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Any behaviour that reduces the chances of one animal being harmed by another animal. Defensive adaptations may be static, such as the spines of a hedgehog (Erinaceus sp.), or active, such as running away. But almost all defence has a behavioural component. Thus, when attacked, the hedgehog rolls into a ball.

While the most common systems of defence are adaptations against predators, animals may also have defences against parasites and against other members of their own species. For example, as well as having a social function, mutual grooming helps defend against ectoparasites. Similarly, territorial behaviour helps defend against rivals.

Defence against predators may be primary or secondary. Primary defences operate regardless of whether or not there is a predator in the vicinity. They reduce the probability that a predator will encounter an animal. They include camouflage and mimicry, group living and some forms of social interactions, and symbiosis.

Secondary defences operate only after an animal has detected a predator. They increase the chances that the animal will escape from the encounter. They include withdrawal, flight (escape), bluff (deimatic display), death feigning, deflection of attack, and retaliation.

Defensive behaviour incurs costs. Escape from one type of predator may make an animal more vulnerable to another. Camouflage often necessitates periods of motionless during which the animal cannot feed. Sticklebacks (Gasterosteus aculeatus) are best protected from predators if they are muted in colour, but the males are more attractive to females if they have conspicuous red bellies. Hence, the colour of male sticklebacks in any population will depend upon the relative strengths of predation pressure and female preference. Thus defensive behaviour is a compromise between the requirements of defence and those of other essential systems.

Subjects: Zoology and Animal Sciences.

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