Predation is the most urgent threat to future reproductive success, and, as a result, powerful behavioral systems have evolved to enable animals to thwart their predators effectively. Viewed within this functional behavior systems perspective, fear evolved as a set of antipredator strategies designed to evaluate and respond to threat. The rapid learning of fear is a component of this system that usually facilitates an animal's ability to deal with the threats it may confront (Fanselow & Lester, 1988). Because failure to defend in the presence of life-threatening danger eliminates future reproductive success, fear evolved to dominate behavior in the face of threat. But the ability of fear to dominate behavior that is normally protective can also lead to devastating consequences if the system is not working adaptively. Inappropriate or excessive activiation of fear responses may lead to the development of psychopathology (Rosen & Schulkin, 1998). One clear example of this is posttraumatic stress disorder (PTSD). In this chapter, we outline the structure of antipredator behavior and then relate this structure to PTSD, which we view as an inappropriate activation of this normally adaptive system.

In response to a cue for danger, animals display unlearned behavior patterns that have a phylogenetic history of protecting that species from danger (Bolles & Fanselow, 1980). These innate behavior patterns have been termed species-specific defense reactions, or SSDRs (Bolles, 1970). Once an animal recognizes a stimulus that is predictive of threat, its range of behavior becomes restricted to a limited repertoire of SSDRs.