First of all it should be pointed out that there is confusion with language use when discussing turtles. American English tends to use ‘turtle’ for all chelonians. British English distinguishes between tortoises, turtles and terrapins, with turtle only being used for marine chelonians. Other forms of English differ from both. In this article British English is being used, although it not an entirely scientific classification it divides chelonians up into convenient groups based on habitat.
Turtles and their relatives are an ancient group. All belong to the order Testudine which separated from the ancestors of all other living reptiles during the early Carboniferous period around 300 million years ago. Birds surprisingly are actually are more closely related to other reptiles than they are. They are characterised by the shells that grow out from their ribs. Therefore they have effectively an endoskeleton (internal skeleton) and an exoskeleton (external skeleton).
Tortoises [land turtles] are mostly herbivorous chelonians that live on land. Representatives include the Greek tortoise and the enormous giant tortoises. Anyone who has heard of tortoises being kept as pets in Europe is probably familiar with them ‘hibernating’. This is not actually true hibernation (this only occurs in some small mammals and one species of bird) but is similar in that they slow down enormously to get through cold winters.
Giant tortoises used to roam Australia and Asia and inhabited several tropical archipelagos. Their range is now far more limited, restricted to the Galapagos (the Galapagos giant tortoise, Geochelone nigra with several subspecies) and the Seychelles with the Aldabra giant tortoise (Geochelone gigantea) occuring in very healthy numbers on the Aldabra atoll. They reach great ages, at least one hundred years and probably much longer in some cases and for chelonians great sizes – well over 4 feet.
The Greek tortoise (Testudo graeca – with a large number of subspecies) is a smaller tortoise widespread through Southern Europe, the Middle East and north Africa. As with other tortoises there are some surprises to the behaviour of these animals. They are not the placid, not very bright, slow moving creatures they first appear and their social behaviour is complex, sometimes very active, and noisy – they squeak. They establish a hierarchy and use various techniques to become top tortoise, including ramming each other. In captivity it has been known for males to kill each other, although this probably does not happen in the wild as there is more space and less compition.
Terrapins [fresh or brackish water turtles]. A term used to apply to various groups of chelonians that are semi-aquatic and live in lakes, rivers and ponds. Familiar terrapins include the diamonback terrapin (also called a terrapin in American English and the red-eared slider.
The diamondback terrapin (Malaclemys terrapin) inhabits swamps and marshes in the southern United States. They can survive in both fresh and salt water but prefer a mixture (brackish water) given the choice. Diamondbacked terrapins are carnivorous, eating small animals such as snails. Unfortunately they are threatened by habitat destruction and rising sea levels.
Not threatened but in some cases causing threats to other species is the red eared slider (Trachemys scripta). These are also naturally found in the southern US and have been kept as a pets worldwide. Escapees, or abandoned terrapins, have established themselves in various places internationally, often at the expense of local wildlife. They spend most of their time in the water leaving occasionally to sunbathe (for the heat) and to lay their eggs. They are fast, in the water and out, and like the Greek tortoise (and many other chelonians) can be surprisingly aggressive.
Turtles [marine turtles] are a distinct family and very competent swimmers, some migrating across the oceans. All are carnivores. They only have to deal with land in two cases, after hatching when they have to make a perilously journey down to the water and the annual trip made by females to lay their eggs.
There are 7 species of marine turtle still surviving, the Leatherback, Green, Olive’s Ridley, Kemp’s Ridley, Loggerhead, Flathead and Hawksbill turtles. They are adapted for an almost entirely aquatic life, with flippers, salt glands [with no access to fresh water it is essential the excrete the excess salt found in seawater] and the ability to dive for long periods of time.
Al marine turtles except the flathead are endangered. They all face similar problems, although some are more vulnerable than others. They get tangled in fishing gear, in plastic bags, their food sources are under threat, they are losing their breeding grounds, they encounter pollution, and yes they are still being hunted and their eggs are still being taken.