Understanding Turtles and Tortoises
When you think of turtles, do you think of the tiny quarter or half-dollar-sized turtles that used to be sold in pet stores (and in some places still are)? If so, you're not alone. That image is the one that comes to mind when many people think of turtles. However, those tiny little turtles (most of whom died shortly after their purchase) are only one of many different types of turtles and tortoises, many of which can be long-lived, healthy, hearty pets.
So what's the difference?
Turtles and tortoises are known as chelonians, from the Greek word for "tortoise," chelona. Chelonian refers to all turtles and tortoises, no matter whether they live in the ocean, in fresh water, or on land.
- The term turtles applies to chelonians that live in or around water. Sea turtles, for example, never leave the ocean except to lay their eggs. Other turtles are more or less aquatic, depending on their species. The sliders, mud, bog, and leaf turtles are all found close to fresh water. Terrapins are aquatic turtles that people frequently eat. Many terrapins live in and close to brackish salt water. Box turtles are primarily terrestrial (land roving) but are almost always found within walking distance of water. See Figure 1 for an example of a turtle.
- The term tortoise refers to chelonians that live on land and rarely venture into water except to drink or bathe. Tortoises range in size from tiny little guys weighing less than 1 pound to gigantic, weighing over 600 (or more) pounds. Size aside, tortoises have many things in common. They are primarily herbivores (plant eaters), although many will scavenge, given the chance. Tortoises also usually have hard shells, often with high domes, to help protect against predators. Take a look at Figure 2.
Although more than 250 different species of chelonians exist, many are impossible to keep as pets. Think of the size aquarium and the filtration system you would need to keep a 200-pound green sea turtle! However, many species of turtles and tortoises do quite well in captivity and make good pets.
Life in the original mobile home
Turtles and tortoises are in the reptile family, which means that they are ectothermic, or cold-blooded. Cold-blooded animals rely on external heat sources, such as warm ground, hot rocks, or sunshine, to warm their bodies. Turtles are the original sun worshippers and can often be found sleeping on rocks or logs, soaking up the sun's rays.
All turtles and tortoises share a similar characteristic: their shell. No other animal on Earth has a shell quite like this. A turtle's shell is a boxlike exoskeleton (a word that refers to a part of an external skeleton) with the spine and ribs fused to the top shell. These bones are, in fact, a part of the carapace, or top shell. The shell itself is made of bone, and the outer covering of the shell is made of keratin, much like human fingernails.
Each shell is made up of sections called scutes. As the turtle grows, new layers of keratin are formed around the outer edges of each scute, looking much like the growth rings of a tree. Some people count each of these rings in an effort to tell how old a turtle may be. This can give only a rough idea, however, because just like a tree, if food is plentiful a turtle may have two growth spurts per year, or in a bad year may grow very little. Also, as a turtle gets older, the shell becomes worn and smooth, and the rings may be difficult to see.
The type of shell and the degree of protection offered by the shell are based on the turtle's lifestyle and habitat. Sea turtles, for example, have a light, streamlined shell covered by a leathery skin. Freshwater turtles usually have a hard shell, but in some species, it's too small to protect the entire body. Land turtles and tortoises that rely on the shell for protection have a hard, domed shell.
Many turtles and tortoises can pull in all four legs and their head so that the shell protects them from predators. When a turtle pulls its head into its shell for protection, its neck either folds to the side or into a vertical S shape, and the skin of its neck bunches up — hence the name turtleneck for shirts and sweaters with bunched-up necks.
With many species, the outer skin of the legs is hard, rough, and, in some tortoises, armored, giving the turtle even more protection. The skin on the legs of tortoises is hard, with scales made of keratin protecting it. Some of the keratin scales are quite large and pronounced; on some species, the scales create spurs or spikes that help protect the tortoise from predators and also help desert tortoises retain water. Because aquatic turtles usually dive into water when threatened, their skin is much softer with fewer protective scales. Most turtles and tortoises have five toes (although they sometimes have four or as few as three) with hard nails on the toes. Aquatic turtles have webbing between their toes.
Some turtles and tortoises can even close their shell, giving additional protection. Box turtles (hence their name) have a hinge across the bottom shell (the plastron). This hinge can close both front and rear, hiding the turtle completely inside. The muscles holding the shell closed are incredibly tough, and after the hinge is shut, you can't open it without harming the turtle. A type of tortoise can close its shell, too, although not as completely as the box turtle. Hinge-back tortoises have a hinge across the top of their top shell (the carapace) and can close in their back legs, protecting them.
Although the shell, made of bone, seems to be the ultimate protection, it is vulnerable. Predators can chew and break the shell. A larger bird of prey can pick up a small turtle and, flying high, drop the turtle on rocks below, breaking the turtle's shell like an egg. The shell can protect a tortoise from a small, fast-moving wildfire, but larger, hotter fires will kill a turtle or tortoise caught in it. Domestic dog, Fido and Fluffy, have been known to treat turtles like chew toys, with disastrous results!