Because octopuses have sophisticated nervous systems and feel pain just as acutely as mammals do...
Octopuses and other cephalopods have sophisticated nervous systems, granted. I’ve written about their wonderful behaviour several times on this blog. But let’s look at the second half of that statement. “Feel pain as acutely as mammals do.” It sounds plausible, but what’s the evidence supporting it?
Pain is a very complicated subject. Comparing pain between humans is tricky, and we have the advantage of being able to talk to each other. Last week’s episode of All in the Mind contained this thought on the difficulty of comparing pain in different people (emphasis added):
Henry Greely: (P)ain is a huge question in law suits. Injury law suits, accident law suits, in workers’ compensation cases, in disability cases. And we don't have great tests for pain: we know that some people lie, that they say that their lower back hurts so badly they can’t go to work, but in fact occasionally a lucky defendant will have a private detective who's got video of the claimant bouncing up and down on a trampoline. But we don’t have a good test for it. ...
I don’t know that I see it ever as being plausible that we could say that the pain you're feeling is greater or less than the pain I am feeling.
We have crude methods of comparing nociceptive behaviour in very similar kinds of mammals. And even those crude measures give us very good reason to think that not all mammals experience pain to the same degree (see, for example, Park et al. 2008 on naked mole rats), which is an underlying assumption of the quote above.
Comparing levels of pain across the broad reaches of the animal kingdom? None that I know of. And given the paper that we just published (Puri & Faulkes 2010), I’ve been looking, trust me.
Let’s look at the more tractable issue of whether octopuses have nociceptors. There’s good physiological evidence that some mollusks have nociceptors; Aplysia californica do, for instance (Illich & Walters 1997). But to the best of my knowledge, nobody has shown nociceptors in octopuses. Varner (2003) wrote:
(Octopuses) have stunning learning abilities, but about whom little is known regarding nociception, endogenous opiates, and responses to analgesics.
I did find a paper on anesthetics in octopuses (Andrews & Tansey 1981), and those authors speculated that octopuses might have sensory neurons like mammalian nociceptors. An even more recent review (Mather & Anderson 2007) did not mention any physiological evidence for nociceptors in octopus, though providing a variety of behavioural reasons to suspect their presence.
Kavaliers (1988) noted that Wells (1978) claimed that there were “pain pathways” in Octopus vulgaris, but I don’t have the book at my fingertips to check what evidence Wells gives to support this claim.
To sum up, there are many good reasons to think that octopuses have nociceptors, but almost no experimental evidence. But is there anyone with the technical skills and the inclination to to to get that evidence? Not sure. While great for behavioural experiments, octopuses can be quite tricky animals to work with for detailed neurophysiology.
Given that cephalopods do have complex nervous systems, and that this has rightly fueled concerns about appropriate care levels for cephalopods, it may be very difficult for anyone to get permission to do the experiments that might determine if octopuses have nociceptors and what their properties are. The nature of octopus nociceptors would be incredibly interesting from a comparative and evolutionary point of view. Octopuses may have a completely different way of detecting tissue damage than is found in those organisms with a backbone.
Octopuses always challenge us, because there is no other organism on the planet that so obviously seems to have some kind of intelligence, but housed in an animal that might as well have come from another planet.
Andrews P, Tansey E. 1981. The effects of some anaesthetic agents in Octopus vulgaris Comparative Biochemistry and Physiology Part C: Comparative Pharmacology, 70(2): 241-247. DOI: 10.1016/0306-4492(81)90057-5
Illich PA, Walters ET. 1997. Mechanosensory neurons innervating Aplysia siphon encode noxious stimuli and display nociceptive sensitization. The Journal of Neuroscience 17(1): 459-469. http://www.jneurosci.org/cgi/content/abstract/17/1/459
Kavaliers M. 1988. Evolutionary and comparative aspects of nociception Brain Research Bulletin 21(6): 923-931. DOI: 10.1016/0361-9230(88)90030-5
Park T, Lu Y, Jüttner R, Smith E, Hu J, Brand A, Wetzel C, Milenkovic N, Erdmann B, Heppenstall P, Laurito C, Wilson S, Lewin G. 2008. Selective Inflammatory Pain Insensitivity in the African Naked Mole-Rat (Heterocephalus glaber) PLoS Biology 6(1). DOI: 10.1371/journal.pbio.0060013
Puri S & Faulkes Z. 2010. Do decapod crustaceans have nociceptors for extreme pH? PLoS ONE 5(4): e10244. http://dx.doi.org/10.1371/journal.pone.0010244
Varner G. 2003. How facts matter. In: Armstrong, S. & Botzler, R. G., Animal Ethics Reader, pp. 92-93. London: Routledge.
Wells MJ. 1978. Octopus: Physiology and behaviour of an advanced invertebrate. London: Chapman Hall.
Photo by Joachim S. Müller on Flickr. Used under a Creative Commons license.