Pain is an unpleasant feeling that happens when there is or might be harm to body tissues. Scientists and philosophers agree that non-human animals, including amphibians, can feel pain.
Pain is a complex experience that involves both how something feels and the emotional reaction to it. Because of this, it is difficult to know for sure if non-human animals feel pain just by watching them. However, scientists often believe animals feel pain based on signs of awareness of feelings, which can be studied by comparing brain structures and observing physical and behavioral reactions.
Amphibians, especially frogs, meet several conditions that suggest they may feel pain. These include having a nervous system and sensory receptors, opioid receptors that respond to painkillers and local anesthetics, changes in body functions when exposed to harmful stimuli, protective movements, learning to avoid harmful experiences, and making choices between avoiding pain and other needs.
Pain in amphibians has effects on society, such as their exposure to harmful chemicals, being used in food (like frog legs), and being used in scientific studies.
Many scientists believe amphibians can feel pain, but this idea is still debated because their brains and nervous systems are different from those of other vertebrates.
Background
The idea that amphibians and other non-human animals might feel pain has been studied for a long time. At first, people talked about this based on ideas and philosophy, but now scientists study it carefully.
A long time ago, in the 17th century, the French philosopher René Descartes believed that animals could not feel pain or suffering because they did not have consciousness. Later, in 1789, the British philosopher Jeremy Bentham wrote that the important question was not whether animals could think or speak, but whether they could suffer.
In 1975, Peter Singer, a bioethicist, wrote that animals with smaller brains or less consciousness might still feel pain. Bernard Rollin, who helped create laws about animal pain relief, said that scientists were unsure until the 1980s whether animals felt pain. He often had to explain that animals could feel pain using scientific evidence.
By the 1990s, scientists and philosophers studied how animals think and feel. Some research suggested that animals, especially amniotes, might have simple thoughts and feelings. It is now less common to believe that animals feel pain in a way different from higher primates.
Animals without a neocortex can still feel emotions. Evidence from brain structure, brain chemicals, and brain activity shows that non-human animals, including mammals, birds, and even octopuses, have parts of the brain that support conscious states. This means humans are not the only animals with brain parts that create consciousness.
In the 20th and 21st centuries, scientists studied pain in non-human animals. For example, studies from the early 2000s showed that rats with arthritis chose pain-relieving drugs on their own. In 2014, a journal article said that all mammals can feel pain. In 2015, research showed that rats, mice, rabbits, cats, and horses make facial expressions when in pain.
At the same time, studies found that birds with walking problems chose a diet with a pain-relieving drug. In 2005, scientists said that birds likely feel pain in ways similar to mammals. In 2014, they confirmed that birds can feel and respond to pain.
Veterinary studies also showed that reptiles feel pain in ways similar to mammals, and pain-relieving drugs work for them. Scientists have also studied whether fish feel pain. In 2004, researchers said that fish might feel pain, fear, and stress in ways similar to other animals. In 2009, a European group said fish can feel pain. In 2015, a scientist wrote that fish likely feel pain like other vertebrates.
In 2012, the philosopher Gary Varner reviewed research on animal pain. He said that animals with certain brain and behavior traits might feel pain. Based on this, he concluded that all vertebrates, including amphibians, probably feel pain, but most invertebrates, except for cephalopods, likely do not.
Experiencing pain
Pain is a complex experience that involves two main parts. The first part is called nociception, which is the body's ability to sense harmful or dangerous stimuli. When something harmful happens, like touching something hot, the body quickly moves away from the source of the harm. This reaction happens automatically and does not involve feeling pain yet. For example, a finger might pull away from a hot object before the person feels pain.
The second part is the actual experience of pain, which is the emotional and internal feeling that happens after the body moves away from the harmful stimulus. This is when the person starts to feel hurt or uncomfortable. Pain is a personal experience and cannot be directly measured in animals other than humans. Scientists use a method called argument-by-analogy to study whether other animals feel pain. This method compares how animals react to harmful stimuli with how humans react, assuming similar reactions might mean similar experiences.
Nociception usually works by sending signals through nerve fibers from the part of the body that is harmed to the spinal cord and brain. This process can cause quick reactions, such as flinching or pulling a limb away, without the brain being involved. Nociception is found in many types of animals. Scientists can observe nociception using imaging tools and by studying how animals behave when exposed to harmful stimuli.
Sometimes, people distinguish between physical pain and emotional or psychological pain. Physical pain happens when the body is hurt, such as from an injury. Emotional pain is the feeling of sadness or hurt that occurs without physical harm, like after losing a loved one. Some scientists believe only primates can feel emotional pain because they have a part of the brain called the neocortex. However, research shows that animals like monkeys, dogs, cats, and birds can show signs of emotional pain, such as losing interest in activities, becoming less active, or not eating.
Nerve signals from nociception can reach the brain, helping the person understand where the pain is, how strong it is, and how unpleasant it feels. This part of pain involves being aware of the sensation and the discomfort it causes. Scientists do not fully understand how the brain processes this awareness.
Several lists of criteria have been created to determine if non-human animals can experience pain. These criteria include:
1. Having a nervous system and sensory receptors to detect harm.
2. Having opioid receptors and showing less reaction to harmful stimuli when given pain relief.
3. Showing changes in body functions when exposed to harmful stimuli.
4. Reacting with protective movements, such as limping or avoiding using an injured body part.
5. Learning to avoid harmful situations.
6. Choosing to avoid harm even if it means giving up other needs.
7. Having high intelligence and the ability to feel.
Adaptive value
The ability to sense pain helps organisms avoid harm. When an organism feels a harmful stimulus, it quickly moves away to prevent more damage. However, in mammals, pain can cause two changes: hyperalgesia, which is increased sensitivity to harmful stimuli, and allodynia, which is increased sensitivity to harmless stimuli. These changes make it unclear how helpful pain is in these situations. First, the pain from these changes may be much stronger than the actual injury. Second, these changes can last long after the injury has healed. This means the pain from the changes, not the injury itself, becomes the main problem. Because of this, the process is sometimes called maladaptive. Some people say hyperalgesia and allodynia help protect organisms while healing, but there has been little scientific proof to support this idea.
In 2014, scientists studied how injury affects behavior in longfin inshore squid and black sea bass, which are natural predators of the squid. When injured squid were attacked by bass, they started defensive actions earlier than uninjured squid, such as moving away sooner. When a numbing medicine (1% ethanol and MgCl₂) was used before the injury, the squid did not show these changes in behavior. The researchers said this was the first study to provide evidence that pain-related sensitivity after injury can be a helpful response.
Research findings
Frogs have special pain sensors in the top and deeper layers of their skin that detect harmful physical and chemical signals. These sensors send signals through the nervous system to the brain, allowing frogs to sense pain. While their nervous system is not as organized as in mammals, scientists now agree that frogs have the necessary brain structures to experience pain fully.
Early studies on frogs showed that harmful physical, thermal, and chemical signals activate nerve fibers that carry signals slowly. These nerve fibers are important for sensing pain.
There are two types of nerve fibers involved in pain in frogs. Group C fibers are unmyelinated (lack a protective covering) and have a small size, which makes them transmit signals slowly. These fibers are linked to pain from burns, toothaches, or injuries. A-delta fibers are myelinated and send signals faster than Group C fibers. These fibers help sense cold, pressure, and some types of pain, and are connected to quick reactions to avoid pain.
Frogs have both Group C and A-delta fibers in their skin.
All vertebrates (animals with backbones) share a basic brain structure divided into the forebrain, midbrain, and hindbrain. Connections to the forebrain suggest that frogs may be able to sense pain.
In 2002, James Rose from the University of Wyoming argued that fish cannot feel pain because they lack a part of the brain called the neocortex. If this were true, it would mean that many animals, including amphibians, could not feel pain. However, other scientists believe that animals can experience pain without a neocortex, using other brain structures. Temple Grandin, an animal behaviorist, suggests that fish and amphibians may still have consciousness using different brain systems.
Studies on frogs show that they have opioid receptors (mu, delta, and kappa) in their spinal cord and brains. These receptors are similar to those in mammals, though differences are less clear. Frog opioid receptors are about 70-84% similar to those in mammals and are found in brain areas linked to pain.
Veterinarians often use the same painkillers and anesthetics for frogs as they do for mammals. These drugs block pain signals in the nervous system before they reach the brain.
Testing the effectiveness of 11 opioid drugs on frogs showed that these drugs reduced pain for at least four hours. The results matched those seen in mice and other animals. Other painkillers, like butorphanol, also work in frogs.
Mixtures of alfaxalone and butorphanol, or alfaxalone and morphine, work similarly in Oriental fire-bellied toads in terms of how quickly they start and how long they last.
A substance called "frog's nociception-related peptide" (fNRP) increases the time it takes for newts to react to heat. This effect is blocked by naloxone, showing that fNRP interacts with opioid systems in newts.
Naloxone and naltrexone are drugs that block the pain-relieving effects of opioids in mammals. These drugs also block the pain-relieving effects of morphine in frogs, showing that frogs use opioid receptors for pain relief.
Injecting certain chemicals like epinephrine, norepinephrine, dexmedetomidine, and clonidine into the spinal cord of frogs increases their pain tolerance without causing sleepiness or muscle weakness.
Drugs like chlorpromazine, haloperidol, chlordiazepoxide, and diphenhydramine reduce pain in frogs when given through their lymph sacs. Other drugs, like indomethacin, ketorolac, and pentobarbital, have weaker pain-relieving effects.
Studies show that stress increases levels of stress hormones like cortisol in frogs. These responses vary by species. For example, American bullfrog tadpoles show stress-related changes in cortisol and white blood cells when exposed to high crowding or low oxygen.
Pain in frogs can be measured by changes in heart rate and breathing rate.
Frogs show protective behaviors, like wiping their skin or pulling away, when exposed to harmful chemicals, heat, or pressure.
Applying acetic acid (a strong irritant) to a frog’s leg causes it to vigorously wipe the affected area. This reaction is used in a standard test to measure pain relief in frogs, called the "acetic acid test."
Newts flick their tails when exposed to heat, similar to how rodents react in the tail flick test.
Pain responses in frogs can be measured using tools like Von Frey hairs (which test sensitivity to touch) and observing how quickly they pull away from painful stimuli.
Early studies showed that African clawed frogs learn to avoid electric shocks in a special test, and cane toads learn to avoid shocks in a maze. American bullfrogs also learn to avoid shocks by stopping their natural reflex to flip back onto their bellies.
Frogs can learn to avoid the chytrid fungus after one exposure.
Pain can change how frogs behave. American bullfrogs learn to avoid electric shocks by stopping their natural reflex to flip back onto their bellies. After repeated shocks, they stay on their backs instead of reacting quickly, showing a trade-off in behavior.
Criteria for pain perception
Scientists have also suggested that when using argument-by-analogy, signs related to how animals' bodies function or how they behave can help determine if non-human animals feel pain. The following is a table showing the criteria proposed by Sneddon et al.
Scientific statements
Several scientists have stated that amphibians may feel pain. For example, after studying the structure of the nervous system in vertebrates, Somme concluded that "most four-legged vertebrates have some level of consciousness." In a paper about amphibian surgery, Gentz wrote that "pain relief should be used after procedures" and that "using cold to sedate amphibians during painful operations is not acceptable." Veterinary articles have been published stating that amphibians feel pain in a way similar to mammals and that pain medications are effective for this group of vertebrates. Shine et al. wrote that most animal ethics committees and the general public believe amphibians can feel pain. Some scientists are more cautious, such as Michaels et al., who wrote that shared pain pathways between amphibians and other amniotes suggest the ability to feel pain, even if in a different and more limited way than in amniote species.
Societal implications
The effects of pain on amphibians in society include short-term and long-term contact with harmful substances, use in food, and scientific studies. For example, changing their genes might harm their health. Scientists may cause harm by making them experience pain, injury, or changes in behavior. Methods like cutting off toes or other procedures that hurt them are sometimes used.
Some people say that frogs killed for food are cut open while still awake and may take up to an hour to die.
In the United Kingdom, the Animals (Scientific Procedures) Act 1986 protects amphibians from the time they can feed themselves. Another law, the Animal Welfare Act 2006, covers most other situations and includes amphibians because they have backbones. The Animal Welfare (Sentience) Act 2022 also includes all animals with backbones in its definition of sentience.
In Norway, the 1974 Animal Rights Law protects mammals, birds, frogs, salamanders, reptiles, fish, and crustaceans.
In the United States, the Animal Welfare Act protects animals during scientific research. However, this law does not include "cold-blooded" animals, which means amphibians are not protected under this law.