Turtles are strange creatures with a unique body plan not seen in any other vertebrates. In fact, they are so different from other animals that scientists have only recently agreed where their most recent common ancestor fits into the tree of life. Through the 230 million years of turtle evolution, some have developed a very hard, protective shell for defense, whereas others have de-emphasized their shell in lieu of other defenses like biting, musking and hiding.
Protective Shells
All turtles have a bony shell that is covered with either scales or leathery skin. A primary defensive tactic for many turtles is to simply withdraw into their shell. Some species, like box turtles (Terrapene carolina) have evolved hinges that allow the shell to close completely, encapsulating the reptile safely inside. Other turtles have reduced shells that don't afford the same level of protection, and these species have to compensate for this loss with other defensive tactics.
Musk or Worse
Some turtle species, notably the Eastern musk turtle or stinkpot (Sternotherus odoratus), will defend themselves by releasing a foul musk if attacked. Two small glands located underneath the carapace produce the musk. Most musk turtles are small, rarely exceeding 6 inches, so the musk is a helpful deterrent to predators. Other species will void the contents of their cloaca if captured by a predator, which can dissuade all but the hungriest predators.
Biting
Though most turtles can bite, only a few use biting as a defense. Common snapping turtles (Chelydra serpentina), alligator snapping turtles (Macrochelys temminckii), softshell turtles (Apalone sp.) and a few other species are known to readily bite in defense. Many species that bite in defense have very long necks, which increases their defensive capabilities.
Hiding and Burrowing
Many species of turtles and tortoises simply try to escape a predator's attention by camouflaging with their surroundings. Aquatic turtles are even countershaded to protect them from predators above and below. (With countershading, an animal is darker above and lighter below; this generally makes it harder to see.) Sometimes hiding in plain sight doesn't provide enough safety and a turtle may choose to dig a burrow for protection. Gopher tortoises are one example of a turtle that is known to dig very long tunnels, which form an entire ecosystem that houses over 360 species. Gopher tortoise burrows offer so much protection that the turtles can survive wildfires by retreating to the depths of the burrow.
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