Chronic pain produces hypervigilance to predator odor in mice.

The adaptive significance of acute pain (to withdraw from tissue-damaging or potentially tissue-damaging external stimuli, and to enhance the salience of the stimulus resulting in escape and avoidance learning) and tonic pain (to enforce recuperation by punishing movement) are well-accepted [1]. Pain researchers, however, generally assert that chronic pain has no adaptive significance, representing instead a pathophysiological state. This belief was recently challenged by the observation [2] that nociceptive sensitization caused by a chronic pain-producing injury reduced predation risk in squid (Doryteuthis pealeii). In that study, injury to an arm (removal of the tip with a scalpel) 6 hours prior led to increased targeting by black sea bass, resulting in decreased survival of the squid in a 30-minute trial featuring free interaction between predator and prey. The surprising finding was that anesthesia during surgery, preventing the chronic nociceptor sensitization associated with such injuries, led to even lower probability of survival. That is, the likely presence of pain increased apparent fitness, and the authors concluded that the chronic pain state and its associated nociceptive sensitization represented an adaptive function. Pain-induced defensive behaviors affecting fitness have also been reported in crustaceans (Gammarus fossarum) [3]. It is, however, currently unknown whether this may also be true in any other species, including in Mammalia.