In simple animals, behavior is governed almost entirely by instinct, meaning that it is pre-programmed by an animal's genes. In more complex animals, instinctive behavior is often modified by learning, producing more-flexible responses to the outside world.
This Egyptian vulture holds a stone in its beak in preparation for smashing an ostrich egg. Egyptian vultures are unusual among birds because they use stones as tools for obtaining food.
Many forms of behavior help animals to survive severe environmental conditions. Two examples are hibernation, which enables animals to survive cold and food shortages in winter; and estivation, which allows animals to survive drought and heat in summer. True hibernators, such as brown bears and some rodents, become completely inactive during winter, and their body temperature falls close to freezing. While in this state, they survive entirely on food reserves stored in their bodies. Estivating animals, which include land snails and some amphibians, seal themselves up when conditions become dry and only become active again when it rains. Between these two extremes, many other animals show less drastic patterns of behavior that are triggered by cold or heat. Winter wrens, for example, often crowd together for sleep when temperatures fall below freezing. On warmer nights, they sleep on their own.
Special forms of behavior also help animals to find food, to avoid being eaten, and to protect their young. One of the most advanced forms of this behavior is the use of tools. Several kinds of animals, particularly primates and birds, pick up implements such as twigs and stones and use them to get at food. More rarely, some tool-using animals seek out a particular object and then shape it so that it can be used. Woodpecker finches probe for insect grubs by making tools from cactus spines, and chimpanzees sometimes dig for termites using specially prepared twigs.
Defensive behavior is exhibited by individual animals and also by animal groups. Group defense is common in herding mammals, particularly in species such as the musk-ox, which form a protective ring around their calves when threatened by wolves. It can also be seen in swallows, starlings, and other songbirds, which instinctively mob hawks and other birds of prey. By grouping together to harass their enemies, they reduce the chances that they or their young will be singled out and attacked.
Individual defensive behavior is often based on threatening gestures that make an animal look larger or more dangerous than it actually is. Sometimes it involves some highly specialized forms of deception. One of the most remarkable is playing dead. Seen in animals such as the Virginia opossum and some snakes, this last-ditch defense is effective against predators that habitually hunt moving prey but leave dead animals alone. After the predator has inspected the "dead" animal and moved on, the prey comes back to life and makes its escape.