Reptiles

Reptiles are a group of cold-blooded vertebrates with scaly skin who mostly lay eggs on land. There are around 11,000 known species of reptile, split between four main groups: snakes and lizards, turtles and tortoises, crocodilians (crocodiles, alligators, gharials), and the rare tuatara of New Zealand. Reptiles dominated the land for over 150 million years during the age of dinosaurs and are still some of the most successful land animals today.

• Known speciesapprox. 11,000Most are lizards and snakes
• Largest reptileSaltwater crocodileUp to 6 m long, 1 tonne
• SmallestBrookesia chameleonapprox. 2 cm long, fits on a fingertip
• Longest snakeReticulated pythonUp to approx. 7 m
• HeaviestGalapagos tortoiseapprox. 250 kg
• Four main groupsSquamates, turtles, crocs, tuataras

What makes a reptile?

• Scaly skin: tough, dry, waterproof scales (not the same as fish scales).
• Cold-blooded (ectothermic): their body temperature follows the environment.
• Vertebrates: backbone with a complete skeleton.
• Lung-breathing: no gills, even in the water-living ones.
• Eggs with shells (mostly): laid on land, with a tough leathery shell to protect them.
• Four legs (or descended from four-legged ancestors): snakes have evolved away from this.

The four main groups

• Squamates (snakes and lizards): the biggest reptile group, with over 10,000 species. Includes geckos, iguanas, monitor lizards, chameleons, cobras, vipers, pythons and constrictors.
• Turtles and tortoises: shelled reptiles, found in both fresh water and on land. Around 350 species. The largest is the leatherback sea turtle (up to 2 m long).
• Crocodilians: the largest reptiles. 27 species of crocodiles, alligators, caimans and gharials, all top predators in tropical waters.
• Tuataras: just two species, found only in New Zealand. They look like lizards but are actually the last survivors of an entirely separate reptile order, the Rhynchocephalia.

Cold-blooded means clever, not slow

Reptiles are ectothermic, sometimes called "cold-blooded". That does not mean their blood is always cold; it means they cannot generate their own body heat from food the way mammals and birds can. So they have to manage their temperature by behaviour:

• Bask in the sun to warm up.
• Move into shade to cool down.
• Slow down or hibernate in winter when it is too cold to function.

This actually has some advantages. Because reptiles do not need to keep themselves warm, they can survive on far less food than a mammal of the same size. A 3-metre python can eat one large meal and not need another for months. A 3-metre lion needs to kill almost every day.

Crocodiles: the great survivors

Crocodilians are some of the most ancient creatures alive. They have hardly changed in 200 million years and lived alongside the dinosaurs. They survived the asteroid that killed off most of their dinosaur cousins because, as water-dwelling cold-blooded carnivores, they did not need as much food and could survive in damp habitats.

Crocodiles are extraordinarily powerful. The saltwater crocodile has the strongest bite of any animal on Earth, measured at over 16,000 newtons (about 3,700 pounds of force). For comparison, a lion's bite is around 4,500 N and yours is around 1,000 N.

Fact Most reptiles have a remarkable feature called parthenogenesis: some females can produce viable young without ever mating with a male. This is well-documented in several species of lizards (including some whiptails and Komodo dragons), some snakes, and certain turtles. In zoos, female Komodo dragons isolated from males have laid eggs that hatched into healthy young, all male. They are essentially natural clones of their mother.

Snakes: lizards without legs

Snakes are technically just lizards that lost their legs around 150 million years ago. Their long, flexible bodies are an adaptation that lets them squeeze into burrows, climb trees, swim, and swallow prey much wider than their own head (thanks to flexibly hinged jaws). Some snakes (like pythons and boas) still have tiny leftover hip bones inside their bodies, evidence of their four-legged ancestors.

There are around 4,000 species of snake. Most are harmless to humans; only about 600 are venomous and only around 200 of those are dangerous to people. The world's most venomous snake (the inland taipan of Australia) is so rarely encountered that there are no recorded human deaths from it.

Did you know? Some sea turtles (like the leatherback and green turtle) navigate across thousands of kilometres of open ocean to return to the exact beach where they were born, decades after hatching. They appear to use Earth's magnetic field as a compass. Scientists are still working out exactly how they remember the beach so precisely after so long.

Deeper dive: why no reptile got really big after the dinosaurs

For 160 million years (the Mesozoic Era), reptiles dominated the land and reached spectacular sizes. The biggest dinosaurs were 30+ metres long and weighed up to 100 tonnes. Mesozoic seas were ruled by huge marine reptiles like the 25-metre Mosasaurus. Even pterosaurs (flying reptiles) reached wingspans of 12 metres, far bigger than any modern bird.

Yet after the asteroid impact 66 million years ago, no reptile ever got that big again. Today's biggest reptile (the saltwater crocodile) reaches a respectable 6 metres, but this is a fraction of the largest extinct reptiles. Why?

Several reasons. First, the asteroid killed off the biggest reptile groups completely; the survivors were all small. Second, the niches that giant dinosaurs had filled were quickly taken by mammals and birds, which evolved much faster than the remaining reptiles. Within 10 million years of the impact, mammals were producing whales, elephants and rhinos. Birds were producing giant flightless predators. The reptile survivors were left as small to medium-sized animals, mostly in warm habitats where their cold-blooded lifestyle worked best.

The world today belongs to warm-blooded animals because mammals and birds can stay active in cooler climates, can sustain higher energy demands, and (in mammals) can suckle and protect their young. Reptiles thrive in warm parts of the world where their cold-blooded biology is an advantage, but the days of dragon-sized reptiles ruling the planet are over for now.

For other vertebrate groups, see mammals, birds, amphibians and fish.