Do you ever wonder if your pet fish feels pain? What about insects that get swatted or spiders that get squished? It may come as a surprise, but there are actually quite a few animals that scientists believe don’t experience pain or at least not in the same way humans do.

If you’re short on time, here’s a quick answer to your question: Some animals like insects, fish, and crustaceans either have very simple nervous systems that likely don’t process pain or lack the brain pathways to consciously experience it, but they still respond reflexively to harmful stimuli.

In this comprehensive article, we’ll explore the evidence behind animals that may not feel pain, examine how their nervous systems work, look at their behaviors, and also consider the ethics around how we treat creatures that cannot suffer.

Which Animals May Not Feel Pain

Insects and Spiders

Many researchers believe that insects and spiders likely do not feel pain. This is because they lack certain anatomical structures that are associated with pain perception in vertebrates, such as advanced nervous systems and nociceptors (specialized sensory receptors that detect potentially harmful stimuli).

Insects and spiders have simple nervous systems and exhibit fixed reflexive responses to external stimuli rather than learned behaviors. For example, an injured insect will continue to move its damaged limb in the normal walking pattern rather than nursing its wound or showing guarding behavior.

This suggests a lack of awareness of the injury.

Some recent studies have raised the possibility that insects may sense some form of nociception (detection of harm), but the bulk of evidence indicates they do not subjectively experience pain.[1]


The issue of pain perception in fish is controversial. Traditionally, fish have been thought to lack the neural complexity to feel pain consciously. But more recent research suggests they may in fact have the capacity, though their experience of pain may differ from humans and other mammals.

Here are some key points on both sides of the debate:

  • Fish do have nociceptors and reactions to injurious stimuli. However, their brains lack certain structures associated with conscious pain experience in mammals, such as the neocortex.
  • Studies show fish do show long-term behavioral changes after painful events, suggesting psychological awareness. For example, fish will rub and avoid painful areas after an injury.[2]
  • Some species of fish even pass pain-related tests, such as conditioned place aversion – associating a certain environment with past pain.
  • Pain medications appear to alter fish reactions to injurious stimuli, though the effects are not entirely consistent.

Crustaceans Like Lobsters and Crabs

There is scientific debate around whether crustaceans such as lobsters, crabs and shrimp feel pain. Here are some key considerations:

  • Crustaceans do have nociceptors that detect harm, and they exhibit avoidance learning in response to negative stimuli. When exposed to an electric shock, prawns showed rubbing and grooming behaviors.
  • However, crustaceans lack certain brain structures found in vertebrates associated with conscious pain perception, like the cortex or limbic system.
  • Some studies show painkillers like morphine reduce crustacean responses to injury, suggesting they may mediate pain. But other studies find analgesics had little effect.
  • Behavioral reactions to injury may be simple reflexes rather than conscious responses. But long-term motivational change after injury hints at a possible higher level experience.

Nervous System Differences

Lack of Nociceptors

Nociceptors are special sensory receptors that detect potentially harmful stimuli and send signals to the spinal cord and brain (1). However, some animals like lobsters and insects lack nociceptors entirely (2).

Without these specialized pain receptors, they may not experience pain in the same way humans do. For example, a study found locusts continued normal behavior after injury, suggesting a missing pain sensation (3).

The absence of key anatomy for detecting threats likely explains reduced pain behaviors in these creatures. Still, more research is needed on their neural processing.

Missing Neural Pathways

Several animal species lack the neural pathways required to process and perceive pain. For instance, fish do not have a neocortex or similar brain region linked to conscious pain perception in mammals (4).

And some insects like fruit flies lack neurons that signal suffering, based on a 2020 study (5). Without these pathways, injury responses may be reflexive rather than consciously experienced as painful.

However, fish demonstrate complex behaviors that may suggest an awareness of harm, and some invertebrates do have nociceptors despite missing key neural anatomy (6,7). More work is needed to understand these distinctions across species.

Absence of a Neocortex

In humans and other mammals, the neocortex plays a key role in subjective pain experience and suffering (8). However, species like amphibians, reptiles, birds, and fish lack this brain structure (9). For instance, one study found rats that had their neocortex removed still responded to painful stimuli but without behavioral signs of experiencing pain (10).

The lack of a neocortex hints that some animals may respond to injury without feeling conscious pain. However, amphibians and reptiles do show opioid receptors that mediate pain responses, suggesting they sense some harm (11,12).

More comparative research on neuroanatomy and behaviors across species is important.

Behavioral Responses

Reflex Reactions

Even though some animals may not feel pain, they still exhibit reflex reactions to harmful stimuli. These involuntary responses are controlled by the peripheral nervous system and spinal cord rather than the brain. For example, an animal may still withdraw its paw if it touches something hot.

However, without an emotional response, the animal may not learn to avoid that harmful stimulus in the future.

Nocifensive Behaviors

Nocifensive behaviors are actions taken by an animal to avoid potential bodily harm. These may look similar to reflex reactions, but they involve more complex motor responses controlled by the brainstem and higher neural circuits rather than just the spinal cord.

For instance, an animal may rapidly flee or attack in response to a harmful stimulus. These defensive behaviors do not necessarily mean an animal can feel pain emotionally, but they do suggest an evolved self-preservation mechanism.

Stress Hormones

Even animals that may not experience pain consciously can still exhibit physiological stress reactions. In response to bodily injury or other threats, the hypothalamic-pituitary-adrenal axis triggers the release of hormones like cortisol and adrenaline.

These hormones prepare the body to fight infection, escape danger, and heal damage. The presence of stress hormones alone does not confirm an animal feels pain subjectively. However, it does indicate a biological system for dealing with potential threats to survival and homeostasis.

Ethical Considerations

Humane Treatment Still Important

Even though some animals may not feel pain, it is still important to treat them humanely. Causing harm or distress should be avoided whenever possible. Here are some key reasons why:

  • There is still uncertainty around pain perception abilities in many species. The precautionary principle suggests we should err on the side of caution.
  • Even without pain, animals can suffer in other ways, such as fear, distress or boredom. Minimizing suffering should be a priority.
  • Humane treatment promotes moral values like compassion in society. Being cruel to any living being can damage human ethics and values.

Arguments For and Against Pain Perception

There is ongoing scientific debate around which animals can feel pain. Some key arguments include:

For pain perception:

  • Many animals show pain-avoiding behavior, suggesting they consciously feel it.
  • Some animals have similar neurobiology to humans, including sensory nerves and brain regions involved in processing pain.
  • Pain perception may have evolved in early animals and be widespread in the animal kingdom.

Against pain perception:

  • Some animals lack key brain regions linked to conscious pain experience in humans.
  • Simple reflexive responses to injury may occur without conscious awareness.
  • Assessing pain in animals relies on anthropomorphic assumptions.

At present there are reasonable arguments on both sides. More comparative research on animal neurobiology will help clarify which non-human species are likely to experience pain.

Precautionary Principle

Given the complexities and uncertainties around animal pain, many argue we should apply the precautionary principle. This means assuming that animals may experience pain unless there is strong evidence to the contrary. Better to err on the side of caution and animal welfare.

As Peter Singer states: “Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation.”

This can guide our treatment of possibly sentient animals even before we fully understand their pain perceptions.

Pain Differences Across Species

There is a wide spectrum of ability to feel pain across the animal kingdom. At one end, humans and other mammals have advanced nociceptors and neural pathways that produce acute pain sensations. At the other end of the spectrum, some animals lack the anatomical and physiological capacity for the conscious perception of noxious stimuli.

Spectrum of Ability

The ability to detect and respond to noxious stimuli is essential for animal survival. However, the psychological experience of “pain” requires complex neural processing. For example, some evidence suggests that fish and amphibians have nociceptors to detect stimuli, but they may lack the neural sophistication to consciously perceive pain like mammals do.

Evolutionary Factors

The evolutionary development of the sense of pain is closely tied to other cognitive capacities. Species with more advanced learning, memory, and behavior complexity also tend to have more developed pathways for central pain perception and behavioral responses to avoid injury.

For instance, the octopus nervous system is quite sophisticated and comparable in complexity to many vertebrates. Their advanced cognitive abilities are associated with nociceptive reflex behaviors indicating centralized mediation of pain-like states in these invertebrates.

Correlation with Intelligence

There seems to be a close correlation between the level of general intelligence demonstrated in animal behavior and the capacity to consciously perceive pain:

Animal Group Indications of Pain Perception Cognitive Capacity
Mammals Extensive Advanced
Birds Moderate Moderate
Reptiles/Amphibians Minimal Limited
Insects/Arachnids None Minimal

As illustrated, mammals demonstrate substantial evidence of conscious pain experience coupled with advanced cognition, learning, memory and complex behaviors. Meanwhile, lower animal forms like insects do not seem capable of feeling pain given their simpler nervous systems and largely innate behaviors.


While the scientific evidence suggests certain animals may not experience pain consciously or in complex ways like humans do, there is still uncertainty around these conclusions. Additionally, it raises questions around how we ought to treat creatures ethically, even if they do not suffer.

This fascinating topic reveals how nervous systems, brains, and the perception of pain itself have evolved differently across the animal kingdom.

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