Researchers have determined that octopuses — the most neurologically complex invertebrates — both feel pain and remember it, responding with sophisticated behaviors and shedding new light on the unsolved mystery of how invertebrate animals experience pain.
After experiencing a short burst of pain, octopuses showed a conditioned preference for locations where pain was relieved, while avoiding the location where the pain occurred. The octopuses also demonstrated pain-specific grooming behaviors. The findings, published Feb. 22 in iScience, demonstrate that the octopus brain is sophisticated enough to experience pain on a physical and a dispositional level.
“This is a highly complex ability — to link a subjective feeling about their bodily state with a novel context, and then remember and avoid that context later,” said study author Robyn Crook, an assistant professor of marine biology at San Francisco State University. “This suggests that the animal is aware of an ongoing pain state. This is the first time such an ability has been shown in cephalopods.”
Invertebrates make up at least 97% of the animals on Earth, but not much is known about their experience of pain. While studies have shown that simpler invertebrates like insects are unlikely to experience pain, at least in the same way humans do, invertebrate pain is a rising topic of debate in the biological community.
Octopuses and other cephalopods are extreme outliers among invertebrates due to their astounding brains. Octopuses are not only more intelligent than their invertebrate cousins, they demonstrate some of the same cognitive abilities as smaller vertebrates such as reptiles and amphibians. The neurological complexity of octopuses makes them an ideal subject for invertebrate pain studies.
“To be honest, I wasn’t terribly surprised by the findings based on my years working with cephalopods,” Crook told The Academic Times. “What I would say is that among the broader community of invertebrate researchers, there is wide divergence of views on the question of pain. Here, I think we move the question of cephalopod pain beyond any reasonable doubt.”
Pain is a multifaceted phenomenon, with both physical and psychological elements. Crook and her team showed in a previous study that cephalopods have the required sensory neurons to experience and locate pain physically, known formally as the discriminatory component of pain.
What was missing was whether that pain can then shape the animal’s state of mind, mood or behaviors, also known as the affective component of pain. While many animals show an immediate reflexive response to pain, making the choice to avoid pain is a much more sophisticated cognitive skill.
“In cephalopods, we do not yet know what the neural circuits behind the affective experience of pain look like or whether they are similar to vertebrates,” said Crook. “However, the behavioral and neurophysiological data in this study suggest so.”
To demonstrate the ability of octopuses to respond to pain, Crook set up conditioned place preference assays, a type of study in which researchers expose animals to a stimulus in a certain location, then measure the amount of time the animal spends in that location.
For this study, eight octopuses received a small injection of acetic acid, producing a sharp but harmless burst of pain “similar to what we would experience if we were to get lemon juice into a paper cut,” Crook said. They were then placed into one chamber of a multi-chamber tank. Octopuses avoided the initial chamber when allowed to move around, indicating that they understood and remembered experiencing pain in that location.
Octopuses also showed a strong preference for a chamber in which a researcher was waiting with an analgesic shot to relieve the pain, showing that in addition to avoiding pain, octopuses are able to seek out sources of relief. Octopuses that did not receive the initial burst of pain showed no location preferences.
The octopuses that received injections also groomed themselves around the injection site, demonstrating ongoing awareness of the irritation. Once the pain was relieved, the grooming stopped.
The study involved inflicting a small amount of pain onto the animals, and they were euthanized at the conclusion of the study. But Crook emphasizes that the larger purpose of this study, and her future work, is to promote animal welfare, including in experiments.
“One of the goals of this study was to develop a robust way of measuring affective state in cephalopods so that we can begin to evaluate analgesics and other interventions that can promote welfare in these animals. To me this is the most important outcome of this study,” she said.
She also hopes that the work will have a wider reach for all who those work with these animals.
“My hope is that with work like this we can make measurable, validated improvements to the welfare of cephalopods as they encounter humans: in research labs obviously, but also in zoos and aquariums, and potentially in fisheries,” she said.
The study, “Behavioral and neurophysiological evidence suggests affective pain experience in octopus,” published Feb. 22 in the journal iScience, was authored by Robyn J. Crook, San Francisco State University.