/ Why have we evolved to itch?

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JuneBob on 30 Jul 2013
Having been biten about 80 billion times by some pesky insects last night (it may be less, but I know I couldn't feel worse) and now writhing around in total agony, I'm wondering why we have evolved to itch?
What on earth is the benefit? It just makes a tiny bite many times worse, possibly resulting in me scratching open the wound, getting infected and dying. How is this a good natural response? It seems like a total fail by evolution (or god fwtw) to me.
I have been unable to sift out a good answer from the dross that google returns, so maybe someone here knows?
Jon Stewart - on 30 Jul 2013
In reply to JuneBob:

That is a good question. We've got specific "itch and tickle" nerve endings and I can't for the life of me think what the point of these is. Pain I get, touch I get, temperature I get, proprioception I get, but "itch and tickle" seems like a completely pointless feature of the afferent nervous system. Interested to hear the responses...
Tom Last - on 30 Jul 2013
In reply to JuneBob:

Encouraged our ancestors to go and live somewhere other than a swamp, thus resulting in them not catching malaria and reproducing etc.?
Timmd on 30 Jul 2013
In reply to JuneBob:

My intuitive answer is it's to remove a bit of foreign matter from the skin before it does something bad, like cause an infection or work it's way into the skin as the skin continues to grow around it, which could be very bad. I guess that applies to bacteria and bites and old skin too. It's a way of keeping the skin clear of things which shouldn't be on it.

It does seem like a flaw when it comes to midge bites though.
ThunderCat - on 30 Jul 2013
In reply to Tom Last:
> (In reply to JuneBob)
>
> Encouraged our ancestors to go and live somewhere other than a swamp, thus resulting in them not catching malaria and reproducing etc.?

Yeah, I've often wondered if it's an evolutionary response to insects. The ones who reacted quickly and swatted away whatever was biting or sucking (*) statistically lived a wee bit longer gaining an advantage over their peers


(*) no laughing at the back, there.

JuneBob on 30 Jul 2013
In reply to JuneBob:
Some interesting points, but none justify me feeling the urge to want to rip off my skin. Something a bit milder would be sufficient I would have thought?
Timmd on 30 Jul 2013
In reply to JuneBob:
> (In reply to JuneBob)
> Some interesting points, but none justify me feeling the urge to want to rip off my skin. Something a bit milder would be sufficient I would have thought?

I guess it does justify it if the biters can kill you or make you very ill though (which would happen around Maleria(sp) infected mosquitos)?

Timmd on 30 Jul 2013
In reply to JuneBob: It might be a bit late, but for another time I've found that if I don't scratch them once they start to itch, the insanely irritating itching doesn't get a chance to start and they just itch for a bit and then stop.
JuneBob on 30 Jul 2013
In reply to Timmd:
hehe, yup. I'm going to go for a bike ride to try to distract myself!
kathrync - on 30 Jul 2013
In reply to Timmd:
> (In reply to JuneBob)
>
> My intuitive answer is it's to remove a bit of foreign matter from the skin before it does something bad, like cause an infection or work it's way into the skin as the skin continues to grow around it, which could be very bad. I guess that applies to bacteria and bites and old skin too. It's a way of keeping the skin clear of things which shouldn't be on it.
>

That's pretty much it - histamine is involved in initiating local inflammatory immune response. Amongst other things it makes your capillaries more permeable to the white blood cells and proteins involved in immune responses so that they can migrate into the area where the foreign substance is to deal with it. It is an important primary immune response at all mucosal surfaces, not just the skin.

The bad news is that scratching will make bites worse because it increases localised inflammation which means that your body thinks it needs more white cells in the area and produces more histamine.

The good news is that a normal antihistamine tablet, like you would take for hayfever, will make the worst of the itching go away.

As an aside, histamine also functions as a neurotransmitter with effects on sleep regulation. This is why older hayfever tablets used to make you drowsy. Modern tablets are more specific in which of the four histamine receptors they block and therefore don't have this problem any more.
JuneBob on 30 Jul 2013
In reply to kathrync & Timmd:
Thanks for that explanation; still seems like a bit of an overreaction to me :-)
I guess I should learn some meditation techniques to block out the itching.
3 Names - on 30 Jul 2013
In reply to JuneBob:

Why have we evolved to itch?

So we know when its time to go climbing.
pebbles - on 30 Jul 2013
In reply to JuneBob: proof that some stuff we have evolved is just a bit random?
Timmd on 30 Jul 2013
In reply to JuneBob:
> (In reply to kathrync & Timmd)
> Thanks for that explanation; still seems like a bit of an overreaction to me :-)
> I guess I should learn some meditation techniques to block out the itching.

Dissociation can work, I don't know if that's the same as meditating, just thinking about the itching being on the end of something further away than right at the front of your thoughts. 'That itch is miles away right where I've been bitten, not here where I'm doing my thinking...'

Another name could be ignoring the itches until they stop. It's impulse control? Good luck. (:-))
Gordon Stainforth - on 30 Jul 2013
In reply to JuneBob:

We haven't 'evolved' at all 'to itch'. This is just how nature happens to be now, for us; and kathrynr has pointed out the very clever things that nature's doing already with that itching. The great paradox of neo-Darwinism, that tried so hard to rid the world of teleology, is that it's still so hung up by grand evolutionary 'purposes' ... with some quite ridiculous teleological theories resulting.
needvert on 30 Jul 2013
In reply to pebbles:
> (In reply to JuneBob) proof that some stuff we have evolved is just a bit random?

As I understood it, the change in DNA between generations involves that random aspect, and selection pressures the not very random at all bit.
Gordon Stainforth - on 30 Jul 2013
In reply to needvert:

Vast amounts of time are wasted talking about randomness rather than looking at the very un-random way nature in fact works. Just forget the word 'random', I suggest. It's just a dead end that, taken too seriously, could waste you months or years of more productive research time.
Timmd on 30 Jul 2013
In reply to Gordon Stainforth:

The word random can probably lead to vague thinking if misapplied.
Timmd on 31 Jul 2013
In reply to Gordon Stainforth: Or certainly can, seem to have the word probably fixed in my head.
teflonpete - on 31 Jul 2013
In reply to JuneBob:

Maybe the insects are further evolved than humans.

They bite you, you scratch the itch, you get an infection and die, their larvae eat your corpse.
USBRIT - on 31 Jul 2013
In reply to JuneBob: Think yourselves lucky you live in the UK with only midges and clegs.Just a few days ago a guy in Arizona was killed by killer bees while cleaning a new route..over a 100 stings..they also killed his dog. Insect repellent containing a LOT of Deet will stop you from being bitten.. But only if you cover ALL exposed areas. This powerful repellent is not great for your skin or plastic but it works.It works not because of the smell but the insects do not like to stand in it.If you sweat or it rains you may have to keep putting on more of the repellent.
pebbles - on 31 Jul 2013
In reply to Timmd: yes, but since evolution operates by selection pressure on randomly generated changes, not everything that evolves is going to be amazingly clever or the best possible solution, and some stuff that is neither specially good or specially bad will persist as well
MG - on 31 Jul 2013
In reply to Gordon Stainforth:
> (In reply to needvert)
>
> Vast amounts of time are wasted talking about randomness rather than looking at the very un-random way nature in fact works. Just forget the word 'random', I suggest. It's just a dead end that, taken too seriously,

What a strange thing to say. I would suggest that the opposite - thinking in a purely deterministic manner - is much more of a danger, and not only when considering natural processes.
kathrync - on 31 Jul 2013
In reply to MG:
> (In reply to Gordon Stainforth)
> [...]
>
> What a strange thing to say. I would suggest that the opposite - thinking in a purely deterministic manner - is much more of a danger, and not only when considering natural processes.

Yeah, I spend a lot of time reading undergraduate essays about how a certain organism is attempting to evolve towards some ideal trait - as if the organism in question had any control over how they are evolving, or even thought about it at all.
Timmd on 31 Jul 2013
In reply to pebbles:
> (In reply to Timmd) yes, but since evolution operates by selection pressure on randomly generated changes, not everything that evolves is going to be amazingly clever or the best possible solution, and some stuff that is neither specially good or specially bad will persist as well

Yes, I tried to explain that to a Christian who believes in intelligent design, after he'd said why he thought evolution wasn't so believable. I'd no desire to 'convert' him as such, it just came up in conversation.
Dave Garnett - on 31 Jul 2013
In reply to pebbles:
> (In reply to Timmd) yes, but since evolution operates by selection pressure on randomly generated changes, not everything that evolves is going to be amazingly clever or the best possible solution, and some stuff that is neither specially good or specially bad will persist as well

But there's a perfectly plausible selective mechanism for itchy midge bites isn't there? Midge bites are intrinsically trivial and could easily go unnoticed. However, there is a risk involved - possibly quite a big risk in areas where they are carrying something nasty. If the obvious symptoms of the infection caused by the bite don't arise for days or weeks, the bite would not be associated with its result and so it would be an evolutionary advantage to directly associate midge bites with unpleasant discomfort and to actively avoid being bitten. Individuals with an inflammatory reaction accompanied by discomfort would avoid being bitten and so have a survival advantage.

Histamine-based inflammation was already being selected for, so individuals with an added pain/discomfort reaction would be additionally selected for. I forget but doesn't histamine somehow lower the threshold of afferent impulses? Anyway, I don't think any anthropic, teleological or even accidental by-product argument is required.
Gordon Stainforth - on 31 Jul 2013
In reply to MG:
> (In reply to Gordon Stainforth)
> [...]
>
> What a strange thing to say. I would suggest that the opposite - thinking in a purely deterministic manner - is much more of a danger, and not only when considering natural processes.

Agreed. My point had nothing to do with determinism whatever.
althesin on 31 Jul 2013
In reply to JuneBob:
If you think itching is funny, where the hell did tickle come from? Now to really make you think, try tickleing yourself.
MG - on 31 Jul 2013
In reply to Gordon Stainforth: Well the opposite to deterministic is probabilistic (ie random, like rolling dice) so I don't follow
Gordon Stainforth - on 31 Jul 2013
In reply to MG:

Only a very long and complex argument would explain what I mean (and I'm still working on it, inter alia). In short (very short), I don't think it's a simple polarity between chance and necessity because living nature is always a mixture of the two, so that no single process can be described as either random or determined. My distaste for simple/crude polarities such as these also leads me to think that (there is a strong possibility at least) that something else is going on as well. The possibility has to be left open that we haven't got the full story yet.

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