Free-living animals attempt to avoid contact with potential predators including man. Wild animals constantly check their surroundings for danger. In the zoo we see this in waterfowl, rodent groups, primates and meerkat sentries. Because man is regarded as a universal enemy by both predator and prey species, we can include enemy-prey relationships with predator-prey relationships.

Normally an animal that senses a predator responds with the escape reaction, aimed at avoiding contact by fleeing, remaining motionless, hiding or using camouflage. The escape reaction is specific for sex, age, the kind of predator and surroundings. For instance a marmot that would ordinarily retreat to its burrow may instead lie motionless if surprised at a distance from the burrow. An enemy, or what is perceived to be an enemy, enters into the sequence of escape reactions specific for sex, age, the kind of enemy, and surroundings.

a) Flight distance: This is the distance at which an animal will begin to flee when approached by an enemy. This distance is related to the perceived significance of the intruder. Individual experiences of an animal may increase or decrease the distance involved. Man could decrease the distance (for example a keeper bringing food) or increase it (a hunter or, initially, the zoo visitors). Sometimes, as when the protective impulse to defend offspring exists, the flight distance can be greatly reduced, possibly eliminated, on the flight reaction may even be modified to an attack.

b) Critical distance: This is a component of the flight distance. A penned or cornered animal without an escape corridor will tolerate the approach of an enemy up to the point where it feels forced to defend itself; the attack/defense takes place within the critical distance range. If an animal feels the keeper is an enemy, and this can change depending upon the circumstances, an otherwise inoffensive animal may attack when you reach the critical distance.

c) Personal or intimate distance is the individual distance within which conspecifics are not tolerated. In contact species, such as horses, that exhibit mutual grooming this distance may be zero. Mates, juveniles and sometimes adolescents may also be allowed within the typical personal distance but other members of the species are rebuffed or attacked when they intrude within this space. These limits must be reduced for mating to occur and males practice many strategies to penetrate the defensive zone. Personal distance is often considered a component of territoriality but often it closely resembles critical distance when an enemy is intruding.