[Pain, perception and expression: an ethological perspective].

To better understand the pain phenomenon, its occurrence and its functional significance, pain should be considered as a part of a behaviour system which activates two functions: defensive and aversive behaviour on the one hand, and recuperative behaviour on the other. In the former especially the more short-lasting primary pain plays a role, in recuperative behaviour more chronic secondary pain is of importance. Pain should not be seen primarily as the unconditioned stimulus in the fear system, but as one of the independent behaviour systems. Within the framework of this ethological model both the facilitative and inhibitive influences which 'pain' and 'fear' exert on each other can be more satisfactorily explained. We assume that animals feel pain on the grounds of the so-called 'analogy decision'. The only objective measure, of course, is the behaviour which is observable as a consequence of harmful influences. This behaviour varies greatly with the nature and localisation of the harmful effects; the variation is dependent on the degree to which behavioural responses can be effective in avoiding damage and promoting recovery, and also on the animal species considered. In some species expressions of pain have been selected for and adapted as social signals which elicit consideration, help and care of conspecifics. In other species social and other factors have precluded stimulation of such a development. All this means that the extent to which spontaneous behavioural expressions can serve as valid pain measures is a complicated question; it leads to consideration of the feasibility of more meaningful experimental measurement methods.