Ethics of Research involving animals
Pain, suffering and distress: meaning and function in animals and humans
The basic evolutionary functions of pain and ways of relieving it
4.11 In evolutionary terms, pain has evolved from nociception as an aversive sensory mechanism that warns of harmful experiences. Pain has three main functions: First, it allows animals and humans to avoid dangerous situations, as painful experiences usually prompt an immediate impulse to withdraw and escape from situations that cause harm, usually in the form of tissue damage. Secondly, as pain is associated closely with the environmental context in which it occurred, its experience can help to prevent repeated damage. Pain-causing experiences will be avoided through learning when a similar environment is encountered again. Thirdly, pain promotes the healing of injuries, as affected body parts are not used in normal activities, as far as possible.
4.12 In the natural environment where there are predators, and competition for mates and food, an overt display of pain-related behaviour could be disadvantageous. For example, an animal showing obvious signs of pain such as lameness or pain-related vocalisation could become a target for predation or aggression which would reduce its chances of mating or survival. Due to evolutionary pressures, many animals have therefore developed mechanisms that suppress signs of acute and chronic pain resulting, perhaps, from injury or an attack. Animals, including humans, produce opioids (natural ‘painkillers’) which may remain effective for a few minutes or several hours.12 These internally secreted opioids are released when chronic pain increases. This occurs through higher levels of activity of the ascending chronic pain pathways of humans and other animals (Figure 4.1). They trigger painsuppressive pathways (known as descending pathways) which originate in the brain stem.
This knowledge has been used to develop means for the alleviation of pain in animals and humans by administering the opiate morphine, which acts on the same receptors. The sensation of pain can sometimes be partly or completely blocked by these natural endogenous pain relieving chemicals which are a physiological response to injury.
4.13 It is also important to note that the capacity for, and nature of, suffering probably depends on specific selection pressures which have acted on different species, favouring certain brain structures and functions over others. This phenomenon can be illustrated by considering the loss of an offspring. In humans, the suffering and distress from the loss of a child is emotionally devastating and debilitating, feelings that may persist for many years, even throughout life. Other species show signs that indicate severe distress at the loss of an infant, such as carrying the body around for several days.13 Rodents, which mate more frequently and produce larger litters, do not display similar behaviours. Even if a whole litter of infants is removed, they return within hours to oestrus and mate again
4.14 Of course, the fact that an animal rapidly returns to mating condition cannot automatically be taken as evidence that it did not experience any form of suffering. Such questions might be elucidated by empirical research into levels of stress indicators. However, it could be hypothesised that evolutionary mechanisms might have favoured the capacity for experiencing relatively greater suffering in the case of infant loss in those species that breed infrequently and produce few offspring. Each infant represents a significant investment of time and resources and therefore individual animals that are motivated to take more care of their offspring are more likely to pass on their genes.
Figure 4.1.
| Figure 4.1. The pain pathway and interventions that can modulate activity at each point Opioids bind to opioid signal receptors in the central nervous system, affecting the descending pain pathway in the brain and the spinal cord.* * Gottschalk A and Smith DS (2001) New concepts in acute pain therapy: preemptive analgesia American Family Physician 63 (10). Redrawn with permission from Kehlet H and Dahl JB (1993) The value of ‘multimodal’ or ‘balanced analgesia’ in postoperative pain treatment Anesth Analg 77: 1049 |