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Pain

Last revised April 9, 2017

Nah, it doesn’t hurt a bit1.

Sufferin’ Catfish

If you go down to the river, you may see a fisherman. Ask him — they are almost always hims — whether a hook in a fish’s mouth hurts the fish, and you’ll hear what I always hear: “No. Fish don’t have any feelings. They don’t feel pain. A hook doesn’t hurt them.” If you go down to the stable, you may see a rider. Ask her — they are almost always hers — whether a bit in a horse’s mouth hurts the horse, and you’ll hear what I hear: “No. Even a very severe bit, in the right hands, can transmit extremely subtle, nuanced signals that cause no pain to the horse.”

Logic tells me that this can’t be true.

Occam’s Razor

First, let’s think about just our own species. I whack my thumb with a hammer or touch it with a flame or poke it with a pin. I immediately experience pain. I jerk my hand back. While the details of the sensation of hammer, flame, and pin may differ, the response to all is the same.

What happens looks like this:

  1. encounter injurious source;
  2. experience pain;
  3. withdraw from injurious source.

If I observe that you burn your thumb on the stove (“encounter injurious source”) and then yank your hand back (“withdraw from injurious source”), shouldn’t I assume that the intermediary was an experience of pain, like my own experience? It would be possible, of course, that you and I are wired differently, and that you actually felt good when you burned your thumb. But we simply have too many people on this planet to assume that everyone’s experience is different. Occam’s Razor2 — the principle of parsimony, of economy, of succinctness — guides us to select the simplest explanation which accounts for the facts.

Pain has two important qualities: it hurts, and it triggers an avoidance reaction.

In our own lives, pain is supremely useful. If we did not experience pain, the market for hot pads would be much diminished, and there would be a burn clinic on every corner.

Pain must feel the same to you and to me, producing the same physiological and metabolic reactions, the same quick withdrawal from its source.

Other creatures will quickly withdraw from an injurious source, too. Should we believe that an entirely different process guides their behavior? Do they withdraw from a flame out of habit? what they learned in school? The best theory of reaction to injury should work for all creatures.

Simple organisms seem very nicely suited for this model. Let a large number of sources of injury trigger a feeling of pain. Let that single feeling of pain guide a standard escape or withdrawal behavior. Contact with a sharp object triggers withdrawal in an earthworm. Contact with a hot or cold object triggers the same withdrawal.

Darwin would have seen the value of pain in the simplest organism: if it injures you, then let it be painful; if it is painful, withdraw. A simple organism, evolving to a more complex one, would have had no need to cast aside a reaction to injury in place of one entirely different. Every organism that can move will need to move away from sources of injury to survive. Long ago, Mother Nature got it right about injury and pain. Pain defends us all from further injury.

There are some organisms that cannot act when they experience pain. In a forest fire, trees are never seen running from the woods. For those species that cannot move, pain would not be useful, and it seems unlikely that anything like pain in them would have ever evolved. But movement to avoid injury is a splendid idea, and so any species capable of movement must be able to experience something like pain. Pain must have evolved at the time that the first one-celled animals were figuring out how to move. Movement, for them, offered a double benefit: not only could they move away from injurious sources, but they could use it to move toward greener pastures.

Sensing Pain: Nociceptors

If a single pain model is the best we can do, then we should expect that the physiological reactions to injurious sources should be the same, whether we are talking about worms, fish, or humans.

A nociceptor is a sensory neural receptor that signals when it senses potentially damaging stimuli. Some nociceptors respond to noxious heat or cold, some respond to pressure or cuts that break the skin surface, and some respond to chemicals. Nociception evokes a reflex that moves the entire animal, or at least the affected part, away from the noxious stimulus.

Nociceptors have been found in leeches, nematode worms, sea slugs, molluscs, fruit flies, birds, horses and other mammals, human embryos, and — sufferin’ catfish — in fish3. I expect that scientists one day will identify them in some protozoa.

The face of a fish must collect information on whether to approach or avoid, so should contain nerves that can taste or smell for food, and nociceptors that can spot trouble and signal escape. The face of a fish is loaded with nociceptors. Some respond to heat, touch, and chemicals, others respond to just two or one. In one study, of 58 receptors tested, 22 were nociceptors4.

Sneddon et al (2003) report “We located 58 receptors on the face and head of the rainbow trout. Twenty-two of these receptors could be classified as nociceptors… Receptor diameter, thermal thresholds and mechanical responses are similar to those measured in higher vertebrate groups… Mechanical thresholds were lower than those found in humans: at least 0.6g is required for noxious stimulation in human skin but many of the nociceptors in the fish skin were stimulated by 0.1g. This may be a consequence of the more easily damaged nature of the fish skin requiring the nociceptors to have lower thresholds. Similar thresholds were found in mammalian eye nociceptors.” In short, a trout’s pain is similar to that of other species, including humans, but the trout skin is more sensitive to pain than the human skin.5 A poke in a trout’s face and head must hurt about as much as a poke in our eye. The face of a fish is very sensitive to pain.

The face of a horse is just as clever as that of a fish. As Leblanc (2002) notes, “The tactile sensitivity of the horse… is especially strong around the lips, nostrils, and eyes, given both the high concentration of receptors and the presence of vibrissae, which … are rooted in many nerve endings6.” The face of a horse is very sensitive to pain. And not merely the face. Saslow’s (2002) study of tactile sensitivity of the horse, using the same methods as those used to test human tactile sensitivity. She notes “We were surprised to find that horse sensitivity on the parts of the body that would be in contact with the rider’s legs is greater than what has been found for the adult human calf or even the more sensitive human fingertip. Horses can react to pressures that are too light for the human to feel7.”

Reacting to Pain

Our first behavioral reaction to pain is to withdraw from the source of the pain. A hooked fish, fighting to escape, shows the same intent. A horse moves its head away from a pulled bit, and tries to move its entire body away from a poke with spurs. Other reactions — also found throughout the animal kingdom — may include trembling, hiding, restlessness, panting, salivation, pupil dilation, aggressiveness, increased heart rate, and lost appetite. Adrenalin, to help promote escape, is produced.

When humans or animals are injured, their blood chemistry changes. Blood sugar, blood cortisol and white blood cell counts all go up.

Opioids are neurochemicals that moderate pain when they reach opiate receptors. The presence of opioids and opiate receptors in a species is good evidence that the species can feel pain. Numerous studies have found opioids and/or opiate receptors in a wide variety of species, including nematodes, molluscs, insects, crabs, shrimp, lobsters, newts, frogs, fish, … and of course humans8. A genetic study has found that the complete vertebrate opioid system was already established in the first jawed vertebrates9.

Analgesics that relieve pain in humans are used by veterinarians to treat animals that have been injured. Vets believe that animals feel pain.

Pain and Suffering

What sort of evidence do we need in order to conclude that an animal does feel pain? When answered, some skeptics always seem to have an objection, eventually arguing that they don’t feel pain the way we feel pain. They aren’t conscious, they don’t have a neocortex, they don’t have a soul, they can’t comprehend mortality, blah blah blah. But you and I can’t know whether these skeptics have a neocortex or a soul, or can comprehend mortality. And they can’t know if some animal feels no pain simply because of its different design.

No doubt, your dog will experience pain and suffering if I put a fish hook in his lip. But there are philosophers out there that think your dog won’t mind this quite as much as you would, and that your fish won’t mind this at all. These philosophers may resort to the concept of sentience — “the ability to feel, perceive or be conscious, or to have subjective experiences”10 Any animal that is capable of sentience is entitled to the same general rights as humans, it seems. But normally, we will work it the other way: any animal we care about will be credited with sentience. If you love lobsters, you will assume that their many heat-detecting nociceptors are working just fine, and you won’t scald them to death. If you don’t love lobsters, but love to eat them, you might prefer not to think about it. Those who think such animals don’t feel pain may not do much thinking at all.

Fish and horses are not exactly like us. Most can’t type. Few can give you change for a $5. But both fish and horses are enough like us to deserve our empathy, our compassion, our best tenderness. They experience pain. It is wishful thinking to pretend otherwise.

Revealing Pain

Dogs are pretty good at expressing how they feel, and we are pretty good at figuring them out. A wagging tail means they are happy. A squeal means they’ve just experienced pain.

Humans sometimes lie about their feelings. When my great grandmother was alive, and in her 90’s, my mother would ask her “Grandma, how are you feeling today?” Grandma always replied “Oh, I’m fine. Getting a little better every day.” A year later, as she continued to decline, she died. I suspect she was one of those people who claimed that she had never been sick a day in her life. If we don’t reveal our emotion through body language or facial expression or communication, then others will have trouble knowing that emotion.

But we should not confuse a failure to reveal emotion with a failure to correctly read emotion. If my great grandmother was in pain, perhaps my young mom did not recognize its subtle signs.

If pain occurs when a dog tries to walk, you can expect a limp. But if the dog has a low-level of chronic pain, you will need some wisdom to determine this. Through our co-evolution, dogs learned to read our facial expressions and behavior much better than we learned to read theirs. And so, with our lack of skill in reading animal emotion, we might conclude that animals are notorious liars about their feelings.

Every night I sit in the woods with some raccoon friends of mine, giving them cashews. One of these raccoons appears to have a broken pelvis — perhaps from a fall from a tree or encounter with a car — and has trouble moving either back leg. When he stops, his back end just flops over. But I can see no emotion on his face, I hear no whimpering. If his movement did not reveal his catastrophe, it would not be evident. But this is not a shortcoming of the raccoon. It is my own shortcoming, my own lack of skill at reading his face and behavior and smell. With our own horses, most of us come up short in understanding their facial expressions.

Behavior Reveals Pain

What behaviors reveal pain? In humans, pain may be accompanied by depression or aggression. Activity level may decrease (to avoid working the affected area and causing further pain) or increase if the pain is non-specific. An animal will move away from whatever will give it pain and will move toward whatever will give it pleasure. In a stall, when a horse is in pain, we might expect the appearance of agitation: incessant movement and pacing.

Pain is not restricted to those who look like us, or soldiers on our team. Bacteria withdraw from repellent chemicals and swim toward attractants.11 Opioids have been found in protozoa,12 suggesting that protozoa can use them to deal with pain. And insects writhe when poisoned. In them, both opioids and their receptors have been identified.13 Various other invertebrates have been found to have endogenous opioid peptides and receptor sites for them14 — useful if an insect or crab is going to deal with pain the way mammals do — by production of opioid analgesia. We know that lobsters struggle to get out of the pot, and can certainly understand why. It appears to me that pain occurs in all animals that can move. This is certainly not a majority view of scientists or sellers of insecticide. But I don’t see a good reason to believe otherwise.

In all animals, pain seems to be coupled with aggression. In a series of experiments with mice, researchers found that if a rat was shocked in a cage with other rats, it might turn on the other rats and bite them. When a horse’s back is in pain from the use of a poorly fitting saddle, an attempt to saddle him may trigger a threat or attempt to bite you.

A horse suffering severe pain from a bout of colic may appear anxious and unusually sweaty. He may paw at the ground, kick or bite at his abdomen or groan or grind his teeth. Less severe pain may show itself through loss of appetite, reluctance to move and general grumpiness15.

Horses and humans will likely share these behavioral responses to pain:

  1. Facial grimacing (see below)
  2. Guarding. Muscles spasm — involuntarily contract — to minimize motion of sites that are affected. You may find this tensing happening when you visit the dentist.
  3. Behavior change. Pain can trigger agitation and arousal (which may increase vocalizations and increase aggressive behaviors) or depression (which may cause social withdrawal, lethargy, and decreased appetite.)

The horse has a wide variety of behaviors that indicate it is in pain. Here is a catalog, adapted from a recent review16. This listing is intended to show the connection between specific behaviors and some common sources of pain. Please consult your vet when you find any of the behaviors listed.

No indication of pain source:

  1. Restlessness, agitation, and anxiety17. A common indicator of severe or acute pain. Easily seen in confined horses.
  2. Rigid stance18. Reluctant to move. A general protective behavior shown for many disorders. Horse may appear anxious, may face away. Does not indicate source or intensity of pain.
  3. Lowered head19. May come with depression stemming from chronic, severe, unrelenting pain.
  4. Fixed stare. Dilated nostrils. Clenched jaw20. This is a general facial expression showing pain and/or fear. Occurs with chronic pain. Does not indicate source or intensity of pain.
  5. Aggression.21 May be displayed toward handlers, horses, objects, or self. Handling or palpating may produce an aggressive reaction. Conditioned fear may exaggerate this aggression. Aggression is often observed in humans, dogs, horses, and other animals that are in either acute or chronic pain22.

Abdominal pain:

  1. Deep groaning.23 Usually indicates visceral pain. May accompany rolling or when horse is recumbent. May not correlate with intensity of pain, but usually indicates source.
  2. Rolling.24 The most common indicator of abdominal pain. May be violent, and may lead to self-inflicted injury. Intensity is likely related to the severity of the lesion.
  3. Kicking abdomen.25 Frequency and intensity relates to pain severity. Likely demonstrated with colic.
  4. Watching flank.26 Horse may turn head and touch flank, or stare at it, or just turn head slightly. Commonly seen.
  5. Stretching.27 Commonly associated with colic. May be accompanied by straining to defecate or urinate.

Limb and foot pain:

  1. Weight-shifting28. Horse shifts weight from one side to the other, alternately loading limbs. Reluctant to stand on the sore leg. Feet may not leave the ground. Ceases with administration of analgesic.
  2. Limb guarding29. Horse tries to stabilize limb to reduce pain. Difficult to detect.
  3. Abnormal weight distribution30. New posture depends on what hurts, how much it hurts, and the cause.
  4. Pointing, hanging and rotating limbs.31 Pointing or lifting (hanging) the limb to a non-weight-bearing level reduces pain. Often found with severe, unrelenting pain. A rotated limb may indicate pain in the pelvis or shoulder.
  5. Abnormal movement.32 Jerky movements may be seen in confined horses. Often accompanied by unsuccessful attempts to lie down.
  6. Reluctance to move.33 A protective behavior suggesting serious skeletal damage and severe limb pain.

Head and dental pain:

  1. Headshaking34. Various sources of pain in the head can produce headshaking. Palpation of the painful area will cause an obvious head toss. Can become an established behavior. Might vary with the season or climate. High individual variation.
  2. Abnormal bit behavior35. Mucosal damage from bit or exacerbation of dental pain by bit may reduce horse’s contact with the bit. Head may tilt.
  3. Altered eating, quidding, food pocketing.36 Quidding is a response to mouth pain in which the horse loses or spits balls of semi-chewed food out of his mouth. The most common cause of quidding is teeth that are uneven or that have sharp points. This does not allow the mouth to close properly and makes chewing extremely difficult. Food pocketing (retaining it in the cheeks) may accompany chewing slowly on the preferred side. Reduced consumption may lead to anorexia. All suggest dental pain.
  4. Depression. Manifests as lethargy, reduced alertness, lowered head, self-isolation, or facing away from handlers37.

 

Facial Expression Reveals Pain

A casual observer would conclude that animals do not show pain through facial expression in the way that humans do. Claim after claim may be found on the Internet that “animals don’t reveal pain” presumably because predators would find them more interesting if they did.

But this thinking is a relic of the days when people believed that most animals had no feelings, and thus would not be expected to reveal them. Back in 1872, Darwin argued otherwise, in his “Expression of the Emotions in Man and Animals”.38 Darwin’s insight that facial expression of emotion are universal in mammals has stood the test of time. Since then, researchers have learned that human facial expressions are not specific to each culture, but are understood across cultures, with common meanings. And today they are learning that all mammals use facial expressions, and that such expressions are similar across species.

For understanding pain, a new body of scientific literature has developed “grimace scales” which use facial expression to score the degree of pain an animal must be feeling. There are now grimace scales for mice, rats, rabbits, cattle, and horses.

Rats reveal pain in their facial expression39, and a Rat Grimace Scale scores orbital tightening, nose/cheek flattening, ear changes, and whisker change to determine a score. Mice reveal pain through orbital tightening, nose bulge, cheek bulge, ear position and whisker position40. Cats reveal pain in their faces with changes in ear position (ears flatten in pain) and muzzle/cheek shape (the nose and cheek flatten).41 Rabbits reveal pain through orbital tightening, cheek flattening, a more pointed nose, and a change in whisker position.42 Cattle reveal pain through “(1) Ears: ears are tense and backwards or low/lambs ears. (2) Eyes: eyes have a tense stare or a withdrawn appearance. Tension of the muscles above the eyes may be seen as ‘furrow lines’. (3) Facial muscles: tension of the facial muscles on the side of the head. (4) Muzzle: strained nostrils, the nostrils may be dilated and there may be ‘lines’ above the nostrils. There is increased tonus of the lips”.43

The Rat Grimace Scale.44

What are the clues that a horse is in pain? Dalla Costa et. al. (2014) developed a Horse Grimace Scale by studying facial expressions in stallions that had or had not just been castrated45. Their scoring assesses six “facial action units” that include stiffly backwards ears, orbital tightening, tension above the eye area, prominent strained chewing muscles, mouth strained and pronounced chin, strained nostrils and flattening of the profile. Scores can range from 0 (none of the facial action units present) to 12 (all 6 are obviously present.)

The Horse Grimace Pain Scale with images and explanations for each of the 6 facial action units (FAUs). Each FAU is scored according to whether it is not present (score of 0), moderately present (score of 1) and obliviously present (score of 2).

Rats, mice, rabbits, cattle and horses show similar facial expressions of pain, making it easier to identify the primary clues for pain in the each of these species. We can summarize such expression like this:

  1. Above the eyes: tension in muscles above the eyes, creating furrow lines and revealing underlying bone. Eyebrows may lower.
  2. Eyes: orbital tightening, reducing the size of the exposed eye. Eyes may close tightly.
  3. Facial muscles: tension of the facial chewing muscles on the side of the head. Chewing muscles become prominent.
  4. Mouth/Nose/Muzzle: Nostrils may flatten, lips may tighten and press upwards.
  5. Ears: tense or stiff and facing backwards.

Each of these aspects of emotional display is worth considering when trying to read a horse’s emotions, for the muscles involved may be used to reveal other emotions under other circumstances.

When we look at the human facial expressions for pain, we find ourselves with a list similar to that of horses, though we fail to change the position of our ears. The face below,46 of a man in pain, shows the first four characteristics of a pain expression from the list above.

When we find that horses, cats, rats, mice, rabbits, cattle, and humans all make similar facial expressions when in pain, we can suspect that such expressions have been stable during mammalian evolution, perhaps over the last 170 million years.

The case is not closed on invertebrates,47 but we’d all do better to assume that they experience pain and don’t like it one bit. Such a belief might serve us well if the rising oceans bring us to the day when lobsters rule the earth.

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1 Source: “Fish Hook Removal” by Silverfish8 at http://blog.clinicalmonster.com/2014/11/fish-hook-removal/

2 Wikipedia. <a href=”http://en.wikipedia.org/wiki/Occam’s_razor” target=”_blank”>Occam’s Razor</a>

3 Pastor, J., B. Soria, and C. Belmonte. 1996. Properties of the nociceptive neurons of the leech segmental ganglion. Journal of Neurophysiology 75: 2268–2279. http://jn.physiology.org/cgi/content/abstract/75/6/2268; Wittenburg, N., and R. Baumeister. 1999. Thermal avoidance in Caenorhabditis elegans: an approach to the study of nociception. Proceedings of the National Academy of Sciences of the United States of America 96: 10477-10482. http://www.pnas.org/cgi/content/abstract/96/18/10477 ; Illich, P. A., and E. T. Walters. 1997. Mechanosensory neurons innervating <em>Aplysia</em> siphon encode noxious stimuli and display nociceptive sensitization. The Journal of Neuroscience 17: 459–469. http://www.jneurosci.org/cgi/content/abstract/17/1/459 ; Robyn J. Crook and Edgar T. Walters Nociceptive Behavior and Physiology of Molluscs: Animal Welfare Implications. Institute for Laboratory Animal Research. http://nas-sites.org/ilarjournal/previous-issues/spineless-wonders-welfare-and-use-of-invertebrates-in-the-laboratory-and-classroom/nociceptive-behavior-and-physiology-of-molluscs-animal-welfare-implications/”>link</a>; Tracey, J., W. Daniel, R. I. Wilson, G. Laurent, and S. Benzer. 2003. painless, a Drosophila gene essential for nociception. Cell 113: 261–273. http://dx.doi.org/10.1016/S0092-8674(03)00272-1 ; Gentle MJ, Tilston V, McKeegan DE. Mechanothermal nociceptors in the scaly skin of the chicken leg. Neuroscience. 2001;106(3):643-52. http://www.ncbi.nlm.nih.gov/pubmed/11591464 ; Sneddon, L. U., V. A. Braithwaite, and M. J. Gentle. 2003. Do fishes have nociceptors? Evidence for the evolution of a vertebrate sensory system. Proceedings of the Royal Society of London. Series B. Biological sciences 270: 1115–1121. http://dx.doi.org/10.1098/rspb.2003.2349 ; Anand KJS, Hickey PR. Pain and its effects in the human neonate and fetus. New England Journal of Medicine. 317:21 (1987) 1321-1329.; Humphrey T. Some correlations between the appearance of human fetal reflexes and the development of the nervous system. Progress in Brain Research. 4 (1964) 93-135.; Valnaan HB, Pearson JP. What the fetus feels. British Medical Journal. 280 (1980) 233-234.

4 Victoria Braithwaite. Do Fish Feel Pain? Oxford University Press, 2010. <a href=”http://books.google.com/books?id=aMvonPqzu_cC&amp;printsec=frontcover#v=onepage&amp;q&amp;f=false” target=”_blank”>link</a>

5 Sneddon, Lynne U., Victoria A. Braithwaite, and Michael J. Gentle. “Do fishes have nociceptors? Evidence for the evolution of a vertebrate sensory system.” Proceedings of the Royal Society of London B: Biological Sciences 270, no. 1520 (2003): 1115-1121.

6 Michel-Antoine Leblanc “The Mind of the Horse” Harvard University Press, Nov 4, 2013 471 pages.

7 Saslow, Carol 2002. Unpublished study cited repeatedly in Leblanc “The Mind of the Horse”

8 Maldonado, H. and Miralto, A., (1982). Effects of morphine and naloxone on a defensive response of the mantis shrimp (Squilla mantis). Journal of Comparative Physiology, A, 147: 455–459; Lozada, M., Romano, A. and Maldonado, H., (1988). Effect of morphine and naloxone on a defensive response of the crab Chasmagnathus granulatus. Pharmacology, Biochemistry and Behavior, 30: 635–640; Dyakonova, V.E., Schurmann, F. and Sakharov, D.A., (1999) Effects of serotonergic and opioidergic drugs on escape behaviors and social status of male crickets. Naturwissenschaften, 86: 435–437; Zabala, N. and Gomez, M., (1991). Morphine analgesia, tolerance and addiction in the cricket, Pteronemobius. Pharmacology, Biochemistry and Behaviour, 40: 887-891; Dalton, L.M. and Widdowson, P.S., (1989). The involvement of opioid peptides in stress-induced analgesia in the slug Arion ater. Peptides:, 10:9-13; Kavaliers, M. and Ossenkopp, K.-P., (1991). Opioid systems and magnetic field effects in the land snail, Cepaea nemoralis. Biological Bulletin, 180: 301-309; Wittenburg, N. and Baumeister, R., (1999). Thermal avoidance in Caenorhabditis elegans: an approach to the study of nociception. Proceedings of the National Academy of Sciences USA, 96: 10477–10482; Pryor, S.C., Nieto, F., Henry, S. and Sarfo, J., (2007). The effect of opiates and opiate antagonists on heat latency response in the parasitic nematode Ascaris suum. Life Sciences, 80: 1650–1655; L. Sømme (2005). “Sentience and pain in invertebrates: Report to Norwegian Scientific Committee for Food Safety”.  Norwegian University of Life Sciences, Oslo.; Cephalopods and decapod crustaceans: their capacity to experience pain and suffering. Advocates for Animals; 2005.; Opiates in the Human Body <a href=”http://www.opiates.com/opiates/” target=”_blank”>link</a>

9 Susanne Dreborg, Görel Sundström, Tomas A. Larsson, and Dan Larhammar Evolution of vertebrate opioid receptors. Proceedings of the National Academy of Sciences of the United States of America. <a href=”http://www.pnas.org/content/105/40/15487.full” target=”_blank”>link</a>

10 Wikipedia. Sentience. <a href=”http://en.wikipedia.org/wiki/Sentient” target=”_blank”>link</a>.

11 Berg, Howard C. “Bacterial behaviour.” Nature 254, no. 5499 (1975): 389-392.

12 LeRoith, D., Shiloach, J., Roth, J., Liotta, A. S., Krieger, D. T., Lewis, M., & Pert, C. B. (1981). Evolutionary origins of vertebrate hormones: material very similar to adrenocorticotropic hormone, beta-endorphin, and dynorphin in protozoa. Transactions of the Association of American Physicians, 94, 52.

13 El-Salhy, M., S. Falkmer, K. J. Kramer, and R. D. Speirs. “Immunohistochemical investigations of neuropeptides in the brain, corpora cardiaca, and corpora allata of an adult lepidopteran insect, Manduca sexta (L).” Cell and tissue research 232, no. 2 (1983): 295-317.; Stefano, George B., and Berta Scharrer. “High affinity binding of an enkephalin analog in the cerebral ganglion of the insectLeucophaea maderae (Blattaria).” Brain research 225, no. 1 (1981): 107-114.

14 Stefano, G.B., and Scharrer, B., High affinity binding of an enkephalin analog in the cerebral ganglion of the insect Leucophaea maderae (Blattaria). Brain Res. 225 (1981) 107- 114.; Gallup, G.G., and Suarez, S.D., On the use of animals in psychological research. Psychol. Rec. 30 (1980) 211-218.

15 Williams, Clint. “How animals express pain”. Mother Nature Network November 26, 2012. Online at http://www.mnn.com/earth-matters/animals/stories/how-animals-express-pain

16 Ashley, F. H., Waterman-Pearson, A. E., & Whay, H. R. (2005). Behavioural assessment of pain in horses and donkeys: application to clinical practice and future studies. Equine Veterinary Journal37(6), 565-575.

17 Taylor, T.S. and Matthews, N.S. (1998) Mammoth asses – selected behavioural considerations for the veterinarian. Appl. anim. behav. Sci. 60, 283-289; Sanford, J., Ewbank, R., Molony, V., Tavernor, W.D., and Uvarov, O. (1989) Working Party of the Association of Veterinarians, Teachers and Research Workers: Guidelines for the Recognition and Assessment of Pain in Animals. Universities Federation of Animal Welfare (UFAW), Potters Bar, UK; Fleming, P. (2002) Non traditional approaches to pain management. Vet. Clin. N. Am.: Equine Pract. 18, 83-105.; Price, J., Catriona, S., Welsh, E.M. and Waran, N.K. (2003) Preliminary evaluation of post-operative pain in horses following arthroscopic surgery. Vet. Anaesth. Analg. 30, 124-137.

18 Sanford, J., Ewbank, R., Molony, V., Tavernor, W.D., and Uvarov, O. (1989) Working Party of the Association of Veterinarians, Teachers and Research Workers: Guidelines for the Recognition and Assessment of Pain in Animals. Universities Federation of Animal Welfare (UFAW), Potters Bar, UK.; Taylor, T.S. and Matthews, N.S. (1998) Mammoth asses – selected behavioural considerations for the veterinarian. Appl. anim. behav. Sci. 60, 283-289.; Taylor, P.M., Pascoe, P.J. and Mama, K.R. (2002) Diagnosing and treating pain in the horse; where are we today? Vet. Clin. N. Am.: Equine Pract. 18, 1-19.

19 Whitehead, G., French, J. and Ikin, P. (1991) Welfare and veterinary care of donkeys. In Pract. 13, 62-68.; Taylor, P.M., Pascoe, P.J. and Mama, K.R. (2002) Diagnosing and treating pain in the horse; where are we today? Vet. Clin. N. Am.: Equine Pract. 18, 1-19.; Price, J., Catriona, S., Welsh, E.M. and Waran, N.K. (2003) Preliminary evaluation of post-operative pain in horses following arthroscopic surgery. Vet. Anaesth. Analg. 30, 124-137.

20 Taylor, P.M., Pascoe, P.J. and Mama, K.R. (2002a) Diagnosing and treating pain in the horse; where are we today? Vet. Clin. N. Am.: Equine Pract. 18, 1-19.; Price, J., Catriona, S., Welsh, E.M. and Waran, N.K. (2003b) Preliminary evaluation of post-operative pain in horses following arthroscopic surgery. Vet. Anaesth. Analg. 30, 124-137.

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36 Lane, J.G. (1994) A review of dental disorders of the horse, their treatment, and possible fresh approaches to management. Equine vet. Educ. 6, 13-21.; Easley, J.K. (1999) Dental and oral examination. In: Equine Dentistry, Ed: G.J. Baker and J.K. Easley, W.B Saunders Co., London. pp 114-117.; Duffield, H.F., Bell, N. and Henson, F.M.D. (2002) Factors associated with impactive colic in the donkey. In: Proceedings of the 7th International Equine Colic Research Symposium, Equine Veterinary Journal Ltd, Newmarket. p 122.; Graham, B.P. (2002) Dental care in the older horse. Vet. Clin. N. Am.: Equine Pract. 18, 509-522.

37 Ashley, F. H., Waterman-Pearson, A. E., & Whay, H. R. (2005). Behavioural assessment of pain in horses and donkeys: application to clinical practice and future studies. Equine Veterinary Journal37(6), 565-575.

38 Darwin, C., Ekman, P., & Prodger, P. (1998). The expression of the emotions in man and animals. Oxford University Press, USA.

39 Sotocinal, S. G., Sorge, R. E., Zaloum, A., Tuttle, A. H., Martin, L. J., Wieskopf, J. S., … & McDougall, J. J. (2011). The Rat Grimace Scale: a partially automated method for quantifying pain in the laboratory rat via facial expressions. Molecular pain7(1), 1.

40 Langford, Dale J., Andrea L. Bailey, Mona Lisa Chanda, Sarah E. Clarke, Tanya E. Drummond, Stephanie Echols, Sarah Glick et al. “Coding of facial expressions of pain in the laboratory mouse.” Nature methods 7, no. 6 (2010): 447-449.

41 Holden, E., Calvo, G., Collins, M., Bell, A., Reid, J., Scott, E. M., & Nolan, A. M. (2014). Evaluation of facial expression in acute pain in cats. Journal of Small Animal Practice55(12), 615-621.

42 Keating, S. C., Thomas, A. A., Flecknell, P. A., & Leach, M. C. (2012). Evaluation of EMLA cream for preventing pain during tattooing of rabbits: changes in physiological, behavioural and facial expression responses.PLoS One7(9), e44437.

43 Gleerup, K. B., Andersen, P. H., Munksgaard, L., & Forkman, B. (2015). Pain evaluation in dairy cattle. Applied Animal Behaviour Science171, 25-32.

44 Photo source: Sotocinal, S. G., Sorge, R. E., Zaloum, A., Tuttle, A. H., Martin, L. J., Wieskopf, J. S., … & McDougall, J. J. (2011). The Rat Grimace Scale: a partially automated method for quantifying pain in the laboratory rat via facial expressions. Molecular pain7(1), 1.

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46 Photo source: “Body Language Quiz. Test your Emotional Intelligence”. Greater Good. Online at http://greatergood.berkeley.edu/ei_quiz/

47 Eisemann, C. H., W. K. Jorgensen, D. J. Merritt, M. J. Rice, B. W. Cribb, P. D. Webb, and M. P. Zalucki. “Do insects feel pain?—A biological view.” Cellular and Molecular Life Sciences 40, no. 2 (1984): 164-167.; Fiorito, G. “Is there “pain” in Invertebrates?.” Behavioural Processes 12.4 (1986): 383-388.; Mather, Jennifer A. “Animal suffering: An invertebrate perspective.” Journal of Applied Animal Welfare Science 4.2 (2001): 151-156.; Stefano, George B., Beatrice Salzet, and Gregory L. Fricchione. “Enkelytin and opioid peptide association in invertebrates and vertebrates: immune activation and pain.” Immunology today 19, no. 6 (1998): 265-268.

 

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