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 scien-

tific 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 be-

lieved 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 verte-

brate 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 col-

leagues(2002) noticed the heart rate in a crayfish decreased following claw autotomy during an aggres-

sive 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 sim-

ilar 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 capability for suf-

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fering. He argued that if a pin is stuck in an animal and they rapidly withdraw, then argu-

ment-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 nocicep-

tion 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 experi-

ments 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 re-

ceptors in some invertebrates.

Some specifications necessary for experiencing pain include a suitable nervous

system and receptors, physiological changes to harmful stimuli and displays of protec-

tive 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 ac-

tual 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). Addi-

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tionally, 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). How-

ever, 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 elec-

trophysiological 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 relation-

ships to the central nervous system than mammalian pain receptors. Nociceptive neu-

rons 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

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responses of leech neurons to mechanical, chemical and thermal stimulation influenced

researchers to write "These properties are typical of mammalian polymodal pain recep-

tors"(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 ab-

dominal 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 modifica-

tions 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 di-

rected 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, de-

creased following claw autotomy during an aggressive encounter. Recording physiologi-

cal 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

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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 be-

tween the parapodia(Carew et al., 1983). It is typically claimed this response is an invol-

untary 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(Wal-

ters 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 condi-

tions, 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 auto-

tomize the leg, indicating it is not the physical act or the injection of fluid that causes au-

totomy. In spiders injected with venom components that cause injected humans to re-

port pain, the spider will autotomize the leg, but if the injection contained venom compo-

nents that does not cause pain to humans, autotomy did not occur.

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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 environ-

ment to realize a positive reinforcement or avoid a negative one. In this manner, ani-

mals learn from the consequence of their own actions, i.e. they use an internal predic-

tor. Operant responses indicate a voluntary act; the animal exerts control over the fre-

quency or intensity of its responses, making these separate from reflexes and compli-

cated 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 verte-

brates 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 defini-

tions 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

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these neurons could be used for different pathways and relationships to the central ner-

vous 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 inverte-

brates which can only mean that invertebrates are capable of feeling pain.

Bibliography

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Log:

Sept 20/2013 3:00 PM 4:20 PM Looked for articles to do with topicSept 25/2013 10:15 AM 11:16 AM Look and read referencesSept 26/2013 7:00 PM 8:08 PM genealogyOct 1/2013 11:00 AM 12:00 PM genealogyOct 10/2013 9:45 AM 10:05 AM outline and researchOct 15/2013 10:30 AM 11:55 AM Writing first draftOct 15/2013 2:00 PM 6:40 PM Writing first draft, fixing references and citations.Oct 16/2013 10:05 AM 11:25 AM Looking through first draft, checking for plagiarism(citations)Oct 30/2013 5:00 PM 6:00 PM Fresh look at draft, minor changes.Nov 19/2013 11:00 AM 12:30 PM Looked over edited paper, fixed changes.Nov 20/2013 4:00 PM 8:00 PM Wrote abstract and conclusionNov 28/2013 4:00 PM 4:30 PM Touch ups

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