Updated: Oct 19, 2019
Animals and pain is a topic that is largely explored to ensure that welfare issues are addressed and animals are not made to suffer unnecessarily, whether during their life as companions or when used in scientific experimentation. The National Research Council Committee (2009) states that pain is experienced by all mammals, and possibly all vertebrate animals; but where does that leave invertebrates?
Invertebrates are categorised by their lack of back bone, and they lack the highly complicated structures of the CNS and PNS that we have discussed in vertebrate animals (Kutsch, 1995). Their anatomy and physiology not only differs from that of vertebrate phyla, but with invertebrate species making up an estimated 97% of the animal kingdom, each class and even species varies greatly from the other (May, 1988).
Is it possible that invertebrates can experience pain and suffering as vertebrate animals do? This report will explore pain as a biological phenomenon, describe why we believe vertebrate animals can experience pain, and discuss why some invertebrates could experience pain, and what legislative protection is available for them.
What is pain?
The Oxford Dictionary (2015) defines pain as “a highly unpleasant physical sensation caused by illness or injury, or mental suffering or distress”, while the International Association for the Study of Pain (IASP, 1994) describes pain as: “an unpleasant sensory and emotional experience associated with actual or potential tissue damage”. Such a negative experience is beneficial to overall survival, whether it manifests as slight discomfort, or severe and chronic. From rapid reflex arcs to a slow inflammatory response, pain works to prevent further harm and protect the body until it heals (Holden and Winlow, 1983). There is plenty of evidence that points to animals physically feeling pain (Rollin, 1985), but it is not exactly known if and how the pain is experienced in the same way as the human species, or if all or some species suffer in the same way in regard to physical or emotional pain.
A brief biology of pain
Biologically, pain can be divided up into categories, but all pain stems from a nervous bodily response. Nociceptive pain is often described as an ache or a throb, and occurs when nociceptors in the skin, viscera and internal organs are triggered by harmful mechanical, thermal or chemical stimuli. Specialised nerve endings detect these stimuli, where impulses are then transmitted from these peripheral nerves, along myelinated or unmyelinated, nerve fibres into the central nervous system (CNS). The ‘pain pathway’ of the CNS consists of the connections between the dorsal horn, spinal cord, brain stem, thalamus and cortex.
A simplified diagram of the pathway in which pain stimuli are detected, transmitted, perceived and modulated. Source: http://neurowiki2014.wikidot.com/group:pain
Neuropathic pain is caused by a lesion or disruption in the nervous system. Often described as burning, tingling, electric shock-like or sharp stabbing sensations (Price and Dussor. 2015), it is an abnormal activation of the pain pathway by nerve compression or destruction, for example, either in the peripheral nervous systems (PNS) or in the CNS (Bennett, 2010).
Inflammatory pain characteristically involves a pain response from typically inoffensive stimuli, usually following tissue damage and associated with heat, redness and swelling as well as pain. The nociceptors in this case are sensitised from the release of proinflammatory mediates from damaged cells and vessels, and immune cells in the affected tissue, therefore giving an exaggerated pain response to stimuli (Price and Dussor, 2015).
Biological pain in animals
The IASP (1994) states that humans generally measure pain via descriptions of how severe their experience of pain is, alas animals cannot translate their suffering verbally, and we must rely on behavioural observation and clinical examination. In additional to the original definitions of pain provided, Zimmerman in 1986 described animal pain as “an aversive sensory experience caused by actual or potential injury that elicits protective motor and vegetative reactions, resulting in learned avoidance and modifying species specific behaviour”.
To experience pain in the same or similar manner as humans, nociceptors are required. As described above, nociceptors will send a signal to the CNS in reaction to a noxious stimulus. Nociceptors types vary and perform different roles in pain reception, but they are exclusively designed for that one function (Sneddon, 2004). Many animals possess identical or very similar nociceptor nerves, as well as the ‘primitive areas of the brain that process nociceptive information, namely the medulla, thalamus and limbic system’ (Sneddon, 2015). This suggests strongly that from an anatomical and physiological perspective, there are animals that experience pain. Mammalian species in particular share the same PNS and CNS systems as humans (Jessell et al, 1991) and react physiologically the same way when presented with harmful stimuli (Apkarian et al, 2006). Other vertebrates such as fish (Sneddon et al, 2003) also have similar pain pathway systems; amphibians are the least similar to humans however, and like birds and fish, have no neocortex along with their complex forebrains (Allen, 2004).
Broom and Johnson in 2000 stated that there is a lack of scientific evidence to contradict the idea that animals feel pain. Brambell reported in 1965 that mammals must suffer in the same way as humans given that their nervous apparatus is the same. Rovainen and Yan in 1985 found that fish have functional pain receptors, and Verheijen and Buwalda (1988) observed physiological responses to painful stimuli that mimic that in humans. The same is strongly presumed for amphibia, birds and reptiles (Broom, 2001). As far as vertebrates go, it is difficult to argue that animals experience pain.
Psychological pain and stress
Psychological pain is received by sensory nerve system as the above, as it related to but research suggests that biologically, psychological and physical pain share some neurological mechanisms (Eisenberger, 2013). Pain and stress are directly related, with stress being caused by both any threat to biological homeostasis (injury, infection, etc.) and psychological threat (fear, anxiety, etc.) (Hadkistavropoulos and Craig, 2012).
There are many documented human cases where emotional trauma has manifested as physical pain, for example the heart problem of Takotsubo cardiomyopathy, where patients were suffering from the associated chest pains due to seeing a loved one die and experiencing emotional stress (Braitman, 2015). Emotional or psychological pain however, will not be further discussed and the main focus will be on the physical sensation of pain cause by noxious stimulus.
Observational behavioural pain detection in animals
Anatomy and physiology alone cannot determine if animals experience pain, how they experience it or if the pain results in suffering. The issue with behavioural observations however, is that they can be deemed unverifiable and anthropomorphic, and therefore unscientific.
A key finding that is seen in numerous species is the suppression of pain behaviours to avoid predation. Coren describes how dogs could be using this exact method of predator avoidance to explain why they have fewer behavioural responses to pain then humans. Dogs have inherited the instinct to hide any pain caused by injury as a way to hide their vulnerability. This means they will suppress many behaviours that would normal signal pain to maintain social standing in their pack and prevent appearing weak to potential predators (Coren, 2015).
Nolan observed that birds who were injured would choose to eat food that contained analgesics over untreated food, then demonstrated behavioural improvements regarding their health. This demonstrates that even in birds, their nervous systems reflect those of humans in some way, as analgesics are known to relieve pain and alter human behaviour in a similar way (Nolan, 2015).
Invertebrate pain biology
Sneddon (2015) describes how nociceptors can differentiate damage from pain, and that these are found in invertebrates as well as vertebrates. This is backed up by a 2009 study that found nociceptors in nematodes, annelids and molluscs (Smith and Lewis, 2009). Sneddon does go on however to explain that vertebrates are capable only of reacting to stimulus responses, and do not possess the necessary brain processes to experience the pain in the same way vertebrates do.
Kutsch described in detail the evolutionary anatomy of the nervous systems in invertebrate classes. While Kutsch fails to define specifically nociceptors or any reference to pain, his book published in 1995 describes cnidarian nervous systems as primitive and used only for basic muscle contraction, and comparable in anatomy and physiology to ‘higher’ invertebrates such as molluscs and insects. He also explains that the ‘brain’ of flatworms, much like many other invertebrates, lacks complexity. It is merely a cephalic ganglion, described as archaic and controls movement and digestion.
Mandal in his 2012 book also describes the brain systems of inverts. Velvet worms, for example, have two large cerebral ganglia, fused on the midline, and sensory cells found at the end of the papilla covering the body. Like all invertebrates described so far, these are no exception in that they possess sensory cells in their periphery and brain like structures, but again, lack the composite brains found in vertebrates that allows animals to process the sensory pain from nociceptors.
In the same book, Mandal describes that arthropods are far more complex than many of their invertebrate counterparts. Their ganglion brains contain bundles of fibres that pass to various body areas, just like a PNS in vertebrates. They have tactile organs such as bristles for sensory input, which does reflect those seen in upper vertebrate. They also possess a varied range of sensory organs such as compound eyes, the otolith and balancing organs, and sound wave receiving organs that do not reflect anything seen in vertebrate animals, despite serving similar functions.
Whilst inverts appear to lack the very anatomy in their version of a CNS or PNS to experience pain, some higher species could possess organs that detect and experience pain via alternative organs. This however, is unlikely, given that they generally possess archaic systems to simplistic at best.
A study published in 2009 discusses the evidence they found that crustaceans can suffer from a pain experience. They stated 7 criteria that were looked at to determine pain and suffering; a CNS and receptors, avoidance learning, protective motor reactions, physiological changes, prioritising stimulus avoidance over other survival requirements, response to analgesics and high cognitive ability and sentience. Elwood et al found that there was a considerable amount of similarity despite there being systems anatomically, and there a huge welfare issue is presented.
Invertebrate pain behaviour
Behaviourally, there are studies which can observe typical avoidance responses to painful stimulus, as well protective behaviours post-contact (Fiorito, 1986).
The longfin inshore squid will perform defensive behaviours sooner if they are injured and approached by their natural predator, the black sea bass. Those who have been administered with an anaesthetic will not perform their defensive behaviour sooner, and will behave just as the uninjured squid to. This demonstrates that the perception of pain or injury in this invertebrate animal increases their vigilance (greater alertness over longer distances) (Crook, et al, 2014).
Cephalopod pain is a subject that is highly contentious. Even as invertebrates, it would be dismissive to state they are simple creatures that lack cognition. Abbott et al states that cephalopods demonstrate phenomenal consciousness when performing tasks and comparatively, their brain physiology makes them more ‘advanced’ than other invertebrates (Abbott et al, 1995).
As well as meeting the criteria described previously for crustacean pain response, cephalopods also demonstrate tool use (Finn et al, 2009), learning (Tricarico et al, 2011) and advanced interaction with environmental enrichment (Mather et al, 2010). Surely such high levels of ‘intelligence’ and cognition means that cephalopods can also feel pain, and suffer?
Suffering and Animal Welfare Law
There are researchers who argue that feelings and emotions are exclusive to humans, and to suffer, these experiences must be present (Braithwaite, 2010). Unfortunately the legislation exists to protect vertebrate animals only.
The Animal Welfare Act 2006, the primarily animal welfare legislation in the UK protects animals that meet certain criteria. This includes all vertebrate animals, commonly domesticated in the British Isles and under the temporary or permanent control of ‘man’. This does not include animals in their foetal form, animals that are living in a wild state or any invertebrates. This act makes it illegal to cause unnecessary suffering including not preventing your protected animal from pain, illness, injury or disease.
The Animals (Scientific Procedures) Act 1986 recognises pain and suffering in animals, but again for vertebrate animals only. This act recognises the system of the 3Rs; replace, refine and reduce. This is so that any animals used for scientific testing are exposed to as little amount of pain and suffering as possible, if they cannot be replaced, have their numbers reduced, and the process has been refined to minimal invasion. A paper produced by the National Research Council in the US states that the 3Rs system should be the standard for all animal testing.
Under The Veterinary Surgeons act 1966, vets responsible for an animal is legally obliged to provide pain relief and prevent further suffering. The Protection of Animals (Anaesthetics) Act 1964 even states that any operation to be performed that interferes with sensitive tissue or bones must be done with appropriate anaesthetics to alleviate pain. Again, this is for vertebrate animals only.
In 2010, however, there was a council directive passed that insists that any cephalopod used for medical or scientific experimentation must be killed humanely to minimise suffering, and it states that there is enough scientific evidence to suggest that cephalopods have the ability to experience pain, suffering and distress (EU Directive 2010/63/EU, 2010).
The strong evidence for the existence of feeling and experiencing pain in mammals and other vertebrates is strong and often overwhelming. Considering that invertebrates are so diverse and have such a vast array of anatomical and physiological systems that vary from the primitive to the semi-complex, it seems redundant to cluster them together and declare them unprotected, purely because the necessary research required to determine suffering in these animals does not exist.
There has been some movement regarding cephalopods, due to the research and work done to determine these invertebrates of a higher intelligence and deeming them able to suffer, but this is not reflected in welfare legislation outside of scientific procedures. Crustaceans have also been seen to demonstrate pain avoidance and meet the criteria that determines pain experience, yet no movements for the farming, handling and treatment have been tackled for those.
The invertebrate phyla is rich and diverse, and starting and finishing with humane octopus killing is not enough when there are potentially thousands more species that require the same levels of scrutiny when it comes to preventing pain and suffering.
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