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An Examination Into the Controversy Behind Whether or Not Invertebrates Feel Pain: With an Intensive Look Into What Pain Is

Rebecca Brown 1782945 Biology 315 November 29/2013

The controversy of whether or not invertebrates feel pain has been debated for quite awhile in the scientific community. First, one must consider what pain is and what organs or tissues are needed to feel pain. There are two important elements, the presence of nociceptors and experience of ‘pain’ itself. One cannot measure pain in other animals or humans but the response to painful stimuli can be measured. It is believed that pain was developed by natural selection by offering the advantage of reducing further harm to the organism. One reason many reject that invertebrates feel pain is that their brains are smaller but the ratio of brain weight to body-weight, invertebrate brains are within the same ratio category as the vertebrate brain. Nicholls & Baylor(1968) was the first to find a nociceptive cell in an invertebrate, this was the beginning of plenty of studies on whether or not invertebrates feel pain. Vertebrates respond to pain with modifications typically changes in blood flow, respiratory patterns, and endocrine. Schapker and colleagues(2002) noticed the heart rate in a crayfish decreased following claw autotomy during an aggressive encounter. In studies that used operant conditioning they found that spiders memorize information concerning their previous movements which is proof of cognitive ability. The research that has been done on invertebrates and pain points toward the capability of experiencing pain.

Although there are various definitions of pain, most involve two key elements. The first element that is required is nociception. Rob Elwood and Appel(2009) explains that this is the ability to notice harmful stimuli that stimulates a reflex response that moves the complete organism, or the affected part of its body, away from the source of the stimulus. The second element is the experience of 'pain' itself, or suffering. Pain cannot be directly measured in other animals, or humans. Responses to painful stimuli may be measured, but not the actual experience. Argument-by-analogy is used to deal with this problem when assessing the capability of other species to experience pain. This is often supported by the principle that if an animal responds to a stimulus in a similar way to ourselves, it is likely to have had an analogous experience. Sherwin(2001) used this reasoning to question whether or not invertebrates have the 2

capability for suffering. He argued that if a pin is stuck in an animal and they rapidly withdraw, then argument-by-analogy implies that like humans, they felt pain. Sherwin explains using the same reasoning, a cockroach experiences pain when it writhes after being stuck with a pin.

The ability to experience pain has been subject to natural selection and offers the advantage of reducing further harm to the organism. It would be expected that nociception is widespread, robust and varies across species. Puri and Faulkes(2010) shows an example; the chemical capsaicin is commonly used as a harmful stimulus in experiments with mammals; but, the African naked mole-rat lacks pain-related neuropeptides in cutaneous sensory fibres, and shows a unique and remarkable lack of pain-related behaviors to acid and capsaicin(Park et al., 2008). Similarly, capsaicin triggers pain receptors in some invertebrates.

Some specifications necessary for experiencing pain include a suitable nervous system and receptors, physiological changes to harmful stimuli and displays of protective motor reactions such as autotomy and high cognitive ability.

One reason for rejecting a pain experience in invertebrates is that invertebrate brains are smaller(Chittka & Niven, 2009). However, brain size does not automatically associate to complexity of function. Cetaceans have big brains, but if you look at the actual structure of the brain, it's not very complex. And brain size only matters if the rest of the brain is organized properly to facilitate information processing(Manger, 2006).


Additionally, in the ratio of brain weight to body-weight, cephalopod brains are within the same ratio category as the vertebrate brain, smaller than birds and mammals, but as large or larger than most fish brains.

Invertebrate nervous systems are not like those of vertebrates and this variation has typically been used to reject the possibility of a pain experience in invertebrates. In humans, the neocortex of the brain has a central role in pain and it has been argued that any species lacking this structure is incapable of feeling pain(Rose, 2002). However, it is possible that completely different structures may also be involved in the pain experience of other animals.

Arthropods and modern cephalopods are two groups of invertebrates that have advanced brains. Cephalopods have a central nervous system that shares prime electrophysiological and neuroanatomical features with vertebrates. Pain Receptors are sensory receptors that respond to potentially damaging stimuli by sending nerve signals to the brain. These neurons in invertebrates may have different pathways and relationships to the central nervous system than mammalian pain receptors. Nociceptive neurons in invertebrates typically fire in response to similar stimuli as mammals, such as high temperature, low pH, capsaicin, and tissue damage.

The first invertebrate in which a nociceptive cell was identified was the leech, Hirudo medicinalis. It has the segmented body of an Annelida and each segment has a ganglion containing the T, P and N cells(Nicholls & Baylor, 1968). Later studies on the


responses of leech neurons to mechanical, chemical and thermal stimulation influenced researchers to write "These properties are typical of mammalian polymodal pain receptors"(Pastor et al., 1996). In addition, there are various studies of learning and memory using pain receptors in the sea hare, Aplysia. Several studies have centered on mechanosensory neurons innervating the siphon and having their somata within the abdominal ganglion. Aplysia show similarities to certain vertebrate pain receptors, together with a property apparently unique to pain receptors, sensitization by harmful stimulation. Pain receptors have been identified in a wide range of invertebrate species, including annelids, mollusks, nematodes and arthropods.

In vertebrates, potentially painful stimuli typically produce vegetative modifications such as tachycardia, pupil dilation, defecation, arteriole blood gases, fluid and electrolyte imbalance, and changes in blood flow, respiratory patterns, and endocrine(Short, 1998). At the molecular level, injury of invertebrates results in the directed migration and accumulation of haematocytes and neuronal plasticity, similar to the responses of human patients undergoing surgery or after injury. In one study by Schapker and colleagues(2002), heart rate in the crayfish, Procambarus clarkii, decreased following claw autotomy during an aggressive encounter. Recording physiological changes in invertebrates in response to harmful stimuli can enhance the findings of behavioral observations and such studies ought to be encouraged. However, careful management is needed as a result of physiological changes can occur due to harmful, but non-pain related events, e.g. cardiac and respiratory activity in crustaceans is highly sensitive and responds to changes in water level, various chemicals and


activity throughout aggressive encounters. When a weak tactile stimulus is applied to the siphon of the sea-hare Aplysia californica, the animal rapidly withdraws the siphon between the parapodia(Carew et al., 1983). It is typically claimed this response is an involuntary reflex, however, the complex learning related to this response suggests this view might be overly simplistic. A report was published that described the escape responses of the Tobacco Hornworm caterpillar, Manduca sexta, to mechanical stimulation. These responses, particularly their plasticity, were similar to vertebrate escape responses(Walters et al., 2001).

Over two hundred species of invertebrates are capable of using autotomy as an avoidance or protective behavior including land slugs, sea snails, crickets, spiders, crabs and lobsters. The prawns specifically groom the treated antennae and rub them against the tank, showing they are aware of the location of the harmful stimulus on their body instead of exhibiting a generalized response to stimulation. Under natural conditions, orb-weaving spiders undergo autotomy if they are stung in a leg. In an experiment by Eisner and Camazine(1983), spiders were injected in the leg with venom and they would shed this appendage. If they are were injected with saline, they seldom autotomize the leg, indicating it is not the physical act or the injection of fluid that causes autotomy. In spiders injected with venom components that cause injected humans to report pain, the spider will autotomize the leg, but if the injection contained venom components that does not cause pain to humans, autotomy did not occur.


Operant conditioning, the type of learning in which an individuals behavior is modified by its consequences, was used on vertebrates and have been conducted for several years. In such studies, an animal operates or changes some part of the environment to realize a positive reinforcement or avoid a negative one. In this manner, animals learn from the consequence of their own actions, i.e. they use an internal predictor. Operant responses indicate a voluntary act; the animal exerts control over the frequency or intensity of its responses, making these separate from reflexes and complicated fixed action patterns. A variety of studies have discovered surprising similarities between vertebrates and invertebrates in their capacity to use operant responses to gain positive reinforcements, but also to avoid negative reinforcement that in vertebrates would be described as 'pain'. It may be argued that a high cognitive ability is not necessary for the experience of pain, otherwise, it may be argued that humans with less cognitive capacity have a lower probability of experiencing pain. However, most definitions of pain indicate some degree of cognitive ability. Many of the learned and operant behaviors described above indicate that invertebrates have high cognitive abilities. An example is iodiothetic orientation, by spiders which means they memorize information concerning their previous movements. Detour behavior in which spiders choose to take an indirect route to a goal instead of the most direct route, thereby indicating flexibility in behavior, route planning, and insight learning.

The two main elements of pain is nociception and the experience of ‘pain’ itself. Nicholls and Baylor(1968) found nociceptors in the leech, Hirudo medicinalis, which eventually lead to more discoveries of nociceptors in invertebrates. In invertebrates


these neurons could be used for different pathways and relationships to the central nervous system than mammalian pain receptors. Nociceptive neurons in invertebrates will fire in response to similar stimuli as mammals. One can not measure pain in any other organism other then themselves, but responses to painful stimuli can be measured. The argument-by-analogy approach is used to assess the capability of others to experience pain. If an animal responds to a stimulus in a similar way to ourselves, it is likely to have had an analogous experience. Plenty of examples have been shown that invertebrates do respond to painful stimulus the same way we would. When a spiders leg is injected with venom it will shed this appendage but if injected with saline the leg is not shed. This proves that the simple act of injection does not cause them to lose the leg, the venom must have been the reason. Both elements of pain have been seen in invertebrates which can only mean that invertebrates are capable of feeling pain.



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Eisner, T., & Camazine, S. (1983). Spider leg autotomy induced by prey venom injection: An adaptive response to “pain�?. Proceedings of the National Academy of Sciences, 80(11), 3382-3385.

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Pain in Invertebrates