Pain can’t be measured. Experience can’t be measured. Many animals obviously experience pain. If crabs feel pain what does that say about other creatures’ experiences?  

Do animals need a particular set of nerves or brain structures to feel pain? Many animals show surprising levels of cognition without specific brain structures. Do they  also experience pain? 

Pain research in animals has always been controversial. But, recent studies demonstrating crabs do feel pain raise questions about other animals.

Since pain cannot be measured, all studies use subjective human reports or observation of animal behavior. In the future brain imaging might help define pain, as some similar EEG patterns have been observed in humans and a variety of animals. But, many important aspects of human pain are difficult to study because they involve the higher cortical centers that are important in a wide range of human emotions.

Reflex or Internal Experience

The study of pain must separate a reflex from an internal experience. Nociception is defined as a neurological circuit that responds to painful stimuli as a reflex. What if the behavior of the animal changes? What if the animal remembers the pain and deliberately attempts to avoid the source of pain? Deliberately avoiding the painful experience implies conscious behavior.

It is obvious to those who have pets, or farm animals, that many animals feel pain. The source of controversy is based on how conscious people believe animals are.

There can be no real question that our close relatives, the primates, experience pain. Most experts now accept that all mammals experience some form of pain.

Many other animals not only experience pain, but they experience psychic pain in the form of grieving. A previous post described grieving in birds, a shocking finding. Elephants have also shown grieving with ritual attention to the dead. Elephants mourn loved ones by returning to the body of the deceased for years to place leaves and sticks on the site.

Reptiles show surprising social intelligence. A previous post shows that rats show empathy to other rats, an advanced form of awareness of other animals’ pain.

What Type of Brain Feels Pain

If humans and other animals with elaborate brains experience pain, at what point in brain development are there enough neurons to experience pain. Or could pain be like cognition, that even a bacterium, a single cell, has it?

Higher-level capacities can occur in animals with totally different brain structures than humans. The neocortex has been assumed to be unique in the higher order mammals with six distinct cortical layers of advanced neurons in humans. But,recent research finds both birds and reptiles have unique centers in their brain housing neurons that appear to be similar to those in the mammals neocortex levels 4 and 5, but in different structures.

It is accepted now that nociception exists as a reflex in most animals. Nociceptors (pain receptors) are found in mollusks (snails, clams, mussles, squid), nematodes (roundworms), annelids(earthworms, leeches), and drosophilia (fruit flies).

Crustacean (crabs, lobsters, crayfish, shrimp, krill) experience of pain has been controversial until now. One way of observing pain has been grooming behavior, known as a sign of pain in vertebrates. Other signs of pain include showing signs of stress and specifically altered behavior to avoid the source of pain.

Styles of Animal Pain

The sensation of pain in sheep and cows is similar to that in humans. But, this pain is often not recognized because of the style of expression of the pain.

Animals that are raised in groups are used to calling out for help, and express noise when feeling pain. These animals include dogs, pigs, rabbits and human beings.

Animals, raised individually, like cows, dear and sheep, do not call out when they feel pain because they don’t expect help. Calling out would allow a predator to know who is hurt and injured, drawing attention to them. Animals that do not expect help do not call out.

Human observers who don’t understand the stoic nature of animals raised separately misjudge the obvious pain experienced by farm animals.

It is certainly clear to the scientific community that primates, dolphins and octopus feel pain. In fact, there are rules to avoid unnecessary pain in experiments. Also, veterinarians have developed a pain patch for dogs after surgery. It has only been in the past generation that veterinarians were trained to understand pain in many animals.

Confounding Factors

While pain induced reflexes occur in humans, such as instantly pulling away from fire, the neo cortex is very significant in the humans nuanced experience of pain. It is now clear that the cortex modulates pain fibers in the spinal cord, and many emotional and social factors are extremely significant to the experience of “real” pain.

Because animals have advanced capacities in structures that are very different from the human, it is not possible to make easy comparisons. For example, fish, lobsters and octopi see without an optical cortex, which is essential for humans.

Another confounding variable is that some animals release damaged parts. This doesn’t mean that they cannot feel pain. For example, it has been assumed that animals like crabs do not experience pain when an appendage is broken off, because they can grow appendages back. But, recent research shows that when some crabs had their legs twisted off, they went into a form of shock and could not grow back the appendage. Some died from this shock.

Crabs Feel Pain

Increasing evidence shows that crabs feel pain, not just nociception. There is now a wide range of studies that demonstrate pain in crabs.

Crabs place a high value on dark hidden places beneath rocks, a place free from predators. The most recent study showed that crabs are willing to give up their highly prized dark shelter to avoid future pain.

Ninety crabs participated in the study. One at a time a crab was introduced to a tank with two areas of dark shelter. After the crab chose their particular dark shelter some were given an electric shock. When again returned to the tank, all of the crabs went to the dark shelter where they had been previously.

Some of the crabs that had received a shock were then given a second shock. The crabs that were shocked twice did not return to their dark shelter the third time, but rather went to the alternative place. The ones that were not shocked continued to go only to their chosen region.

This study clearly showed the awareness of where the shock occurred and the deliberate avoidance of the shock. This study definitely showed cognitive awareness and experience of pain.

Shocks Inside the Shell

A different experiment showed that crabs would even give up their shells to avoid pain.

The crabs in the study were given shocks within the shells.

Later, the ones that had been shocked, but not the others were offered and accepted a new shell. They left the shell in which they were living, where they had been shocked and moved to a new shell. The crabs that had not been shocked didn’t do this. Crabs also demonstrated behaviors that are known to express stress such as grooming their abdomen or banging their abdomen against the shell.

Importantly, the crabs were only given the option of the new shell after the shocks had ended. This means that they remembered the pain and acted deliberately to avoid future pain, showing conscious choice, not a simple reflex.

Other important facts are consistent with crabs experiencing pain. Crabs are noted to have appropriate receptors for pain in their primitive brain. They show reactions that are protective such as limping or rubbing after being hurt.

Research shows that hormones similar to our adrenalin are released at the time of the pain. When shocked crabs are given pain medications they don’t respond as much to the painful stimuli.

In another study the antennae of prawns were rubbed in acetic acid. These prawns started grooming the particular area and rubbing it on the side of the tank. If a local anesthetic was given, they didn’t do this behavior.

The understanding that crustaceans do feel pain, hopefully, will alter how they are treated. No one would assume, anymore, that primates’ pain should be ignored.

Neuroanatomy and Pain

There are other physiological reasons to assume crabs and lobsters experience pain.

Opiates and an opiate antagonist, naloxone, worked in a similar manner with crabs and humans. There was a dose response relationship with avoidance of electrical shocks. Crustaceans have similar endogenous opiods to those that moderate pain in humans.

Many assume lobsters violent reaction to being put into boiling water is a reflex. But, lobsters respond to opiod medications in the same way as mammals with less pain behavior. With opiods they don’t have the violent reaction. Some attempts to give lobsters less pain have first used a stab in their brain before the boiling water. The problem with this approach is that the lobsters have multiple ganglia and disabling only the highest one doesn’t cause death or unconsciousness.

Chronic pain is stopped in humans who remove the anterior cingulate cortex (ACC.) Similarly, rats with ACC damage were less affected by pain, and even chose to accept shocks rather than leave their comfortable dark chamber.

Ritual Slaughter

A study showed that if a calf is first stunned, signs of pain do not appear during religious ritual slaughter where the throat is cut. Previous study showed that slaughter does cause pain for the calf and therefore many countries have laws where stunning must occur first. But, this rule is exempted for religious practice.

New EEG studies show a similar pain pattern in humans and at least eight other animal species. Even with mild anesthesia these patterns could be seen, which implies that if awake the animals would feel the pain.

When animal’s throats were cut, the EEG patterns lasted for 2 minutes. The animal lost consciousness after 10 to 30 seconds. The pain was shown to be from cutting the nerves in the throat, similar to a neuropathy, where a damaged nerve sends pain signals. The pain was not related to the blood loss. Research where stunning occurred seconds after the throat is cut, EEG pain signals immediately stopped. This disproves the theory that cutting the throat serves as a stunning mechanism and shows that stunning first would avoid the pain. 

Fish Pain

Another controversy is whether fish feel pain. Fish show surprising intelligence (see post on Brains at Sea). The Tusk fish uses rocks to crack open shells to get the meat. The Tusk fish must perform very difficult maneuvers to accomplish this, picking up the shell and then finding a large rock. Holding the shell in its mouth, the fish repeatedly twists its head back and forth to smash the shell against the rock. Also, research with African cichlids show inference ability, understanding A fights better than B, who fights better than C.

There is reason to believe that fish experience pain, as do birds and mammals.

Fish appear to have the same nervous system structures as birds and mammals, including specialized pain nerve fibers for nociception from tissue damage and pain. They show the same responses to pain as human beings with behavioral pain reactions and reduction in the behaviors with pain medications.

A report showed that goldfish consciously experience pain, rather than a reflex reaction. The fish knew the spot where they had experienced the pain showing avoidance behavior. When the fish were given morphine they did not do this.

In killing fish for eating, clearly electrical stunning would stimulate less pain than dying slowly from loss of oxygen.

Is it Pain?

Many animals use nociceptor nerves to detect noxious stimuli. Most researchers believe that these nerves are not sufficient to assume the animal experiences pain, but rather represent an unconscious reflex to noxious stimuli. The animal must also be aware enough to show a behavioral reaction, more than a simple reflex, and perhaps future avoidance behavior.

Study of whether an animal is conscious is complex and confusing with some researchers assuming the animal must show mental representations. But, recent studies show remarkable cognition in birds, reptiles, and insects. In fact, bees have shown the ability to use abstract concepts (see post, The Remarkable Bee Brain.) Previous posts have even described decision-making,and group communication in microbes.

In humans, pain is further complicated by the ability to unconsciously suppress the experience of even very severe pain. In battle, some pain doesn’t reach consciousness immediately, but is suppressed to help emergency survival. A soldier’s leg is blown up, but he still pulls his comrade to safety and only notices the injured leg when he sits down in a safe spot. It is only then that he experiences the severe pain.

Also there is referred pain, where people feel pain in a limb that doesn’t exist.

Where is Pain?

Just as cognition and decision-making are seen at every level of creatures, even unicellular bacteria, is it possible that pain is a natural part of experience and is felt by any creature with cognition? 

Specific human brain structures do not necessarily define the possibility of pain as much as previously thought. Many animals show advanced abilities without human apparatus, instead having different types of brain structures with advanced neurons.

Pain can’t be measured. Many animals obviously experience pain. Many other animals show surprising levels of cognition and may experience pain.

New research shows crabs feel pain. If crabs feel pain what does that say about the experiences of other creatures?