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Can Fishes Feel Pain?

Review and Commentary

James D. Rose. The neurobehavioral nature of fishes and the question of awareness and pain. Reviews in Fisheries Science, vol. 10 (2002) pp. 1–38.

 

Many of us would claim that fishes’ behavior clearly demonstrates that they can suffer. Yet, James D. Rose argues that scientific evidence strongly favors the conclusion that fishes do not feel pain. If valid, this has implications for animal advocacy.

Rose’s argument rests heavily on the observation that human consciousness depends on the neocortex, which is absent in fishes. While humans, fishes, and other vertebrates share more primitive spinal cord and brainstem structures, neuroanatomic studies show that the fishes' cortex is far less developed than that of humans and does not include a neocortex. Decorticate humans, who have no input from the brainstem to the cortex due to trauma, stroke, or other damage, can still exhibit behavioral responses to stimulating nociceptors (nerve receptors that are stimulated by injury). Such humans will withdraw a stimulated limb, vocalize, exhibit a facial grimace, and release hormones and neurotransmitters associated with pain, even though there is no evidence that they are conscious. Even people under general anesthesia receive analgesics to block the hormonal responses that unconsciously accompany nociception.

The spinal cord and brainstem, which are much older structures than the cortex in terms of evolution, mediate the primitive withdrawal responses to nociception in all vertebrates. Spinal cord and brainstem activity occurs unconsciously in humans and, presumably, other animals. When you step on a tack, high velocity neurons to and from the spinal cord mediate rapid withdrawal of the foot, and slower impulses go the brain where, about a second later, you experience a decidedly unpleasant sensation.

Humans and fishes have evolved independently for 400 million years. In humans, consciousness, including conscious perception of pain, requires multiple inputs from frontal and parietal neocortex structures that fishes lack. Further, frontal cortical structures that are absent in fish mediate the unpleasant aspect of pain perception in humans. People lacking input from the frontal cortex (due to disease, trauma, surgery, etc.) relate that they can feel painful stimuli, but it doesn't bother them.

Fishes do have a cortex, which is anatomically far less complicated and much smaller in proportion to the brainstem than that of humans. The cortex in fishes is involved in sensory reception (particularly smell) and modulates responses to nociception, but fishes’ cortex is not essential for their normal responses to noxious stimuli.

Rose addresses several possible objections to his thesis:

Fishes react to injurious or threatening stimuli. The ability to response to such stimuli is also seen in unicellular organisms and multicellular organisms without brains, and we do not attribute consciousness to them. And, humans have unconscious brainstem- and spinal cord-mediated responses to such stimuli.

Fishes have nerve receptors that respond to opiates, which deaden pain in humans. Like humans, fishes have opiate receptors in their brainstems. Their presence in human brainstems demonstrates that opiate receptors have functions besides modulating conscious awareness of pain. It is easy to envision their use in modulating behavior without requiring consciousness. For example, an animal with an injured leg benefits from nociceptive reflexes that encourage keeping weight off the leg, but the animal is best served by suppressing this reflex (with opiate-like neurotransmitters) when being chased by a predator.

Noxious stimuli alter fish behavior. While learning would appear to be compelling evidence for consciousness, it appears that humans gain considerable learning without conscious awareness. Perhaps humans require consciousness in order to learn about and make subtle discriminative choices in much more complex situations than fishes experience. Just as a computer can be programmed to unconsciously learn from experience, fishes may exhibit analogous abilities. Fishes can exhibit simple associative learning, and humans seem to have this capacity at an unconscious level as well. Furthermore, fish behavior is largely preserved after their cortex has been removed, with the principle deficit being loss of responses to smell, because their sense of smell is mediated by their cortex.

While inconclusive, it appears that fishes have analogous structures to the human limbic system, which is involved in emotions and in life-sustaining behaviors in mammals, including reproduction, aggression, defense, feeding, and drinking. In humans, consciousness of emotions requires the neocortex that, again, is absent in fishes. Furthermore, a component of the limbic system known as the cingulate gyrus appears to be essential for the human emotional response to pain. This structure has been identified only in mammals, and researchers have done extensive neuroanatomic research on fish.

Rose argues that, if fishes were conscious, their experiences would differ so fundamentally from those of humans that any attempts at empathetic understanding would be futile. Neuroanatomy research, including studies on injured humans and invasive experiments on animals, has shown that specific functions require specific neuroanatomic structures. Lacking the human structures involved in consciousness and emotional perception, Rose asserts that it is unreasonable to believe that fishes experience noxious stimuli in analogous ways to humans. Similarly, we can't prove that plants are insensate, but we strongly doubt that they experience damaging events as we do, because they lack the anatomic components we regard as essential to conscious experience.

I think that the burden of proof is on those who would harm fishes, particularly if the damage to fishes’ well-being were unnecessary. It seems most reasonable to avoid harming fishes because there is no way to know for certain whether or not they can have some kind of unpleasant sensation. Just as we can’t empathize with a bat’s echolocation, we may be unable to empathize with what a fish feels when hooked, but the possibility that fish find the experience subjectively unpleasant is good reason to refrain.

 

Stephen Kaufman, M.D., is Clinical Assistant Professor at Case Western Reserve and Northeastern Ohio Universities College of Medicine, cochair of both the Medical Research Modernization Committee and the Christian Vegetarian Association, President of Vegetarian Advocates, and Advisor to Vegan Outreach