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Tell Me if it Hurts: Investigating Whether Animals Feel Pain

Caroline Feldman's picture

For many years, the question of whether animals can feel pain has been a very controversial issue. Humans and animals both exhibit similar mechanisms of pain detection, show similar pain behaviors, and they both have similar areas of the brain involved in processing pain. However, it is notoriously challenging 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 with the fact that animals are capable of feeling pain according to the first definition, but it is less apparent whether animals also feel emotional pain.

One of the functions of pain is to warn against damage and to act as an alarm system so that action could take place in order to avoid or minimize injury. This usually takes the form of a withdrawal reflex. This sensory capacity is called nociception, a simple detection and reflex response to damage, to distinguish it from pain. Nociceptive nerves, which preferentially detect injury-causing stimuli, have been identified in a variety of animals, including invertebrates. The sea slug and leech are classic model systems for studying nociception. However, many people believe that invertebrates are only capable of stimulus response reactions and lack the necessary brain system that vertebrates have to process pain (7).

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 have the “primitive areas of the brain” to process nociceptive information, namely the thalamus, medulla, and limbic system. However, one area og significant importance for pain perception is humans is the cortex and the fact that 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 only possess a rudimentary cortex.

Many animals exhibit reflex responses similar to our own. For example, when one accidentally touches a hot stove or iron, they respond almost immediately by retracting their hand. There is a lag period following this when no adverse sensations are felt, but if left untreated, the burn begins to throb and one alters their behavior to guard the affected area.

Other animals respond to painful damage in a similar way. However, their responses are composed of several behavioral and physiological changes: their normal behavior is disrupted, they eat less, their social behavior is suppressed and they may adopt unusual behavior patterns, they may emit specific distress calls, and they may experience cardiovascular and respiratory changes, and also 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.

Fish have recently been shown to have sensory neurons that are sensitive to damaging stimuli and are physiologically identical to human nociceptors (7). Fish show several responses to a painful event: they take on guarding behaviors, their respiration increases, and they become unresponsive to external stimuli. When the fish are given morphine, these specific responses disappear. This evidence reveals that they are, mechanistically at least, directly analogous to pain responses in more complex animals.

Are animals capable of feeling emotional pain? Humans can certainly feel pain without physical damage – after the loss of a loved one, or the break-up of a relationship. Some scientists suggest that only humans and primates can feel emotional pain because they are the only animals that have a neocortex, the “thinking area” of the cortex only found in mammals. However, research has provided evidence that dogs, monkeys, cats, and birds exhibit signs of emotional pain. Also they can display specific behaviors associated with depression during a painful experience, i.e. lack of motivation, lethargy, anorexia, and unresponsiveness to other animals (3).

Although many modern philosophers have debated whether animals experience emotional pain, we simply do not know the answer. In his essay “What is it like to be a bat?”, Thomas Nagel concluded that unless humans can actually get inside the head of an animal and actually “be it”, one will never know exactly how that animal feels (1). An important issue surrounding animal pain is empathy, and many arguments about what animals feel can only be based on human experience and, therefore, maybe tainted with anthropomorphism.

Another argument against animals experiencing pain is the question of whether or not animals are conscious. For example, James Rose, 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” and the “I-function”, an awareness of how certain things affect me and how “I” feel (6). The question of whether animals possess some degree of consciousness has been debated for a long time. It is such a subjective experience that it is hard to define and assess. Fish for example can certainly learn complicated tasks – measuring their size relative to an opponent’s to decide whether or not to fight them. Therefore, at the very least, they must have a sense of how big they are.

Higher invertebrates show more significant signs of consciousness. Robert Hanna, author of “What it is like to be a bat in pain?”, suggests that animals may have a conscious, but it is not as developed as human consciousness (2). Many people argue that consciousness is essentially dependent on language, something that no animal has yet been shown to possess. In contrast, however, a bioethist named Peter Singer, who has advocated animal rights for many years, suggests that consciousness is not even the main issue: just because animals have smaller brains, or are “less conscious” than humans, does not mean that they are incapable of feeling pain. Singer stated that we do not assume that newborn infants, people suffering from neurodegenerative brain disease, or people with learning disabilities, experience less pain (5).

Welfare scientists, who assess animal wellbeing in various contexts including intensive farming, try to be unbiased and objective when monitoring behavioral and physiological responses to potentially painful events. If an animal shows the same unpleasant reactions after a painful stimuli as compared to humans, it is assumed that the stimulus is also painful for the animal. Inevitably, however, all welfare science on pain is essentially an interpretation based on indirect measurements.

It is currently impossible to prove whether animals are capable of experiencing emotional pain, but it is equally impossible to disprove it. This debate has been around for many years and has largely been a moral one. Ultimately it has come down to personal perspectives. Many hunters support the opinion that animals are physiologically dissimilar to humans and are not conscious, therefore they do not experience the “suffering” parallel to human pain. However, scientific evidence reveals 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’s accepts the fact animals experience some type of suffering when they are injured, then it is inevitable that a fox or deer during a hunt, or a fish attached to a fishing line, is going to have some form of pain inflicted on it. The question then comes to whether the hunter’s or fisherman’s enjoyment outweigh the cost to the animal? If the fish is eaten after being caught, do the nutritional benefits make a difference?
As humans, we cannot get into the minds of animals, or meaningfully measure emotional pain in animals. Perhaps we should just accept the fact 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 biologically or ethically?

"... If one desires to judge a particular kind of behavior, one has to consider the aims which underlie it. I hope by means of these experiments to obtain certain results by which many wounded people might be helped."

The previous quotation is a representative argument from scientists involved in animal experimentation. When I heard this argument for the first time it seemed naturally valid to me. If the experiment could help many wounded people then the experiment should be allowed. Then I noticed that the quotation belongs to Fritz Ernest Fischer, a scientist of the Third Reich, and was stated during his trial for the Ravensbruck experiments.

"A range of experiments were conducted at various camps: sterilization studies at Auschwitz-Birkenau, in which victims genitals were irradiated or removed; high-altitude studies at Dachau, where prisoners were subject to pressure so extreme that eardrums burst and organs ruptured; hydrothermia experiments, also at Dachau, in which prisoners were immersed in freezing water or tied naked to boards covered with only a sheet and repeatedly doused with freezing water before being left outside during winter; gangrene and sulfonamide experiments at Ravensbruck in which women's legs were cut to the bone and infected with glass and wood splinters ..." (8).

So after hearing the description of the experiments, I thought that no matter how many wounded people could be helped, these experiments should not be allowed. The previous scenario strikes me while I am deciding whether to possibly pursue research on animals in the future. Biology tells me that we human beings belong to the same family as the rest of the animals. My experience tells me that every form of life is incredibly wonderful. I believe that my decision depends on how far I am willing to extend moral treatment. On one extreme, if I were a scientist of the Third Reich I would have probably said that Jewish people are inferior than Aryan people, therefore they do not deserve moral treatment and I would have performed the previously described experiments.

On the other extreme, if I were an animal protectionist I would say that animals deserve the same moral treatment as human beings and therefore I would reject any kind of animal experimentation. I do not feel that killing an ant for research is as bad as killing a frog, killing a frog does not feel as bad as killing a dog, killing a dog does not feel as bad as killing a monkey and killing a monkey does not feel as bad as killing a person. In all cases we are dealing with wonderful pieces of life. Where to stop? How far shall I extend moral treatment? Is human health a good reason? Is knowledge a good reason? Is there a good reason?

7) ‘Do Fishes have nociceptors? Evidence for the evolution of a vertebrate sensory system’ Sneddon L U, Braithwaite V A and Gentle M A Proc. R. Soc. Lond. B (2003) 270, 1115-1121.
8) Deborah Rudacille. The Scalpel and the Butterfly. University of California Press, 2000.


Serendip Visitor's picture

College is wonderful

I found this article online. I also found Lynne U Sneddon's paper. It seemed really familiar...

Paul Grobstein's picture

research and animal suffering

"while I am deciding whether to possibly pursue research on animals in the future" you should indeed be posing the questions you are. Glad you're doing it, whatever you decide. For more to think about/with, see the 18 March 2008 session of the senior seminar in neural and behavioral science, and links from there.

Jane's picture

Whether animals feels pain

If animals are so different from humans that they cannot feel something so basic as pain... How can experiments on them be useful to "human" healthcare? Yet they are useful. Maybe because animals are not so different from us.

Jeremy Bentham, the founder of the reforming utilitarian school of moral philosophy, stated that when deciding on a being’s rights, “The question is not ‘Can they reason?’ nor ‘Can they talk?’ but ‘Can they suffer?’” In that passage, Bentham points to the capacity for suffering as the vital characteristic that gives a being the right to equal consideration. The capacity for suffering is not just another characteristic like the capacity for language or higher mathematics. All animals have the ability to suffer in the same way and to the same degree that humans do. They feel pain, pleasure, fear, frustration, loneliness, and motherly love. Whenever we consider doing something that would interfere with their needs, we are morally obligated to take them into account.