TurtleTurtles, Tortoises, and Terrapins are reptiles of the Order Testudines (all living turtles belong to the crown group Chelonia), most of whose body is shielded by a special bony or cartilaginous shell developed from their ribs. The Order Testudines includes both extant (living) and extinct species, the earliest known turtles being from around 215 million years ago, making turtles one of the oldest reptile groups, and a much more ancient group than lizards and snakes. About 300 species are alive today; some are highly endangered.
Like other reptiles, turtles are ectothermic (or "cold-blooded"). Like other amniotes (reptiles, dinosaurs, birds, and mammals), they breathe air and don't lay eggs underwater, though many species live in or around water. The largest turtles are aquatic.
Ecology and life historyEven though many spend large amounts of their lives underwater, all turtles and tortoises are air-breathing reptiles, and must surface at regular intervals to refill their lungs with fresh air. They can also spend a lot of their lives on dry land. Aquatic respiration in Australian freshwater turtles is currently being studied. Some species have large cloacal cavities that are lined with many finger-like projections. These projections, called "papillae", have a rich blood supply, and serve to increase the surface area of the cloaca. The turtles can take up dissolved oxygen from the water using these papillae, in much the same way that fish use gills to respire.
Turtles lay eggs, like other reptiles, which are slightly soft and leathery. The eggs of the largest species are spherical, while the eggs of the rest are elongated. Their albumen is white and contains a different protein than bird eggs, such that it will not coagulate when cooked. Turtle eggs prepared to eat consist mainly of yolk. In some species, temperature determines whether an egg develops into a male or a female: a higher temperature causes a female, a lower temperature causes a male. Large numbers of eggs are deposited in holes dug into mud or sand. They are then covered and left to incubate by themselves. When the turtles hatch they squirm their way to the surface and make for the water. There are no known species wherein the mother cares for the young.
Sea turtles lay their eggs on dry sandy beaches, and are highly endangered largely as a result of beach development and over hunting. Immature sea turtles are not raised by either parent. Turtles can take many years to reach breeding age. Often turtles only breed every few years or more. Researchers have recently discovered a turtle’s organs do not gradually break down or become less efficient over time, unlike most other animals. It was found that the liver, lungs and kidneys of a centenarian turtle are virtually indistinguishable from those of its immature counterpart. This has inspired genetic researchers to begin examining the turtle genome for longevity genes
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