Crocodile

They are separate families within the crocodilians. A crocodile has a narrower, more V-shaped snout, and when its mouth is closed the large fourth tooth in the lower jaw is visible, sitting in a notch on the outside of the upper jaw. An alligator has a broader, more U-shaped, rounded snout, and its lower teeth are hidden when the mouth is shut, fitting into pits in the upper jaw. Crocodiles also tolerate salt water far better, thanks to salt-excreting glands on the tongue, so they live in estuaries, mangroves, and the sea, while alligators are freshwater animals. There are around a dozen or more crocodile species worldwide, but only two alligators, in the south-eastern United States and China.

The two actions use different muscles and are under different pressures. Closing the jaws is done by huge muscles at the back of the skull, and for an ambush predator that must hold onto struggling prey with no chance to re-grip, more closing force is always better, so evolution has pushed it to extremes. Opening the jaws, by contrast, is not a job that requires much force, since gravity and a small pull are enough, so the opening muscles have stayed thin and weak. The practical result, used by researchers and wildlife handlers, is that a few turns of tape or a strong hand can keep a crocodile's mouth shut, even though nothing can pull it open once it has clamped down.

Yes, far more than most reptiles. A female crocodile builds a nest, either a mound of vegetation or a hole in a sandy bank, and stays close to guard it against monitor lizards, pigs, and other egg thieves for the two to three months of incubation. As hatching nears, the young call from inside the eggs, and the mother digs them out, sometimes gently rolling eggs in her mouth to help crack them. She then carries the hatchlings, a few at a time, to the water in her jaws, and stays with the group, responding to their distress calls, for weeks and in some species months. The father may help defend the area. This level of parental investment is one reason crocodilians have survived so long.

Two species account for almost all serious attacks, the Nile crocodile in Africa and the saltwater crocodile across South and Southeast Asia and northern Australia. Between them they are responsible for hundreds of human deaths a year, mostly of people fishing, bathing, collecting water, or working at the water's edge in poor rural areas, which makes crocodiles among the deadliest large animals to humans. Other crocodile species rarely attack people. The risk is highly predictable and largely avoidable by keeping away from the water's edge in crocodile country, especially at dawn and dusk and during the breeding season, and by not being complacent in water that looks empty, since a crocodile gives almost no warning before it strikes.