"When can we stop worrying about animal suffering - Do fish feel pain?."

Dr Neville G. Gregory
Research Scientist

This presentation will start by considering which species have a capacity to suffer. This topic is important because it allows us to decide when to stop being concerned about potential suffering. When deciding whether a particular species has the capacity to suffer, it is helpful to make it clear which form of suffering is being considered. Some species can experience complex forms of suffering such as despair, whereas in others suffering is probably limited to rudimentary features, such as osmotic disturbance and thirst. Cognition is presumably a prerequisite for all forms of suffering.

Scientifically, a convenient starting point is to assume that animals that show limited ability to learn probably have limited cognitive capacity. It follows that inability to learn is likely to be a sign of reduced ability to suffer, especially when the learning schedule involves a potentially unpleasant stimulus. Some of the early work which assessed ability to learn in primitive aquatic life forms was conducted by Wilfred Agar at the University of Melbourne (Agar 1925). His work, and some subsequent studies, will be described. The conclusion that will be drawn is that it can be difficult to distinguish between learnt responses and adaptive responses in primitive life forms, and behaviour tests often fail to distinguish between conscious and sub-conscious learning. We need a different approach, and one which is particularly relevant to caring about suffering, is to test for the ability to experience pain.

The Institute of Medical Ethics' criteria for deciding whether or not a species can experience pain are demanding. The species must

  • possess receptors sensitive to noxious stimuli
  • have brain structures which are analogous to the human cerebral cortex
  • possess nervous pathways which link the receptors to the higher brain
  • respond to noxious stimuli by consistently avoiding them
  • show modification of the avoidance responses when given painkillers
  • be able to associate neutral events with noxious stimuli, through learning
  • demonstrate an ability to choose a painkiller when given access to one, and when pain is otherwise unavoidable

According to these standards very few species have been shown to definitely feel pain. To broaden this picture, we have to accept less rigorous standards and form impressions instead of conclusions. Some findings which may help to influence your impressions include

  • there are a number of invertebrate species which are used in experimental models for pain. They include Caenorhabditis elegans, Aplysia californica, and Hirudo medicinalis, all of which are used as models for hyperalgesia, and sometimes for neuropathic pain
  • the land snail is sensitive to heat (40C), and that sensitivity is reduced by morphine. In addition, the snail is more sensitive to heat when given the opiate antagonist naloxone
  • goldfish show similar pharmacological effects as the land snail in the context of their response to an electric shock
  • paradise fish learn to avoid a particular environment where they experienced an electric shock. They also learnt to activate an escape hatch in order to avoid the shock

It has been argued that not all fish can experience pain (Rose 2002). This is based on the following points:

  • fish do not have an isocortex
  • there is no lamina I in the spinal cord of sharks and rays
  • there are few unmyelinated afferent nerve fibres in elasmobranchs
  • stingrays do not have typical nociceptive sensory neurones
  • severe laceration in sharks do not interfere with feeding behaviour

Each of these points will be discussed. It will be concluded that

  • pain perception is probably different and may be reduced in agnathans and elasmobranchs
  • the suggestion that teleosts do not have the necessary neuroanatomical structures for pain perception is unconvincing

The Animal Welfare Science Centre was established by the University of Melbourne, Monash University and the Department of Primary Industries, Victoria. The Centre focuses and coordinates the research and academic resources of the three collaborating organisations, providing the animal industries, animal users, the farming community, Government, the general community and the academic community with an internationally competitive research, teaching and training resource in animal welfare.