What, if any, is the reason that when humans are terrfied our bladders let go?
For example: I’m out on a hike. I see a bear. The bear sees me and gives chase. I could run and hide but, oops, I wet myself! Now no matter where I go I’m essentially holding up a big, neon “here I am!” sign.
One of my friends says it’s to let go of excess weight so you can run faster but that makes no sense.
Any thoughts?
This reminds me of something my dad used to say: ‘I’d have a dry track to run on. You’d be slipping in sh*t!’
Unless you’re really holding in a full bladder I doubt many people actually “wet their pants” when scared. Similar to the stereotypical “fainting when frightened” for ladies.
Neither makes any sense if they were truely common. More likely a movie construct (I’m sure a few people actually do faint/pee their pants but it’s probably rather rare).
Another why in science question…
The best theory I’ve heard states that it’s a thing great apes do. Since we’re great apes, we may do it. Gorillas will sometimes urinate as part of their scary dominance display. This display happens when the gorilla feels threatened, but before he actually charges and attacks. Perhaps it’s to to enhance territory marks, or simply to appear batshit insane and therefore not to be messed with.
It is true that blood flow and muscle tone are re-routed away from central organ function and to the periphery (arms and legs, feet and fists) when the sympathetic nervous system is active due to stress, but I don’t know if it’s really enough to account for bladder voiding. Since pissing yourself is not actually that common, but sympathertic nervous response is, I’d have to think not.
(While I’m sure this isn’t what you’re looking for: In Traditional Chinese Medicine, the Kidney/Urinary Bladder system is associated with fear. People suffering constant or near constant fear often develop kidney problems, and people with kidney problems are often more fearful. One day in class I had an a-ha moment about the kidney/fear connection and loss of bladder control when frightened. My teacher confirmed the theoretical connection. Doesn’t explain why, but the connection is more than a western construct.)
Some dogs will do submissive peeing when they perceive that they are being challenged. The trait may just be a common socialization thing, done by several speceis of mammals.
I don’t have a real answer to the question, but let me point out that clothing is extremely recent on evolutionary timescales, and so this mechanism (whether or not it exists) would have begun when there were no pants to be wet.
Some semi-random thoughts:
Some predators dislike dead prey, so perhaps a strong scent would help drive them away. I would think voiding bowels would fit this theory better (with feces having a similarity to offal) than urination would, though, and the latter seems to be the more traditional fear reaction (at least in folklore).
I really doubt this would “lessen the load” enough to make a human run faster, but that’s a half-way plausible reason.
There is a similar mechanism which can cause humans to vomit when visual stimuli greatly differ from inner-ear stimuli (i.e., when the felt position of the body is radically different from what the person sees) – this, as I understand it, is because this kind of discrepancy can, in the wild, be an early symptom of poisoning. So the body quickly gets rid of what might be poisoning it. I don’t really see a connection here, but it’s an interesting side point.
On preview, I see WhyNot has answered the question with much more rigor than my pissy guesses, but I guess I’ll post this anyway.
There’s a bit somewhere in Steven Pinker’s “How the Mind Works” about this. The interesting part is that our fright response is usually somewhat appropriate to the threat. For example, when we find ourselves on a high ledge, we don’t wet our pants – we freeze in place. Perfect automatic response. darnit… lent the book out… can’t remember the others well… in the fight/flight situation, our mouth goes dry and our face gets pale, because the blood is being routed to our core muscles to get ready for a big exertion.
This line just forced a Gary Larson image into head. I can see a Gorilla standing face-to-face with a large rhinocerous in the jungle, and a thought bubble that says, “Maybe if I act batshit insane…”
I’d heard that this was a response to at least some predators not liking to eating the stuff. Basically the argument went, " this thing has crap all over itself. Crap is bad. I don’t want it anymore".
Logically it may make sense for a predator to not want to eat something covered in its own excreta given the amount of nasty bugs in poo.
Course given that sanitary conditions in the wild aren’t exactly hospital clean I’m not sure if this theory can hold water. 
Another possible reason is that feces is a serious infection risk. If you’re in a situation where bodily trauma is a possibility it makes sense for your body to release feces so it isn’t present in case you’re wounded in the gut.
Considering that predators eat intestines, I don’t think it makes any sense at all. It also makes no sense from the evolutionary persepctive, since your ability to reproduce is pretty much nil once the predator has gotten that close.
I personally think that actual pissing is rare, and restricted to very-full-bladder situations wherein one is strongly resisting the urge. One good startle, and thar she blows!
Don’t you generally soil yourself when you die anyway, though? I’m sure that wouldn’t deter Mr. Sabertooth Tiger if that’s what he’s used to eating. “Mmm, sauce!”
The body’s response to extreem stress is a chemical dump of hormones that have all sorts of effects. Here is a good description although it does not mention incontinance directly.
http://www.phobologia.com/Physiology.htm
This one does:
http://www.killology.com/art_psych_combat.htm
It seems that battle incontinence is related to the body’s shutting down of the digestive tract. There is also a general loss of muscle control as the body goes into flight or fight mode.
So far I have gotten through life with nothing beyond the butterflies in the stomach feeling, I can only try to imagine that feeling a thousand times worse.
I am sure I remember discussing this subject with a military type, though it would have been a long time ago. He told me that soldiers were trained to expect a high probability of incontinence, of both types, under extreem stress. The important thing was to understand the natural responses of yourself and your comrades and get on with the job at hand. Sounds like good advice.
I can tell you’ve never actually been caught by thunderstorms on an alpine ridge miles away from treeline in Roosevelt National Forest and the tallest thing on the ground by two orders of magnitude and then had lightning strike first 50 feet to one side of you, and then 50 feet to the other.
I didn’t know when it had happened, but when our hair stopped standing on end and we felt as if it was safe enough to stand up again and hike out, I noticed I had pissed my pants like a three-year old.
My Highschool Chem teacher’s explaination:
Your bladder is basically a bag in your midsection that is full of chemicals that your body wants out of your body. If this bag becomes ruptured by say a fight with an animal the urine leaks into your system causing tons of damage and infection. When the attack didn’t kill you, the after effects probably will. By flushing out urine before an attack takes place the risk of your bladder popping and wrecking your insides is signifigantly reduced. Its basically a defense mechanism leftover from our ape days.
Band Name! 
This sounds the most reasonable to me, except for one thing - urine is sterile. No risk of infection there. It’s either a side effect of relaxing the urogenital muscles for fecal dumping, or just to make room for more urine (urine overproduction is a side effect of adrenaline).
Funny that someone else mentioned wetting the knickers after a lightning strike… I knew someone who experienced the exact same thing. It’s the only time I ever heard of someone voiding from fear.
The only problem with this is now all the feces will now be able to infect any external wounds you may recieve.
Any wound which punctures the gut will run a very high risk of infection, regardless whether or not the colon is full. Feces consists of bacteria which are always present in the gut. It isn’t the fecal matter which infects, but the bacteria which inhabit it.
Plus, any wound serious enough to puncture the gut probably means that you are screwed to the point to where infection will be the least of your problems.
I think the whole voiding thing, bladder or bowels, is just the body saying that it has more important things to worry about than keeping various sphincters slammed shut.
It does happen, and I’ve seen it once. One night I was going after a group of guys who had robbed a man at gunpoint. Several officers covered the exits while my partner and I knocked at the door. When one of the suspects opened the door, he had two Glocks pointed in his face. He threw his hands in the air, pissed his pants and started crying (this guy was about 25 years old and sober). I guess he wasn’t cut out for a life of crime.
I read this book awhile back that made the claim that 25% of soldiers will crap their pants in combat situations. Can any of our esteemed military/ex-military comrades confirm?