Can animals feel pain?
Lynne U Sneddon
Whether animals can feel pain has been a controversial issue for many years. Animals and humans share similar mechanisms of pain detection, have similar areas of the brain involved in processing pain and show similar pain behaviours, but it is notoriously difficult to assess how animals actually experience pain.
Pain can be considered to have two components: (1) physical hurt or discomfort caused by injury or disease; and (2) emotional suffering. Most people would agree that animals are capable of feeling pain according to the first definition. But it is less clear whether they also feel emotional pain.
Nociceptive nerves, which preferentially detect injury-causing stimuli, have been identified in a variety of animals, including invertebrates. Indeed, the leech and sea slug are classic model systems for studying nociception. However, it is believed that invertebrates are capable only of stimulus-response reactions and lack the necessary brain system that vertebrates have to process pain.
In vertebrates, nociceptive information is collated and augmented in the brain and signals are relayed down the nervous system to alter the intensity of pain. All vertebrates possess the primitive areas of the brain to process nociceptive information, namely the medulla, thalamus and limbic system.
However, one area of great importance for pain perception in humans is the cortex and its relative size decreases as we descend the evolutionary tree. For instance, in relative terms, the cortex gets smaller going from humans, through primates, mammals, birds, reptiles, amphibia and finally to fish, which possess only a rudimentary cortex.
Other animals respond to painful damage in a similar way. Their responses comprise several behavioural and physiological changes: they eat less food, their normal behaviour is disrupted, their social behaviour is suppressed and they may adopt unusual behaviour patterns (typically, highly repetitive or stereotyped behaviours, such as rocking to and fro), they may emit characteristic distress calls, and they experience respiratory and cardiovascular changes, as well as inflammation and release of stress hormones.
As these responses are complex and coordinated, it is likely that the brain is involved and they are more than just simple reflexes.
Although comparatively simple, fish have recently been shown to possess sensory neurons that are sensitive to damaging stimuli and are physiologically identical to human nociceptors. Fish show several responses to a painful event: they adopt guarding behaviours, become unresponsive to external stimuli and their respiration increases. These responses disappear when the fish are given morphine evidence that they are, mechanistically at least, directly analogous to pain responses in more complex animals.
Although modern philosophers have debated this issue, we simply do not know whether animals experience emotional pain. In his essay What is it like to be a bat?, Thomas Nagel concluded that unless we can get inside the head of an animal and actually be it, we will never know exactly how that animal feels. An important issue in animal pain is empathy, and many arguments about what animals feel have can only be based on the human experience and, therefore, may be tainted with anthropomorphism.
Another argument against animals experiencing pain is the question of whether animals are conscious. James Rose, for example, has argued that no animals, except primates, are capable of feeling pain, as they are not conscious. In essence, consciousness is a sense of I, an awareness of how things affect me and how I feel. Whether animals are conscious, or possess some degree of consciousness, has been endlessly debated, but consciousness is such a subjective experience it is hard to define and to assess. Fish can certainly learn complicated tasks, remember approximately 40 individuals, and measure their size relative to an opponents to decide whether to fight them. Therefore, at the very least they must have a sense of how big they are.
Higher vertebrates show even more significant signs of consciousness. Robert Hanna has suggested that animals may be conscious but that this is not as developed as human consciousness. Many argue, however, that consciousness is fundamentally dependent on language, something no other animal has yet been convincingly shown to possess. In contrast, Peter Singer, a bioethicist who has championed animal rights for many years, suggests that consciousness is not even the key issue: just because animals have smaller brains, or are less conscious than humans, this does not mean that they are not capable of feeling pain. After all, says Singer, we do not assume that newborn infants, people suffering from neurodegenerative brain diseases or people with learning disabilities experience less pain than we would.
In practice, welfare scientists, who assess animal wellbeing in various contexts including intensive farming, try to be unbiased and objective when monitoring behavioural and physiological responses to potentially painful events. If an animal shows the same kind of adverse reactions as humans after a painful stimulus, it is assumed that the stimulus is also painful to the animal. Inevitably, however, all welfare science on pain is essentially a interpretation based on indirect measurements.
Weighing the evidence
Many hunters and anglers adopt the opinion that animals are physiologically dissimilar to us, are not conscious and so do not experience suffering akin to human pain. The scientific evidence, however, shows that animals have the hard wiring to perceive and react to sensory pain and injury, and at least some of the brain structures that process pain in humans.
If one accepts that animals experience some kind of suffering when they are injured, then it is inevitable that a fox during a hunt, or a fish during angling, is going to have some form of pain inflicted upon it. The question then is, does the hunters or anglers enjoyment outweigh the cost to the animal? There may be other factors to consider. If the fish is eaten after being caught, for example, do the nutritional benefits make a difference?
As we cannot get into the minds of animals, or meaningfully measure emotional pain in animals, perhaps we should accept that animal pain is different from human pain, and is something we will never be able to describe fully. Nevertheless, even if animal pain may be distinct from human pain, is that a reason to consider it less important either biologically or ethically?
Lynne U Sneddon is in the School of Biological Sciences at the University of Liverpool.
What is it like to be a bat in pain? By Robert Hanna, University
Animal Rights, Anthropomorphism and Traumatized Fish, by
Do Fish Feel Pain? By Stuart Brown, Editor, FirstScience.com
Do fish feel pain? By James Hamilton-Paterson. From Granta Vol.
Hooked! Ouch? By James A. Swan
Do fishes have nociceptors? Evidence for the evolution of a vertebrate