Several animals can and do prey on humans. To reassure readers and promote conservation efforts, many sources state that these animals only do so when they are hungry and there is nothing else available, but this isn't always the case. Animals that have been reported to attack humans for food include three of the four big cats (lion, tiger, and leopard; jaguars are not known to eat people, but will attack them if disturbed), the American Black Bear, the polar bear (world's largest land predator), wolves (especially the Grey Wolf), jackals, large crocodiles and alligators (especially the saltwater and Nile crocodile), large sharks (especially the great white shark, tiger shark, and bull shark), and the Komodo dragon.
All of these animals have been around for millions of years, surviving through difficult times like Ice Ages. To survive in challenging conditions, predators must eat anything they can overpower, so evolution has crafted these animals to be efficient killers. What's more, predators exist on top of the food chain, meaning they depend on a substantial supply of prey. When conditions get harsh in a region, apex predators are usually the first to die off, because they are more likely to starve by being unable to find prey than prey is likely to starve from being unable to find its own food, like smaller prey or plants. All these forces optimize predators for aggression and dietary flexibility.
All instances where animals prey on human depend strongly on the species involved and the size of the humans. Jackals, for instance, are small enough that they usually only attack children, since attempting to prey on adults would probably be too difficult. Grey wolves, on the other hand, are larger and may attack humans for food, especially when hungry, as is highly documented in European folklore. The human fear of wolves led to massive extermination campaigns that wiped them out across much of their former range within Europe.
Most bears only attack humans to protect their cubs (especially grizzly bears) or for territorial reasons, but predatory kills have been reported, most frequently from the American black bear. Pepper spray is recommended as protection from these animals, though usually yelling loudly is enough to scare them off.
The most lethal predators of humans are tigers and saltwater crocodiles. The Champawat tiger, a female Bengal tiger shot in 1907, was responsible for over 400 documented deaths in Nepal and India before she was shot by Jim Corbett. One saltwater crocodile, Gustave, a 20 foot (6 meter) Nile crocodile living in Burundi, Africa, has been rumored to have killed over 300 people. Although this figure is exaggerated, the crocodile has probably killed at least 100.