What is a reptile?
Reptiles are vertebrates (they have a backbone). They have scaly skin and toes with claws. Reptiles have lungs and breathe air. Reptiles are hatched from eggs. Most reptiles depend upon their surroundings for warmth. Living reptiles include: snakes and lizards; turtles; the crocodilians; the tuatara and birds (dinosaur's descendants).
Reptile Evolution. Evolving from amphibians, the first reptiles appeared on Earth about 340 million years ago. Unlike amphibian eggs, reptile eggs are shelled and amniotic. The shell and amniotic fluid prevent the developing embro from drying out. Thus, reptile eggs do not need to be laid in or near water. Early reptiles had an advantage over amphibians of being able to adapt to a variety of habitats on an ever changing earth. Today, reptiles are found on land, in salt and fresh water and in the air (birds) from cool areas to tropics to deserts.
Reptile Eggs and Young
Reptile eggs and young. Reptile eggs are generally soft and flexible. But some, like crocodiles, are hard. A yolk provides food for the developing embryo. Moisture and oxygen are absorbed through the porous shell. Animals whose eggs develop outside the mother's body are called oviparous. In some species of reptile the egg develops inside the body of the mother and the young are born alive (ovoviviparous). Animals whose unborn are nourished directly from the mother's bloodstream are viviparous.
Although some reptiles tend their nest, most reptiles do not take care for their eggs. Young reptiles are able to feed and care for themselves.
Reptile Skin. Reptiles have dry, scaly skin. They do not feel slimy. The scales are thickenings of the outside layer of skin and are mostly made of keratin, much like your fingernails. The outer skin is shed periodically allowing for growth. Most lizards shed their skin in patches, snakes usually shed the entire skin at one time. Scale patterns in the skin are often used to identify different species.
Animals need warmth to convert food to energy. "Warm-blooded" (endothermic) animals generate heat from the food they eat. They have a high rate of metabolism and require approximately ten times more food than "cold-blooded animals." Most reptiles are "cold-blooded" (ectothermic). They depend on the environmental temperature for the warmth that they need. They usually have a low metabolic rate, at least until they have warmed up. However, it is thought that some dinosaurs were warm-blooded and, of course, modern day birds are warm-blooded.
From bony shells to sharp teeth, camouflage and warning colors, reptiles protect and defend themselves in a variety of ways. Some lizards are known for being able to shed their tail which enables them to get away from a predator. New tails are grown to replace the old but are composed of cartilage instead of bone. Playing dead, emitting a foul smell (stink pot turtle) or possessing venom are other forms of protection.
Sea snakes, tiger snakes and the taipan are some of the most venomous of snakes. While there are numerous venomous snakes there are only two species of venomous lizards: the Gila Monster (found in the southwestern United States) and the Mexican Beaded Lizard (found in Mexico).