Persistence of pain in humans and other mammals.

Evolutionary models of chronic pain are relatively undeveloped, but mainly concern dysregulation of an efficient acute defence, or false alarm. Here, a third possibility, mismatch with the modern environment, is examined. In ancestral human and free-living animal environments, survival needs urge a return to activity during recovery, despite pain, but modern environments allow humans and domesticated animals prolonged inactivity after injury. This review uses the research literature to compare humans and other mammals, who share pain neurophysiology, on risk factors for pain persistence, behaviours associated with pain, and responses of conspecifics to behaviours. The mammal populations studied are mainly laboratory rodents in pain research, and farm and companion animals in veterinary research, with observations of captive and free-living primates. Beyond farm animals and rodent models, there is virtually no evidence of chronic pain in other mammals. Since evidence is sparse, it is hard to conclude that it does not occur, but its apparent absence is compatible with the mismatch hypothesis. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.