Dear Dr. Universe: Why can’t sea turtles pull back into their shells? I was at the Saint Louis Zoo and they said tortoises and other turtles can pull back into their shells. — Tyler, 7, Missouri

Dear Tyler,

There are more than 350 species of turtles that live on our planet. While many of these turtles live on land, others live in the sea.

Depending on where a turtle lives, its body will be a little different. Turtles have adapted to live in different environments and they have traits that help them survive in those places. As you mentioned, a lot of turtles and tortoises on land have the ability to pull, or retract, their head into their body.

Because turtles move really slow, the ability to retract their head offers them protection from more speedy predators. Their hard shell can help shield them from danger and prevents them from becoming someone’s dinner.

My friend Abbey Hayes is a graduate student at Washington State University who mainly studies insects, but she’s also studied green sea turtles off the coast of Mexico. Green sea turtles are just one of seven different species of sea turtles. The others are hawksbill, Kemp’s ridley, leatherback, loggerhead, olive ridley and flatback.

Sea turtles have certain traits that help them live in the sea. Instead of having an arched shell like a land turtle, their shell is flattened to help them move more quickly and easily through the ocean. They also have longer legs than land turtles and have evolved paddle-like flippers. Hayes said that sea turtles just don’t have as much space inside their bodies to retract their limbs and heads.

I also found out that a turtle’s hard shell is actually made up from about 50 bones. Scientists used to think that turtle shells evolved from scales, but some scientists now think those shells evolved from ribs. One sea turtle called the leatherback is a kind of sea turtle that doesn’t have a hard shell, but instead a thin layer of rubbery skin that has thousands of tiny bone plates.

It turns out that a lot of these sea turtles are threatened or endangered because of human actions. Because sea turtles can’t pull their heads into their bodies, they can be more vulnerable to predators and other kinds of threats, such as getting tangled up in fishing nets and the trash people toss into the sea.

And while it may be tempting to pet a sea turtle if you see one, Abbey reminded me that it is important to respect animals, especially in their natural habitat. She also told me that if you go swimming in the ocean, it is important to pick reef-safe sunscreens that are safe for animals like corals and turtles.

Instead of hiding in a shell, a sea turtle has the ability to move with speed and also has a lot of agility that lets it quickly escape predators, like whales or sharks. These skills can help them stick their necks out and survive in the ocean.

Sincerely,

Dr. Universe

“Ask Dr. Universe” is a science-education project based out of Washington State University. Send questions to Dr.Universe@wsu.edu.