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Which animals feel pain?

The following table from Gary Varner, In Nature's Interests? Interests, Animal Rights and Environmental Ethics (Oxford University Press, 1998) summarizes six comparisons that were made in most or all of the following studies:

    Jane A. Smith and Kenneth M. Boyd, eds. Lives in the Balance: The Ethics or Using Animals in Biomedical Research (New York: Oxford University Press, 1991).

    This study was researched and written by a working party convened by Britain's Institute of Medical Ethics consisting of eighteen scientists, medical researchers, and philosophers.

    David DeGrazia and Andrew Rowan "Pain, Suffering and Anxiety in Animals and Humans," Theoretical Medicine 12 (1991):193-211.

    Patrick Bateson, "Assessment of Pain in Animals," Animal Behavior 42 (1991):827-39.

    Margaret Rose and David Adams, "Evidence for Pain and Suffering in Other Animals," in Gill Langley, ed., Animal Experimentation: The Consensus Changes (New York: Chapman and Hall, 1989), pp. 42-71.


Key:
    "+" denotes a confirmed comparison between the animals in question and normal humans
    "-" denotes a disconfirmed comparison between the animals in question and normal humans
    "?" denotes that insufficient data are available to evaluate the comparison

INVERTEBRATES VERTEBRATES
Earth-
worms
Insects Cepha-
lopods
Fish Herps Birds Mammals
Nociceptors
present
? - ? - / ? - / ? + +
Central
nervous system
- - + + + + +
Nociceptors connected to central nervous system - - + + + + +
Endogenous opiods
present
+ + ? + + + +
Responses modified by analgesics ? ? ? ? ? + +
Response to damaging stimuli analogous to humans' - - + + + + +
Note: The cells under fish and herps regarding the presence of nociceptors are yellow for the following reasons. Rose and Adams conclude that "Evidence supports the existence of nociception in all vertebrates" (p. 49), suggesting that a "+" belongs here. But they base this on avoidance behavior rather than identification of nociceptors, which as Smith and Boyd note, have been sought without success in these animals.
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