Reptiles
Turtle
Testudines · also called chelonian
A turtle wears its skeleton on the outside. The shell that everyone knows is not armour it puts on but its own ribs and backbone, flattened and fused into bony plates, with the shoulders and hips tucked inside the rib cage — an arrangement found in no other animal. That strange body plan appeared more than 200 million years ago and has scarcely changed since.
Overview
Turtles are the order Testudines, around 360 species, split loosely into the sea turtles, the land tortoises, and the many freshwater turtles and terrapins. All share the fused shell, a toothless beak, and egg-laying on land. They range from the 8 cm bog turtle to the leatherback, which reaches 2 m and 700 kg. Turtles live on every continent except Antarctica and throughout the warm oceans.
A skeleton turned inside out
The domed carapace is built from about 50 bones — broadened ribs and vertebrae — usually covered by horny scutes. The flat underside, the plastron, protects the belly. Because the ribs can no longer move to draw breath, a turtle breathes by pumping its internal organs against its lungs with sheets of muscle, and many freshwater species top up their oxygen through the skin and the lining of the throat and cloaca, which lets them stay underwater for hours or overwinter at the bottom of a pond.
Slow lives, temperature-set sex
Turtles grow slowly, breed late, and live a long time — over a century for the big tortoises. In most species the temperature of the nest during a middle window of incubation decides whether an egg becomes male or female, with warmer nests producing females. A female may lay several clutches a year and then leave; there is no parental care.
Deep time, and a modern crisis
The turtle body plan is ancient and conservative, which is why turtles are often called living fossils. Yet roughly half of all turtle species are now threatened, wrecked by the food and pet trade, drained wetlands, developed and lit beaches, fishing bycatch, and warming sand. A slow-breeding animal cannot replace its adults quickly, so once a turtle population starts to fall it is very hard to bring back.
Status
Turtles as a group are Not Evaluated, but the trend is decreasing and the order is one of the most endangered among vertebrates. Several species, including the Yangtze giant softshell turtle, are down to the last one or two known individuals.
Turtle: common questions
What is the difference between a turtle, a tortoise, and a terrapin?
They are all turtles in the scientific sense, members of the order Testudines, and the other words are informal groupings. In British usage turtle tends to mean the sea species, tortoise means the land species with high domed shells and stumpy elephant-like feet, and terrapin means small freshwater species. In North American usage turtle is often used for the whole group, tortoise still means the land species, and terrapin refers to a few specific brackish-water species such as the diamondback terrapin. None of these names marks a clean branch on the family tree, so the safest general term for any of them is simply turtle.
Can a turtle really not leave its shell?
No, and the idea that it can is one of the most common misunderstandings about them. The shell is built from the turtle's own ribs and backbone, broadened and fused into bony plates, and covered either by horny scutes or by leathery skin. The turtle's spine runs along the inside of the domed top, and its shoulder and hip bones sit within the rib cage rather than outside it. A turtle can no more crawl out of its shell than you could step out of your rib cage. Damage to the shell reaches living bone and blood vessels and can be serious or fatal.
How long do turtles live?
Longer than almost any other backboned animal on land. Small pond turtles often live 30 to 50 years, box turtles can pass 80, and the large tortoises of the Galapagos and Aldabra routinely live beyond 100, with well-documented individuals over 175. Sea turtles are harder to age but are thought to live 50 to 100 years or more, reaching breeding age only at 20 to 50. This slow pace of life is one reason turtles are so vulnerable now, because a population cannot quickly replace adults lost to hunting, roads, or fishing gear.
Why are so many turtles endangered?
Several pressures hit at once, and turtle biology makes recovery slow. They are collected in huge numbers for food, traditional medicine, and the pet trade, especially in Asia, where whole species have been eaten close to extinction. Their nesting beaches and wetlands are drained, built on, and lit up. Sea turtles drown in fishing nets and on longline hooks and swallow plastic bags that look like jellyfish. And because a turtle takes decades to mature and temperature sets the sex of the young, populations respond to all of this slowly and are increasingly skewed towards females as beaches warm.