Crocodiles are large reptiles found in tropical regions of Africa, Asia, the Americas and Australia. They are members of the order Crocodilia, which also includes caimans, gharials and alligators. There are 13 species of crocodiles, so there are many different sizes of crocodile. The smallest crocodile is the dwarf crocodile. It grows to about 5.6 feet (1.7 meters) in length and weighs 13 to 15 pounds (6 to 7 kilograms). The largest crocodile is the saltwater crocodile. The largest one ever found was 20.24 feet (6.17 m) long. They can weigh up to 2,000 pounds (907 kg). Crocodiles are carnivores, which mean they eat only meat. In the wild, they feast on fish, birds, frogs and crustaceans. At the zoo, they eat small animals that have already been killed for them, such as rats, fish or mice. They also eat live locusts. In the wild, crocodiles will clamp down on their prey with their massive jaws, crush it, and then they will swallow the prey whole. They do not have the capability to chew or break off small pieces of food like other animals. During the Mesozoic Era, about 100 million years ago, the Crocodilia order was one of the top animals on the food chain. Today, crocodiles are found in the tropical habitats of Africa, Asia, Australia and the Americas. They normally live near lakes, rivers, wetlands and even some saltwater regions. Saltwater crocodiles have very large heads. A pair of ridges runs from the eyes along the centre of the snout. The eyes, ears and nostrils are located on the same plane on the top of the head, allowing it to see, hear and breathe while almost totally submerged. The eyes have a special second pair of eyelids known as the nictitating membrane. These eyelids are clear and protect the eyes while underwater. The ears, situated behind the eyes, have flaps which also close while underwater. The jaws are heavyset and contain 64-68 teeth. The teeth in the upper jaw are perfectly aligned with those in the lower jaw. The fourth tooth on each side of the bottom jaw is larger than the other teeth and is visible when the mouth is closed. The size of the saltwater crocodile prey is only limited by the crocodiles own size and strength. Saltwater crocodiles mostly eat fish, but will eat almost anything that they can overpower which can include turtles, goannas, snakes, birds, livestock (cattle), buffalo, wild boar and mud crabs. Hatchlings and juvenile crocodiles feed on insects, crustaceans, small reptiles, frogs and small fish. Saltwater crocodiles reach sexual maturity at 10-12 years for females and males at 16 years. Females build mound nests from vegetation between November and May, and an average of 50eggs are laid. If the nest is around 31.6°C, all the eggs will hatch as males. Above 33°C or below 31°C the crocodile eggs will hatch as females. The eggs hatch from 75 days. Only 1% of the hatchlings are thought to survive to maturity in the wild.