Why do you sometimes lose bowel function when scared?
Letting it all go
Also in this week's column:
- What happens when you are executed by electrocution?
- What type of person is accident-prone?
- At what height can you survive a dive into water?
Why do you sometimes lose bowel function when scared?
Asked by Peter N, of Albury, New South Wales, Australia
Humans sometimes defecate at times of extreme fear due to the acute stress response (aka the "flight or fight" response).
When we are experiencing extreme fear, the sympathetic nervous system goes into overdrive and produces a state wherein we are better prepared to engage the source of the fear in a struggle or better prepared to flee from it.
A number of temporary physiological changes occur in the acute stress response. For example, more adrenaline is pumped through the system. This boosts heart and lung activity and thus aides physical functioning needed in fighting or running.
Other temporary physiological changes include dilation of blood vessels for muscles, constriction of blood vessels in parts of the body not needed for fighting or fleeing, liberation of nutrients needed for muscular action, inhibition of tear glands and salivation, dilation of the pupil, inhibition of stomach and intestinal action, inhibition of erection in males, and relaxation of elimination control. Thus, under circumstances of acute stress such as in extreme fear, the bladder and bowels can "let go".
Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au
COMMENTS
What I've been told...
....is that an empty bowel is less likely to lead to infection than a full one in the case of injury.
Some bits make sense...
... like the inhibition of erections. In evolutionary terms it doesn't bode well for running away from attacking cavemen or dinosaurs, either practially or socially.
However it is very effective Darwinism when applied to speed of escape from an angry husband returning to find you doing a bit of 'natural selection' with his wife.
Because, because, because...
"Side-effect of other things" is certainly one answer. The "fight or flight" response starts with a big shot of adrenaline, which puts the sympathetic nervous system into overdrive. This causes bandwidth problems for the central nervous system - no, seriously - and the signalling rate from the internal anal sphincter to the central nervous system drops suddenly, which in turn causes the sphincter to relax.
This mechanism would seem to be a straightforward side-effect.
There's another part to this though; adrenaline has an effect on the external anal sphincter, causing it also to relax and one's bowels to involuntarily evacuate through peristaltic contraction. This latter effect makes me wonder if there could be survival advantages to the response.
Conjecture 1) The sudden evacuation of faeces and the accompanying odour act as a signalling mechanism to nearby members of one's own species that a threat is at hand...
The problem with this idea is that the overwhelming response in mammals to the smell of faeces (particularly of one's own species) is disgust. Soiling yourself when the tiger leaps out of the bushes is more likely to send people downwind of you scurrying off in the opposite direction, not running to help.
Conjecture 2) If you reek of poo, you don't smell like a good potential meal...
Possibly, although mammals - and in particular, carnivores - show a decreased level of disgust response to faeces produced by species other than their own.
Conjecture 3) It makes you lighter and you can therefore run away faster...
This is too silly for words.
I'm going with:
Conjecture 4) Sometimes, sh*t just happens!
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