It seems that, as humans, we relate better to animals if we can see that they share the same (or similar) emotions to us – that they experience fear, pain happiness and sadness. Unfortunately, inferring any form of emotion on an animal based solely on its behaviour is fraught with danger and bias from the observer. However, a picture is beginning to emerge that many animals do indeed experience some similar emotions to humans, with many creatures capable of learning and even showing signs of boredom if something fails to stimulate them sufficiently. In a fascinating paper presented to the American Board of Veterinary Practitioners Symposium back in May 2002, Temple Grandin and Mark Deesing of the Colorado State University summed up that which is currently known about the perception of distress in animals. That which follows is a short summary of their paper.
There is a general feeling that the ability of an organism to perceive pain is directly related to the size and complexity of their brain -- so “small-brained” animals potentially feel less pain and suffering than “larger brained” animals -- inferring a sharp demarcation between animals capable of feeling pain and those that are not. Some consider that the ability of an animal to feel pain may be related to the amount of ‘wiring’ linking the nervous system to the areas of the cerebrum (effectively the whole brain). It does, however, seem that fear operates on a more primitive level. Indeed, as Grandin and Deesing note, that if the cerebral cortex (the outer layer of the brain) is removed, the animal fails to show any signs of pain, but can apparently still learn a conditional fear response. A paper published in the 1997 book Animal Consciousness and Animal Ethics, suggested that only humans, some apes and possibly dolphins were capable of suffering. Such an inference was based on a belief that animals must possess a complex and well-developed part of the front brain (the prefrontal cortex) and the grey covering of the cerebrum possessed by only a few vertebrates (referred to as the neocortex) in order to interpret painful stimuli. Several studies have shown that an intact cortex is required for full suffering to be possible – removal of this part of the brain in rats counters the feeling of suffering and pain. An intriguing experiment in 1992 found that scarring of an almond-shaped lobe of the cerebrum called the amygdala caused a loss of both learned and unlearned fear responses, suggesting that the amygdala is the fear-processing center. Similarly, a paper in the journal Cell in 1996 showed that, for an animal to associate a certain place with fear, the hippocampus (a structure on the floor of the left part of the brain that – in cross-section – is shaped like a sea horse) must be intact.
Several recent studies on pain perception in humans have linked the prefrontal cortex with the emotional component of pain, and experiments on rats have suggested that this cortex has a similar role in rodents too. Indeed, it seems that even the smallest mammals have the necessary neural circuitry to be able to learn from adverse situations. Experiments on rats in the late 1970s and a series of follow-ups in the early 1980s found that rats made ill through an injection of lithium chloride would become ill at the sound of the same tone (beep) that they learned to associate with the lithium chloride spiked food, even without food actually being present. The rats also learned to drink an unpalatable analgesic solution because they found that drinking it would make them better. Mammals are also well known to, in Grandin and Deesing’s words, “pain guard” – i.e. they will limp in order to prevent putting pressure on an injured limb and lick wounds.
Grandin and Deesing concluded that all mammals and birds have a sufficiently developed prefrontal cortex to suffer pain. They also hypothesize that there is likely to be a minimum amount of circuitry involved with the processing of emotion, and cerebral cortex that is required for an animal to feel pain, and this number of circuits is likely to be less for fear perception. Basically, it seems that an animal will suffer from pain if it has enough neural circuitry to combine pain signals with the parts of the brain involved in formation of emotions.
Henceforth, the discussion has focused on mammals, as this is where much of the funding for research has been directed. However, the question of whether fish and amphibians feel pain or experience fear is still largely enigmatic. Amphibians and reptiles are known to respond to painkillers, and amphibians will respond to painful stimulus applied to the skin. What is not known, however, is whether this response is true suffering or a reflex reaction.
Recently the debate as to whether fish are capable of feeling pain and suffering has opened up a metaphorical ‘can of worms’ between neurologists and the angling community. Since the late 1980s, papers on the observation of hooked fish began appearing in the popular science literature. One particular paper in New Scientist back in 1987 by a group of Dutch scientists reported on the various responses of hooked fish – they looked at fish that, when hooked, were kept on a slack line and ones that were exposed to ‘line pressure’ (i.e. they were landed or fought). The study found that fish hooked on a line began spitting air from their swim bladder (the sac in their body cavity that gives them lift – this is not present in elasmobranchs, which rely on an oil-filled liver for the primary component of their buoyancy) and a violent shaking of their head. Moreover, the fish that were exposed to line pressure took longer to resume feeding than those held on a slack line. Another experiment in the same journal more recently, by John Hayes and Roger Young at the Cawthron Institute (New Zealand) in August 2000, found that trout learned to avoid anglers. The study found that once the fish had been caught, they were more difficult to catch next time; on the first day in a newly fished site, each angler caught about ten fish, a number that declined to almost zero by the third day (despite the fish having been released after their initial capture). Indeed, it seems that the fish ‘went to ground’ after the first capture and failed to appear again until several trips later.
The neurological study of whether fish feel pain and are capable of suffering has divided those who care into either the Rose camp or the Sneddon camp. Professor James Rose of the University of Wyoming wrote a 38 page paper about the perception of pain in fish for the journal Reviews in Fisheries Science last year (2002). Rose compared the nervous systems and responses of fish to those of mammals, concluding that fish didn’t have the neurological ‘hardware’ needed for the perception of pain. Pain, Rose considers, depends on specific regions of the prefrontal cortex (primarily the neocortex) that fish just don’t have. Rose puts the escape response seen in hooked fish down to a reaction of the brainstem and spinal patterns elicited by a stimulus. However, many feel that Rose’s paper is long on the conclusions and short on the investigation.
Earlier this year, a team of scientists from the University of Edinburgh, led by Lynn Sneddon, reported in the Proceeding of the Royal Society how trout displayed behaviour that is similar to that associated with stress in mammals. This behaviour consisted of a “rocking motion” and was incited by the injection of bee venom and acetic acid into the lips of he trout. The scientists also witnessed a rubbing of the lips on the gravel and that the injected trout took longer to return to feeding than the control group. Further, the team identified 22 nociceptors (thin, un-insulated nerve cells that only respond to stimuli severe enough to cause tissue damage), 18 of which were polymodal nociceptors (similar to those found in amphibians, birds and mammals). They also found a further 36 receptors involved in the detection of physical stimuli. It seems, however, that Sneddon failed to read Rose’s paper on the subject, and as such there was a clash between the two authors. Rose rebuked -- quite accurately -- the findings of Sneddon et alii, stating that their experiments did not deal with pain (i.e. conscious experience) but nociception (i.e. an unconscious response to noxious stimuli), which is true. The presence of nociceptors is not sufficient evidence for the existence of pain reception. Similarly, anglers were quick to join the cause and point out that the paper offered no explanation of how fish might interpret any noxious stimuli. In return, Sneddon questioned Rose’s definition of pain.
Much of the study looking at whether fish experience pain has focused on the bony fishes, very little has been done to establish where the sharks and rays fit into this perplexity. Interestingly, a 1993 paper in the Journal of Comparative Neurology reported that elasmobranchs lacked the “C-fibers” (i.e. nociceptors) that are known to be involved in the pain interpretation and response in mammals.
It is difficult to see how any organism could survive without being able to feel and interpret a major way that our body tells us something is wrong. One thing that does seem quite clear from the currently available research is that if fish do feel pain, then they (and especially elasmobranchs) do so using a mechanism that is different from that seen in the birds and mammals.
[You must be registered and logged in to see this image.]
Wed Nov 07, 2012 9:59 am
xx
