Other characteristics of birds include hollow or partially hollow bones with very thin braces for extra strength, keen eyesight with the ability to focus each eye independently, a keen sense of hearing, and a beak. There are over 9,000 species of birds worldwide, from the three hundred pound ostrich to the wasp-sized hummingbird. Reptile skin is a major adaptation to life on land. Besides protecting them from other animals, it also prevents the loss of water, allowing them to live in a wider range of environmental conditions. Scales have many forms, from the thin and flexible scales of lizards and snakes to the hardened scutes of crocodiles and turtles.
Reptiles lay eggs protected by leathery shells.
Most reptiles lay their eggs in places having the proper warmth and moisture for their incubation. However, many snakes, including boas and rattlesnakes and some lizards, bear live young. Reptile young are hatched as miniature replicas of adults and can care for themselves. A major exception is found with many crocodiles and alligators which provide some care for their young. There are about 6,300 species of reptiles, from the thirty eight foot anaconda to the one-half inch least gecko.