Fear is the key emotion of post-traumatic stress disorder (PTSD). Fear's evolved function is motivating survival via defensive behaviours. Defensive behaviours have been highly conserved throughout mammalian species; hence much may be learned from ethology. Predation pressure drove the early evolution of defences, laying foundations in the more ancient brain structures. Conspecific (same species) pressure has been a more recent evolutionary influence, but along with environmental threats it has dominated PTSD research. Anti-predator responses involve both avoiding a predator's sensory field and avoiding detection if within it, as well as escape behaviours. More effective avoidance results in less need for escape behaviours, suggesting that avoidance is biologically distinct from flight. Recognizing the predation, environmental and conspecific origins of defence may result in clearer definition of PTSD phenomena. Defence can also be viewed in the stages of no threat, potential threat, encounter and circa strike. Specific defences are used sequentially and according to contexts, loosely in the order: avoidance, attentive immobility, withdrawal, aggressive defence, appeasement and tonic immobility. The DSM-IV criteria and PTSD research show substantial congruence with the model proposed: that PTSD is a disorder of heightened defence involving six key defences used in conjunction with vigilance and risk assessment according to contexts. Human research is reviewed in this respect with reference to laboratory and wild animal observations providing new insights. Understanding individual perceptual issues (e.g. predictability and controllability) relevant to these phenomena, combined with defence strategy recalibration and neuronal plasticity research goes some way to explaining why some traumatized individuals develop PTSD when others do not.