Do Sharks Feel Pain?
Of all the stirring tales of maritime lore, few are as spectacular and chilling as the shark feeding frenzy. During a full-blown frenzy, sharks are reputedly driven 'mad' by an insatiable blood lust. According to whalers' wide-eyed but seemingly earnest accounts, in the throes of violent gluttony, sharks feeding at a whale carcass are undeterred from their bloody feast by even the most serious of injuries - such as being savagely stabbed and sliced by flensing knives (used by whalers to strip blubber from whale carcasses) or bitten repeatedly by other sharks. There are even reports of sharks that have been split in two by whalemen or disemboweled by other sharks continuing to feed, apparently ignoring their mortal wounds as though insensate to them. From vivid stories such as these, it came to be believed that sharks do not feel pain.
A precise neurological understanding of pain remains elusive. We do know that, in vertebrates, sensory neurons (nerve cells) penetrate most tissues and are responsible for relaying information to the central nervous system from the rest of the body. In mammals, two morphologically distinct populations of sensory nerve cells have been identified: relatively thick neurons having axons sheathed with myelin (a fatty, electrically insulating layer) and thinner neurons without. These thinner, unmyelinated neurons respond only to stimuli sufficiently intense to cause tissue damage and are termed nociceptors. In humans, tissue damage activates nociceptors, evoking a sensation of pain. Conversely, people who are born insensitive to pain virtually lack such nociceptors. Therefore, nociceptors are essential to the ability to perceive pain.
A fascinating 1993 paper by Australian neurophysiologists Peter Snow, Mark Plenderleith, and Layne Wright examined the sensory neurons of three species of elasmobranchs. Electron micrographs (images from an electron microscope) were used to count the number of myelinated and unmyelinated neurons in a key portion (the dorsal root ganglia) of the nervous systems of the Giant Shovelnose Ray (Rhinobatos typus), the Honeycomb Whip-tail Ray (Himantura uarnak), and the Blackfin Reef Shark (Carcharhinus melanopterus). Snow and his co-workers found that less than 1 percent of the neurons of both ray species were unmyelinated, while from 14 to 38 percent of the shark neurons were unmyelinated. In humans, by comparison, about 50 percent of neurons are unmyelinated.
Snow and his co-workers concluded that elasmobranchs lack the neuronal machinery absolutely essential for the perception of pain. The researchers went on to speculate that, for sharks and rays, the ability to perceive pain might have little relevance to survival. It is difficult to imagine how this could be. We humans rely on the sensation of pain to indicate that our bodies are not functioning properly, so that we might take action to limit the damage caused by injuries. It is remarkable that sharks, which have proven to be extremely sensitive to many other stimuli, would be insensate to sensory cues that could warn of injuries that could prove life-threatening in a dangerous world.
Yet, according to the available neurophysiological and behavioral evidence, sharks do not seem to feel pain. If sharks do feel pain, then their mechanism and experience of this noxious sensation must be very different from our own.