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.”

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 feel 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 Razor1 — the principle of parsimony, of economy, of succinctness — guides us to select the simplest explanation which accounts for the facts.

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 injury to survive. Long ago, Mother Nature got it right about injury and pain.  Pain defends us all from further injury.

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, mammals, and — sufferin’ catfish — in fish.2.

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 nociceptors.3  The face of a fish is very sensitive to pain.

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.  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 humans.4.  A genetic study has found that the complete vertebrate opioid system was already established in the first jawed vertebrates.5.

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”6  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.

Not Like Us

Fish are not exactly like us. But those who think that our reaction to pain is part of what separates man and beast are wrong.  It is one of the things that binds us together.  As a warning of trouble  — hot, cold, sharp, crushing — it has guided all animals since they first had a choice of which way to travel.  Pain is a fundamental of learning, of adapting, and of survival of both an individual and a species.  Pain is not good. It hurts.  It is a quick teacher.

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

End Notes

Show 6 footnotes

  1. Wikipedia. Occam’s Razor
  2. Pastor, J., B. Soria, and C. Belmonte. 1996. Properties of the nociceptive neurons of the leech segmental ganglion. Journal of Neurophysiology 75: 2268–2279. link; 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. link; Illich, P. A., and E. T. Walters. 1997. Mechanosensory neurons innervating Aplysia siphon encode noxious stimuli and display nociceptive sensitization. The Journal of Neuroscience 17: 459–469. link; Robyn J. Crook and Edgar T. Walters Nociceptive Behavior and Physiology of Molluscs: Animal Welfare Implications. Institute for Laboratory Animal Research. link; Tracey, J., W. Daniel, R. I. Wilson, G. Laurent, and S. Benzer. 2003. painless, a Drosophila gene essential for nociception. Cell 113: 261–273. link; Gentle MJ, Tilston V, McKeegan DE. Mechanothermal nociceptors in the scaly skin of the chicken leg. Neuroscience. 2001;106(3):643-52. link; 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. link
  3. Victoria Braithwaite. Do Fish Feel Pain? Oxford University Press, 2010. link
  4. 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 link
  5. 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. link
  6. Wikipedia. Sentience. link.