When faced by a sudden, as yet unidentified threat, such as a loud noise, a bright flash, or a shock, we have been adapted by selection to perform some of the main defensive measures in anticipation, until at least we know what it is that we are defending ourselves against. We crouch and cover ourselves with our arms, maybe covering our ears as well. And we blink, because some of our most vulnerable parts are our eyes. 

When there is a longer-lasting threat, someone who is frightened also might keep his or her eyes closed just to deny the unpleasant reality of danger.

Presumably rated by a troll who lacks the courtesy and wit to argue any case of their own.

Auxiliary query, though, Jon (nice to see you back, by the way.)

Do you have a view on the evolutionary benefit of the relaxation of various sphincters under extreme physical danger? I hypothesise either improving your capability for flight, or decreasing your attractiveness on a plate, but these seem marginal compared to the time and effort involved.

By voiding pee or poo you reduce the weight you have to carry, so enabling you to flee faster. Maybe before we came down from the trees, it helped us climb back up them quicker too. The threat need not have been a large predator. Elephants will trumpet when themselves alarmed and primed for a stampede. But we cannot test this; we cannot re-run our evolutionary past. And if we could go back in time, what would serve as the experimental control? So, albeit plausible and commensurate with the voiding in question, for now this remains another evolutionary 'Just So' story.

Some butterfly and moth larvae will incorporate their faeces or frass into their silken communal webs or cocoons. Caterpillar frass is by and large odour free and non-toxic or infective. But a mouthful of granular and partly digested plant matter is unlikely an attractive prospect. Quite apart from its unpalatability, the incoporated frass can help disguise the web's or cocoon's true nature.

By voiding pee or poo you reduce the weight you have to carry, so enabling you to flee faster. Maybe before we came down from the trees, it helped us climb back up them quicker too. The threat need not have been a large predator. Elephants will trumpet when themselves alarmed and primed for a stampede. But we cannot test this; we cannot re-run our evolutionary past. And if we could go back in time, what would serve as the experimental control? So, albeit plausible and commensurate with the voiding in question, this remains another evolutionary 'Just So' story.

Some butterfly and moth larvae will incorporate their faeces or frass into their silken communal webs or cocoons. Caterpillar frass is by and large odour free and non-toxic or infective. But a mouthful of granular and partly digested plant matter is unlikely an attractive prospect. Quite apart from its unpalatability, the incoporated frass can help disguise the web's or cocoon's true nature.

Loud noises in nature often come with dust from shock waves etc. So it makes sense to close your eyes to protect yourself from this possibility. It is hard to flee with grit obscuring your vision if the cause of the sound requires it. To bear this out we don't just close our eyes we at least turn our heads away if not our whole bodies. The back being less vulnerable than the belly. This behaviour also has you facing away should you subsequently decide you need to flee so serves both purposes. Behaviours that fulfil more than one function would seem to be more likely to survive natural selection. Animals with clear vision fleeing away from a threat are simply more likely to survive.