Purpose of Itchiness
March 22, 2011 1:47 PM   Subscribe

Why does itchiness exist? It doesn't seem helpful.

Seems like it's always best to go against what itchiness is spurring you to do. Actually scratching worsens whatever the underlying problem is. I can't think of a situation where giving in to the itchiness is helpful. So why did the phenomenon of itchiness ever come into existence in our species? (And same for other species too: thinking of cats and dogs wearing cones on their head to prevent them from biting their skin open when they have fleas.)
posted by Paquda to Science & Nature (37 answers total) 14 users marked this as a favorite
 
Imagine sleeping outside. Now, imagine a spider or other insect, potentially poisonous or disease-carrying, crawling on you. Imagine the tickling sensation of its legs. See how it might be advantageous to want to swat or scratch it away?
posted by Gordafarin at 1:50 PM on March 22, 2011 [5 favorites]


Itchiness is a fantastic way to alert you that there's something crawling on your skin that shouldn't be, and it needs to be slapped/bitten/shuddered off.
posted by restless_nomad at 1:51 PM on March 22, 2011 [1 favorite]


Because:

1) Irritation is irritating and there are some pretty good reasons for our bodies to alert us to the presence of irritating things.

2) Immunity to irritation or ability to ignore and not scratch apparently did not present a reproductive advantage in our distant ancestors. As long as our ancestors were able to reproduce before being overtaken by the irresistible urge to scratch itches to the point of impotence or death, there wouldn't be any evolutionary pressure against itching, would there?
posted by The World Famous at 1:52 PM on March 22, 2011 [2 favorites]


Personalopionfilter. Get the hell off me, flea!

Furthering the personalopionfilter, I have the probably unproven opinion that sometimes 'connections drop out' and so we are compelled to check them.
posted by run"monty at 1:58 PM on March 22, 2011 [1 favorite]


Interestingly, itchiness and pain are governed by the same nerves. Pain, ironically, is a good thing, as it can be devastating to children who suffer a rare genetic disorder called CIPA, who grow up without the sensation of pain.
posted by Blazecock Pileon at 2:00 PM on March 22, 2011 [1 favorite]


+1 on itchiness and pain being a necessary signal that something is wrong and needs attention.
posted by bartleby at 2:02 PM on March 22, 2011


I understand the need to signal trouble, but my question is about the urge to action that comes bundled with the signal: the urge to scratch. The urges bundled with pain on the other hand do seem helpful: remove your hand from the pot handle or whatever.

The idea of dangerous bugs crawling on you does seem reasonable, but would apply to a tiny proportion of the incidents of itchiness in our lives. (Also not sure that that's itchiness, maybe more of a tickle? Or maybe there the same thing.)
posted by Paquda at 2:11 PM on March 22, 2011


I just want to point out that even though people have contributed some excellent reasons for itchiness to exist, it is not necessary for there to be a selective advantage for itchiness. It could be an epiphenomenon that simply arose in the presence of lots of nerves.
posted by milestogo at 2:12 PM on March 22, 2011 [20 favorites]


The idea of dangerous bugs crawling on you does seem reasonable, but would apply to a tiny proportion of the incidents of itchiness in our lives.

Exactly!

The cost of itching when there's no real risk: none, really
The cost of not itching when the risk is fatal: death

So, itching wins.
posted by General Tonic at 2:15 PM on March 22, 2011 [6 favorites]


In other words, the cousins of our ancestors who simply had no itching instinct are now extinct. We are descended from those who itched.
posted by General Tonic at 2:18 PM on March 22, 2011


Your question assumes purpose for itchiness when evolution needs none - evolution does not design. Our bodies are saddled with things that are stupid, or ridiculous, or counter-productive ways for things to work, because of this. Itchiness may be one of them.

That's not an answer, but it's a likelyhood you didn't seem to consider.
posted by -harlequin- at 2:19 PM on March 22, 2011 [2 favorites]


Evolution doesn't always produce traits that are perfectly obviously beneficial to an organism at all times. Here are a few reasons why:

1) Environmental change including climate change, predator/prey mutation/extinction/prevalence/scarcity, or in humans, cultural and technological change

2) Sexual selection, which sometimes means that things are sexy because they're dangerous or costly, and then there is runaway sexual selection (the gene for a big X is bundled with the gene that finds X sexy)

3) Useless or harmful traits that are sometime or always bundled with adaptive traits

4) No fitness difference as a result of the trait (this seems rare)

I'm sure there are others!
posted by the young rope-rider at 2:21 PM on March 22, 2011


The cost of itching when there's no real risk: none, really

I think there are risks: you get some minor fungal infection or something like that causing a maddening itch, you scratch it violently over the course of time, laceration of the skin causes a a bacterial infection to develop which requires treatment by antibiotics. I assume before the relatively-recent invention of antibiotics the bacterial infection had the potential to become life-threatening.
posted by Paquda at 2:22 PM on March 22, 2011


The cost of itching when there's no real risk: none, really

The cost of itching when there if no real risk is very high - broken skin leading to infection leading to injury or death. We don't live that way any more, but it was only 200 years ago that infection was a death sentence.
posted by -harlequin- at 2:23 PM on March 22, 2011


The relative rarity of needing to tend to an itch could be an environmental change. We didn't evolve with modern housing, pesticides, public health measures...even in recorded history there was a time when flea = plague = death. (Even now mosquito can = malaria = possible death)
posted by the young rope-rider at 2:25 PM on March 22, 2011


Paquda: "The idea of dangerous bugs crawling on you does seem reasonable, but would apply to a tiny proportion of the incidents of itchiness in our lives. (Also not sure that that's itchiness, maybe more of a tickle? Or maybe there the same thing.)"
I agree; the feeling of a bug crawling on your skin is entirely different from the itchy feeling of a mosquito bite. According to this article:
Giesler theorizes that the itch sensation creates an excited state in the STT neurons that scratching inhibits — as if our fingernails were sending a message to spinal-cord neurons to cool off. Scientists are still a long way from understanding the itch-scratch phenomenon, and while Giesler's study gives them a good place to start, neuroscientists caution that in humans, the mysteries of itching and scratching may go beyond the physiological: emotional and psychological factors are also often at play, especially in cases of unexplained, unremitting itching or itching of phantom limbs.
posted by Rickalicioso at 2:26 PM on March 22, 2011 [1 favorite]


you scratch it violently over the course of time, laceration of the skin causes a a bacterial infection to develop which requires treatment by antibiotics

Right, but that doesn't usually happen unless something else has gone wrong. If you are itching one spot so continuously that you break your skin, then our itching instinct is not to blame.
posted by General Tonic at 2:29 PM on March 22, 2011 [1 favorite]


Also, from the same article I linked to before:
The common itch isn't so benign in many conditions, including shingles and AIDS, which can cause uncomfortably severe itching. Sometimes itching can occur inexplicably, without any apparent physical cause, and a patient's unchecked scratching can lead to excessive skin damage or worse.
posted by Rickalicioso at 2:31 PM on March 22, 2011


Y'all are looking at this backwards. Humans didn't evolve "itchiness". Other organisms evolved secretions etc that trigger out inflammatory and pain responses so we'll stop messing with them. Most people will only try weed whacking poison ivy once. Same with touching stinging nettles etc.

Rashes are really just your immune system overreacting to a stimulus.

The sensation of things crawling on your skin is stimulation of the little hairs. Not pressure. Spiders aren't that heavy.
posted by fshgrl at 2:38 PM on March 22, 2011 [3 favorites]


This is somewhat analogous to breathing being useless when you're underwater. There's always going to be a downside to to the survival traits we've developed over the millions of years things like "itchiness" saved our species from extinction. I'm speculating we notice those downfalls more than their benefits, as the downfalls are a slight bit more inconvenient..
posted by samsara at 2:39 PM on March 22, 2011


Other organisms evolved secretions etc that trigger out inflammatory and pain responses so we'll stop messing with them.

Interesting, but I'm thinking of problems caused by micro-organisms: e.g., pink eye: the germs that pink eye infections consist of don't get any benefit from our scratching our eyes. But it does damage our delicate eyes. So I do think it's our species that evolved this response. Same with, as I said, fungal infections. Doesn't do the little fungi any good if we scratch the skin where they're living.
posted by Paquda at 2:43 PM on March 22, 2011


Your question reminded me of a wonderful article in the New Yorker, circa 2008, which addressed some of the more recent science framed in a morbidly interesting story of a patient with an (irrational) itch reflex so severe that she at one point itched through her skull and to her brain... in her sleep.

Ther article deals with it tastefully though, as expected of the New Yorker, and it's fascinating.
posted by gilrain at 2:43 PM on March 22, 2011 [5 favorites]


Btw, the sensation of burning you get in your mouth if you eat a poisonous plant* or animal is a variation on itching (pain and inflammation) reaction to epidermal** exposure but more severe since its a mucous membrane. So are plants/ animals that are too spiny to chew properly or that produce any other kind of pain/ inflammation reaction.

*except things like yes which just kill you immediately.
**amusingly auto correct wanted to make this "spiderman". It's reading the thread!
posted by fshgrl at 2:46 PM on March 22, 2011 [1 favorite]


If you are itching one spot so continuously that you break your skin, then our itching instinct is not to blame.

I, and many people I know, will happily scratch until the skin breaks (and continue even then) just with any old regular insect bite, if we didn't know better.

In animals that don't know better, it's not uncommon to see problems develop from itches that were hardly above-and-beyond at the beginning.
posted by -harlequin- at 2:49 PM on March 22, 2011


Pinkeye and other infections are spread through our itching at them. Thee get a very clear benefit. Besides which your own immune system is more than capable if triggering your pain/ inflammation response, either on purpose or by accident.
posted by fshgrl at 2:49 PM on March 22, 2011 [1 favorite]


Scratching causes the release of pre-stored TNF from mast cells and other cytokines from the keratinocytes that then lead to a local inflammatory response (influx of immune cells from the blood). If there's nothing noxious in the skin, then this response will resolve quickly, because many of those immune cells need to "see" pathogens to promote further inflammation. If something nasty is there, however, then scratching will have kick-started an ultimately beneficial immune response that might otherwise have taken many more hours.

Collateral damage is always a secondary consideration in these discussions. Better to have a scar than a persistent infection. And I think there's little benefit for scratching other than it's role in preventing infections.

On Preview: In animals that don't know better, it's not uncommon to see problems develop from itches that were hardly above-and-beyond at the beginning

Evidently, the evolutionary cost of such eventualities is lower than the benefits of itching-scratching behaviour.
posted by kisch mokusch at 2:54 PM on March 22, 2011 [7 favorites]


Judging from the large number of animals which scratch/itch, the itching reflex has been around for a long time... probably developed in some ancestor which had thicker skin and more hair, so it was less dangerous and also useful as a mechanism for grooming/shedding. (This has been on my mind a lot since spring shedding is here.)
posted by anaelith at 3:17 PM on March 22, 2011


I always thought of itching as a warning sign. Sort of your body saying, "Hey! Look over here!"

The first time I ever interacted with a cat I was 11... I spent a good 15 minutes petting the cat while scratching my face. I went to tell my mom that my face was itchy and it turned out my chin and jaw had broken out in hives. Now whenever my face itches I know I'm in the presence of a cat and need to get away ASAP.

I have mild psoriasis and when I realize I'm scratching my arms I know a flare up is coming along and I need to up my moisturizing.
posted by simplethings at 4:13 PM on March 22, 2011


The common itch isn't so benign in many conditions, including shingles and AIDS, which can cause uncomfortably severe itching.

Imagine both of those conditions in paleolithic times: an autoimmune disease is going to kill you anyway, and you'll never live long enough to see the shingles.

Scratching is also a way to remove sub-dermal parasites.
posted by clarknova at 4:55 PM on March 22, 2011


When I was in grad school in the 1980s, there was a theory that allergies are the price we pay for the ability to fight off parasitic infections. A quick search turned up this excerpt from a 2004 book on asthma. The idea is that one class of antibody molecules, called IgE, are usually developed in response to antigens found on parasites. IgEs are also the antibodies that trigger the allergic response, including itching.

Which still doesn't really answer the question, but at least pushes it down a step, to the molecular level. (Is this theory still current? My google-fu is weak today.)
posted by Quietgal at 5:01 PM on March 22, 2011


Evidently, the evolutionary cost of such eventualities is lower than the benefits of itching-scratching behaviour.

I think this over-reaches into begging the question, by implying (incorrectly) that the fact that evolution hasn't removed itch-scratching suggests it is more beneficial than not.
It might be true that scratching has more gain than cost, (or it might not be), but that line of reasoning is not sound - evolution doesn't work like that. The evolutionary persistence of a trait does not mean the trait must be beneficial.
posted by -harlequin- at 5:05 PM on March 22, 2011


but would apply to a tiny proportion of the incidents of itchiness in our lives.

1) A parasite-free life is a relatively modern condition. Most animals have many parasites in the wild, and humans were covered with crawlies once upon a time.
2) Bathing and not having a protective layer of dirt, sebum, hair, and dead skin is also relatively modern.

We did not evolve under current conditions, so I wouldn't expect all our behaviors to make sense in a modern environment.
posted by benzenedream at 5:35 PM on March 22, 2011 [1 favorite]


I know it's common for parents to tell children not to scratch itches like bug bites, rashes, or chicken pox because "it'll get infected", but how often does that really happen to the extent of "...and then you will die"?

My parents gave up on the No Scratching thing (I'm extremely sensitive to mosquito bites and grew up in a swamp). I scratched to my heart's content - still do, in fact. I think I got maybe one infected bite per summer. And it was "infected" in the sense that it took longer to heal and maybe left a scar. I never ended up on antibiotics or anything.

I have a strong feeling that, back in Ye Olden Proto-Hominid Tymes, people weren't so worried that *horrors!* one might get a small scar from scratching a bug bite too much.
posted by Sara C. at 7:59 PM on March 22, 2011


Something missing from this thread is the fact that scratching an itch feels good. Which means there's a pull to the behavior, not just a push. That indicates greater complexity to the phenomenon, at least from a neuro standpoint, which in turn suggests a purposeful origin, as opposed to it being something incidental.

Also, itch is second only to pain in terms of demanding conscious attention. Not many sensations drive you crazy until you acknowledge them. That should give you some idea of its relative import.

I'm going to second the issue of parasites. benzenedream is absolutely right—most people don't realize how common parasites are in the wild. Live in the dirt or the jungle for a while and you soon find yourself covered in things trying to actively colonize you. It remains a serious problem in the developing world.
posted by dephlogisticated at 8:05 PM on March 22, 2011 [2 favorites]


I think this over-reaches into begging the question, by implying (incorrectly) that the fact that evolution hasn't removed itch-scratching suggests it is more beneficial than not.

Yeah, without clarification I can see why you'd take it that way. The "persistence doesn't equal beneficial" line has qualifiers too. For unique traits, that aren't evolutionarily conserved across different species, and for which one cannot see beneficial (or harmful) consequences of that trait (think eyebrows, for example), one can say that the persistence of the trait is not necessarily mean that it's beneficial.

But scratching is performed by so many different species, hundreds of millions of years apart, which means it is pretty highly conserved behaviour. And, from an immunological point of view at least, we can see a potential benefit for scratching. Also, because it has a cost, as you quite rightly pointed out, it in fact more likely to have a benefit in order to be maintained in so many populations.
posted by kisch mokusch at 10:23 PM on March 22, 2011


more likely to have a benefit in order to be maintained in so many populations.

Even with all the reasons you gave, you still cannot conclude that itchiness has some benefit. It could be part and parcel of the nerve/brain hardware. It may not be a thing that can be removed at all. As far as scabs go, for instance (which the OP asked about) -- one could imagine that rebuilding the nerve cells in damaged tissue simply does irritate the brain in the 'itchy' way as a function of the biological hardware. Then it isn't a question of descending from those who evolved the itichiness mutation at all, its just a physical constraint on the nervous system.
posted by milestogo at 3:34 PM on March 23, 2011


It could be part and parcel of the nerve/brain hardware. It may not be a thing that can be removed at all.

Well, I can't speak for the behavioral side (i.e. whether we must scratch if we feel an itch), but that isn't true at all at the detection level. People with Congenital insensitivity to pain, for example, do not itch. Unfortunately, that's a severe scenario, since we can all see the benefit of being able to feel pain. But it does show that the signalling pathways that exist to notify our brains that there is something itchy are not so robust that they can't occasionally fail due to mutations.

I might alos point out that people with Congenital insensitivity to pain are also prone to infections.
posted by kisch mokusch at 1:52 PM on March 24, 2011


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