Turtle

C

Limb Structure

Turtle limbs, used for locomotion, are adapted to their particular habitat. Land-dwelling tortoises have strong, thick legs to support their heavy shells. They typically move at slow speeds of less than 0.5 km/h (0.3 mph). The gopher tortoise of the American southeast has flattened front limbs that function as scoops for digging the deep burrows in which it lives.

Aquatic turtles move either by swimming or by walking on the bottom of a body of water, such as a pond. Many aquatic turtles, including painted turtles, sliders, and soft-shelled turtles, have long toes connected by webbing. These turtles spread out their toes to obtain a large surface area for pushing against the water, which helps them to dive and to swim quickly to escape predators. Soft-shelled turtles are the fastest freshwater turtles and can swim faster than most fish.

Sea turtles are the most specialized swimmers of all turtles. Their forelimbs are modified into flipper-shaped blades. These turtles practically fly through the water, using their hind feet primarily as rudders. Despite turtles’ reputation for being slow-moving animals, sea turtles can achieve swimming speeds of more than 30 km/h (19 mph), a speed an elite sprinter might reach for a short distance on land.

D

Tail

Most turtles have rather short tails, but the Asian big-headed turtle has an extremely large, muscular tail covered with protective scales that it can use to climb steep rocks and logs in mountain streams. While climbing, these turtles press their tails onto the climbing surface to support their weight. The American snapping turtles also have very long tails. Among most turtle species, males tend to have longer, thicker tails than the females.

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E

Physiology

Turtles breathe air with lungs, as do other reptiles and all land-living vertebrates. Since turtle ribs are part of their shell, turtles cannot move their ribs in and out to expand or deflate their lungs. Instead, turtles alternately expand and contract various groups of muscles, including those in their abdomen and above their front and hind legs, to change the amount of space within the shell. When these muscles expand, less space is available and the lungs are compressed, permitting the turtle to exhale. A turtle takes in air as these muscles contract to provide more space into which the lungs can expand. Many aquatic turtles remain submerged in water for periods of several hours to several days—and for many months during winter hibernation. Many turtles are able to take in oxygen from water through the linings of the mouth, throat, and an internal chamber called the cloaca, as well as through the skin. However, when they are active, aquatic turtles need to rise to the surface periodically to breathe air with their lungs.

Like most reptiles, except for crocodiles and their close relatives, turtles have a heart with three chambers. A turtle’s heart operates almost as if it had four chambers, however, because one of its chambers, called the ventricle, has an incomplete divider, or partition. This divider helps prevent the blood that has received oxygen from the lungs and is ready to circulate through the turtle’s body from mixing with blood that is depleted in oxygen and needs to travel to the lungs for a fresh supply.

In a turtle’s digestive system, food passes from a turtle’s mouth through a tubelike esophagus to the stomach, where digestion begins. Food passes from the stomach into the intestine, where nutrients can be absorbed into the bloodstream. Wastes from the intestine are emptied into the cloaca, from which the wastes leave the turtle’s body. Turtles also have a urinary system, which filters waste products from the blood and excretes them through the cloaca.

Turtles have a central nervous system and a well-developed brain. They have keen senses that they use to interpret their world. Most turtles have sharp vision and can recognize patterns and colors. The eyes of sea turtles are adapted for seeing underwater, but they can see only short distances when they are on land. Most turtles, including sea turtles, have a good sense of smell. Both the shells and the skin of turtles are sensitive to touch. Turtles’ ability to hear sounds that travel through the air is limited to low frequencies, but they can perceive vibrations transmitted through the ground or water.

III

Turtle Behavior

A

Temperature Regulation

Like all reptiles, turtles are ectothermic, or cold-blooded, animals that control their body temperature by moving into or out of warm or cool places. Unlike endothermic, or warm-blooded, animals, such as mammals, turtles do not generate heat in their bodies from digesting food. Leatherback sea turtles are an exception, as they can produce internal heat in their muscles while swimming, and their huge size, together with a thick layer of oily fat under their skin, helps them retain this heat. As a result, they can range into such extremely cold areas as the North Sea. Other marine turtles can survive only in warmer waters.

Many activities of turtles help regulate their body temperatures. Aquatic turtles often leave the water to bask in the sun on logs or rocks or along the banks of lakes and streams to warm their bodies. In winter, turtles that live in seasonal climates enter a dormant state resembling hibernation. In this state, called torpor, the turtles stop feeding and their oxygen needs become very low. Aquatic turtles usually remain underwater in winter, relying on their ability to obtain oxygen from water through their skin, throat linings, and sacks within their cloaca. In contrast, land-living turtles burrow into the soil. Eastern box turtles may spend the winter in shallow burrows; they are able to survive partial freezing of their body fluids for several days. Young painted turtles often spend their first winter in an underground nest. These infant turtles have the ability to survive sub-freezing temperatures for several days.

B

Feeding

Most turtles are omnivores—animals that eat both plants and other animals. Most of the smaller pond-, marsh-, and stream-dwelling species, including the American painted and slider turtles and the European pond turtle, eat insects, snails, worms, minnows, and tadpoles, as well as aquatic plants. The terrestrial box turtles of North America commonly eat small animals, but when ripe berries are available, they may eat so many that they become too fat to close their hinged shells.

Some turtles, such as the South American side-necked turtles and American cooters, are largely herbivorous, eating soft water plants and fruits that fall into the water. Tortoises, which move too slowly to capture most types of animals, are almost entirely plant and fruit eaters, although many species scavenge the remains of dead animals on occasion.

Many species of aquatic turtles are strictly meat-eaters, or carnivores, and some of them specialize in eating certain types of prey. Malaysian snail-eating turtles and American map turtles, especially the larger females, eat mostly snails and clams. These turtles have bony ridges in their mouths that help them crush the shells of their prey. The alligator snapping turtle attracts fish to its open mouth by wriggling an appendage on its tongue that looks like a worm.

The matamata, a side-necked turtle from South America, has an unusual strategy for catching fish. Attached to the matamata’s head and neck are numerous flaps that move in the water, possibly causing fish to mistake them for worms or other food. These flaps contain abundant nerve endings that alert the matamata to disturbances in the water, including those caused by the approach of prey. When a fish comes within range, the matamata lunges with its jaws, sucking the fish into its mouth in a rush of water.

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