinge allows the hinge-backed tortoises to close their shells over their hind legs and tails to protect them against predators. The hinge does not allow them to close the front of their shells--instead, they use their scaly front legs to cover and protect their heads.
Other Turtles
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Several other species of turtles, including gopher tortoises (family Testudinidae, genus Gopherus polyphemus) and musk turtles (family Kinosternidae, genus Sternotherus) may also have weakly hinged plastrons.
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What Is a Turtle Hinge?
A turtle hinge is a specialized adaptation of the turtle's anatomy that provides it some additional protection from predators. The shell of the turtle is made up of many smaller bones that are usually tightly bound together, forming a rigid structure--these bones are covered by smaller plates called scutes. In a few turtle species, some of the bones are only loosely bound by connective tissue, creating a movable hinge.