This table is an update of Varner's 1998 version. The most significant ambiguity in this table is in the insects, as there is some evidence that pain perception is different across insects with exoskeletons versus those with endoskeletons. I marked "some" insects as having nociceptors, as I was unable to find anyone who would make the categorical statement "All insects have nociceptors."
A "Y" means yes, blank means no, and question mark means the evidence is equivocal.
|Earthworms||Insects||Cephalopods||Fish||Amphibians / Reptiles||Birds||Mammals|
|Central Nervous System?||Y||Y||Y||Y||Y|
|Nociceptors Connected to CNS?||Y||Y||Y||Y||Y|
|Has Endogeneous Opiods?||Y||Y||?||Y||Y||Y||Y|
|Response Affected By Pain-killers?||?||?||?||Y7,8||Y8,9||Y||Y|
|Behavioral Pain Response?||Y||Y||Y||Y||Y|
Unless otherwise stated, all data comes from Varner, G. E. In nature's interests?: Interests, Animal Rights, and Environmental Ethics. Oxford University Press, USA, 2002. Other sources given by superscripts:
- Allen, C. “Animal pain.” Noûs 38.4 (2004): 617-643. Print.
- Sneddon, L. U., V. A. Braithwaite, and M. J. Gentle. 2003. Do fishes have nociceptors? Evidence for the evolution of a vertebrate sensory system. Proceedings of the Royal Society of London. Series B. Biological sciences 270: 1115-1121. http://dx.doi.org/10.1098/rspb.2003.2349
- Pastor, J., B. Soria, and C. Belmonte. 1996. Properties of the nociceptive neurons of the leech segmental ganglion. Journal of Neurophysiology 75: 2268-2279. http://jn.physiology.org/cgi/content/abstract/75/6/2268
- Wittenburg, N., and R. Baumeister. 1999. Thermal avoidance in Caenorhabditis elegans: an approach to the study of nociception. Proceedings of the National Academy of Sciences of the United States of America 96: 10477-10482. http://www.pnas.org/cgi/content/abstract/96/18/10477
- Illich, P. A., and E. T. Walters. 1997. Mechanosensory neurons innervating Aplysia siphon encode noxious stimuli and display nociceptive sensitization. The Journal of Neuroscience 17: 459-469. http://www.jneurosci.org/cgi/content/abstract/17/1/459
- Tracey, J., W. Daniel, R. I. Wilson, G. Laurent, and S. Benzer. 2003. painless, a Drosophila gene essential for nociception. Cell 113: 261-273. http://dx.doi.org/10.1016/S0092-8674(03)00272-1
- Sneddon, L. U. “The evidence for pain in fish: the use of morphine as an analgesic.” Applied Animal Behaviour Science 83.2 (2003): 153-162. Print.
- Machin, K. L. “Fish, amphibian, and reptile analgesia..” The veterinary clinics of North America. Exotic animal practice 4.1 (2001): 19. Print.
- Machin, K. L. “Amphibian pain and analgesia.” Journal of Zoo and Wildlife Medicine 30.1 (1999): 2-10. Print.
Why these criteria?Whatever the qualitative experience of pain comes from, it seems almost tautologically true that you first need to sense a stimuli in order to find it painful. So the requirement for nociceptors (or a direct analogue) is relatively straightforward.
The requirement of a centralized nervous system is probably equally straightforward at first glance: in order to be in pain, there needs to be a singular, distinct entity who is in pain. Varner notes that "Insects do not favor damaged limbs or become less active after internal injuries," implying that the "pain" insects feel, if any, is heavily localized.
If you have a mechanism to regulate pain, whether endogenous (i.e. created by the body) or through external drugs, that seems to indicate that the species has evolved not just to feel pain, but also to end pain that it feels. (Implying that not only is pain there, but it can also in some circumstances be "bad.")
Most theories for the evolution of pain assume that at least part of its function is to move away from noxious stimuli. Therefore, a lack of response seems to suggest a lack of pain sensing.