Animal Senses #5: Pain

Ed Yong has this to say in Immense World:

“Pain is the unwanted sense. It is the only one whose absence (in some organisms) feels like a superpower.”

 

How does the experience of pain occur? This may seem like a stupid question, but its answer leads to many things. The experience of pain depends on a class of neurons called nociceptors. The naked tips of these neurons pervade our skin and other organs. They are loaded with sensors that detect harmful stimuli – heat, cold, pressure, acid, toxin, injury and inflammation.

 

Nociceptors can be partially disabled. In mole rats, for example, the nociceptors are blocked wrt detecting acid. Why? Because they sleep in large groups huddled with each other. The ones that at the bottom would experience high levels of carbon dioxide. High CO2 levels in turn makes the blood acidic. If a mole rat reacted to acidity levels the way we do, it “would probably lead to an agonized sleep”. Hence, the nociceptors in mole rats are disabled wrt acidity.

 

The mole rats example leads to two interesting conclusions about pain: (1) there is nothing fundamentally painful about acids; and (2) pain, like color, is a subjective sensation. In more general terms:

“Pain, in warning animals of injury or danger, is crucial to their survival… (but) they differ in what they must avoid and what they must tolerate.”

Conversely:

“That makes it notoriously tricky to tell what an animal might find painful, whether an animal is experiencing pain.”

 

Scientists differentiate these two things: (1) the ability to detect harmful stimuli (nociception); and (2) the suffering that ensues (pain). To us, the two feel inseparable, but they’re not. Even in humans. Amputees can experience pain in the phantom “limb” that doesn’t even exist anymore (pain without nociception). And a small number of people don’t experience pain at all even when body parts are bleeding or inflamed (that’s nociception without pain).

 

When most species are injured in one part of the body, they touch, cradle or groom their wounds. And they feel pain in the injured part, not everywhere in the body. The squid, however, doesn’t groom its wounded area. Why not? The question led to a strange finding during experiments:

“The nociceptors on the opposite fin (as the injured fin) were just as excitable as those on the wounded side… When they’re injured, their whole body becomes hypersensitive.”

That may explain why squids don’t groom the wounded part.

“They can sense that they’ve been hurt, but they might not be able to tell where.”

 

All of which leads to an interesting point:

“Things hurt so that animals can do something with that information. And without understanding their needs and their limitations, it’s hard to interpret their behavior correctly.”

 

Have you heard of those horrifying patterns in nature where a praying mantis will continue to mate with the female even while she devours him? Or caterpillars that continue munching on leaves while parasitic wasp larvae eat them from inside? Maybe the answers lie in how these species experience (or rather, don’t experience) pain…