No Brain, No Pain?

In general, humanity treats invertebrates with very little regard. Lobsters and crabs are boiled alive. At seafood processing plants the claws, tails, and heads of living lobsters are ripped apart and sorted so the meat can be extracted and canned. Pesticides are used that cause insects to convulse and die. Scientists perform experiments on snails and roundworms with no ethical considerations. Very few countries have animal welfare legislation that protects any invertebrate species [1].

This is all done because invertebrates have long been thought to be unable to experience pain. Instead, invertebrates are thought to only be capable of nociception. Nociception is the ability to detect and react to noxious stimuli, which may endanger an organism [2]. Pain, on the other hand, is a subjective emotional experience associated with real or perceived tissue damage [3]. To help distinguish the two, recall the last time you accidentally touched something hot. Your hand instantly recoiled, but the pain arrived a few moments after the fact. That instant recoil is a reflex that is the result of activation of specialized sensory neurons called nociceptors [4]. These cells have the ability to sense the potential for tissue damage and they initiate mechanisms to protect against it. Pain only comes into the picture when the sensory information from these cells reaches the brain and is interpreted by structures of the cerebral cortex as a negative mental state.

Evaluating pain is very difficult, as there is no good way to measure it. Pain is an emotion, and there is no stethoscope for feelings. If a doctor wants to know how intensely their patient is hurting, the only thing that can be done is to ask the person. Since this approach does not work with creatures that cannot talk, when scientists try to gauge pain in animals they rely on comparing animal behaviour to human experience. If a chimpanzee exposed to electrical shock is observed reflexively pulling its hand away, most would agree that the chimpanzee experienced pain. We assume that the chimp’s experience is similar to ours. However, when the same argument is applied to crustaceans, molluscs, insects, and worms, fewer people find the argument convincing. A crab recoiling from an electric shock is chalked up as nociception without the involvement of pain [5].

It is claimed that invertebrates lack a nervous system complex enough to support an experience like pain. Dr. James D. Rose, a biologist at the University of Wyoming, writes that the experience of pain in humans is dependent on the capacity for consciousness [6]. Since consciousness is supported by the functions of the neocortex, which is unique to mammals, Rose argues that creatures without a neocortex are unlikely to experience pain. Others contend that invertebrates may simply process pain in a different way [7]. After all, mammals and crustaceans have totally different visual systems, yet both are capable of vision.

Some biologists are beginning to be swayed by increasing evidence for invertebrate suffering. Nociceptors and nociceptive behaviours have been identified in several invertebrate species [8-12] and opiates have been shown to influence those behaviours. When Neohelice crabs are treated with morphine, they exhibit decreased defensive behaviours in response to electric shock [5]. Similar effects can be observed in shrimp [13], roundworms [14], and snails [8]. Morphine acts directly on the central nervous system to relieve pain in humans. So the effects of the drug in invertebrate species suggest that information from nociceptors is being processed and integrated within higher circuits of the nervous system, which is akin to pain in mammals.

A biologist at Queen’s University Belfast, Dr. Robert W. Elwood, believes that the best evidence for invertebrate pain comes from studies that show that invertebrates can learn to avoid harmful stimuli. A recent study from Elwood’s research group involved exposing shore crabs to a brightly lit tank with the choice of two shelters to get out of the light [15]. Shore crabs avoid brightly lit open spaces in order to prevent being easily spotted by predators, so they quickly scuttle into a shelter. Entering one of the two shelters results in electrical shocks. When given the same choice again and again, the crabs learned to avoid the ‘bad’ shelter. Avoidance learning has also been demonstrated in fruit flies [16], crayfish [17], and hermit crabs [18, 19]. According to Elwood, this behaviour demonstrates that, in certain invertebrates, central processing is involved in adapting to the hazards of the environment, which is consistent with the experience of pain [7].

However, avoidance learning does not necessarily require the experience of pain. Imagine a student has class in a poorly maintained lecture hall. On the first day of class, the student discovers that they chose a seat underneath a ceiling vent, which blows cold air down on the student. On the next day of class, the same student remembers to avoid that seat and checks for ceiling vents before choosing a new place to sit. The student is avoiding ceiling vents, but they did not have to be hurt to adopt this behaviour. Perhaps to shore crabs, being shocked is more annoying than it is painful. Stimuli that are painful to humans are not necessarily painful to other creatures. Capsaicin, the molecule that gives chili peppers their heat, reliably activates human nociceptors, but naked mole rats show no sign of pain when exposed to the chemical [20]. Naked mole rats are also totally unfazed by contact with acid, demonstrating the need to be careful about assumptions in science.

The truth of the matter is far from clear, but at the very least we should acknowledge that there is a possibility that invertebrates suffer. If we do discover that flies, worms, snails, and crabs do feel pain, an enormous re-evaluation of how humanity should interact with the world will be required; new ethical standards will have to be set. Or perhaps the hypothesis will be disproven. In any case, think about that the next time you have a lobster dinner.


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Image Credits

  • Anderson G. 2004. Aplysia californica. (accessed February 28, 2014).
  • Zalewski M. 2009. A Shore Crab in Western Washington. (accessed February 28, 2014).
  • Karwath A. 2005. Drosophila melanogaster – side. (accessed February 28, 2014).
  • Goldenstein B. 2007. Caenorhabditis elegans, adult hermaphrodite. (accessed February 28, 2014).

Alexander B. Kim is a student at the University of Calgary majoring in neuroscience. Alexander has spent his summers working in neuroscience labs at the Hotchkiss Brain Institute. Follow The Triple Helix Online on Twitter and join us on Facebook.

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