Why is your snake fainting?

A Youtube video of the death feint https://www.youtube.com/watch?v=9pm6hsIDXVc

The snake you are handling or are about to handle, suddenly goes limp and faints. It looks completely dead but it isn't. This is what Darwin called "the death feint." Or playing dead - though this is far from being a game for the snake.
The modern scientific term for it is "tonic immobility." Many animals including other reptiles and rabbits faint like this.
If an animal faces intense danger then an automatic reaction kicks in. There is a huge surge of adrenaline (epinephrine) and a reaction with what is commonly known as the five "Fs" - flee, fight, freeze, faint or fool about (ie do something odd which distracts the predator). Faced with being handled, a captive snake hasn't got the option to flee or to fight a large human. So fainting is one of the options left to it.
This death feigning reaction sometimes looks like a relaxed animal and ignorant rabbit owners think the rabbit is enjoying it.  Far from it. Rabbit owners are beginning to recognise this, as should snake owners. If your snake goes limp it is in utter terror.
Fainting is a last resort for an animal faced with a dangerous predator. With luck the predator will think the snake is dead and for a moment shift it's attention elsewhere. Then the snake has a chance to escape. This may work, for instance, with predators like the domestic cat.  If you watch a cat "playing" with a living mouse, when the mouse goes immobile the cat stops play for a little while. A cat's predatory drive is turned on by movement so sometimes the cat will poke the mouse to make it move so the play can continue. Cats that catch snakes often do a similar kind of playing around.
Patrick Gregory of Kent University tested the response of grass snakes to being captured. Snakes that were held by the tail rarely fainted when first grasped. Those held by the head were more likely to feign death. The larger the snake the more likely it was to remain immobile after being released. Perhaps because bigger snakes have less to fear about remaining still. The snakes that didn't faint would usually slither away as soon at they were released.
* More info at  Gregory, P. T., (2008),  'Bluffing and waiting: Handling Effects and Post-Release Immobility in a Death-Feigning Snake (Natrix natrix)', Ethology, 114, 768-774.

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