I know that shivering, in mammals, causes the musculature to create heat, and so is a useful defense against exposure to cold.
But reptiles are cold-blooded. As far as I know, they don’t shiver when they’re cold, they just move more slowly.
Does this mean that reptiles also don’t shiver/shake when they’re nervous or threatened, like mammals do?
We have pet Russian Desert Tortoise. When he is nervous, he kind of twitches his legs. Not nearly as rapid as shivering, but a similar motion.
I have a bearded dragon. I’ve never seen her shake. When she’s nervous, her color darkens and she stays stock-still.
You might be able to make the case that a rattlesnake is nervous when he’s rattling to warn someone/something away. 
In Russia, desert tortoise shake…oh, never mind.
Hmmm … interesting.
I’m assuming (due to my utter lack of training in macrobiology*) that the shaking a mammal does when it’s nervous or scared is a means of keeping the muscles at the ready, so that it can leap into action the instant it sees or hears something move. It would make sense that reptiles would do the same thing, even if shaking in a reptile muscle doesn’t produce heat.
Which leads to the next obvious question: Does shaking/shivering produce heat in a reptile’s muscles? I’d guess that reptilian muscle cells aren’t too different from mammalian muscle cells, and if the same enzymatic reactions are used in both kinds of muscles to produce motion, they’d logically create the same amount of heat.
*) Which I have so little training in that I just made up the word right now!
The tail of some snakes does shake when they are threatened. Obviously, a rattlesnake does this, but I was surprised to find a Rat snake doing it. I’d guess that a great many snakes do that.
We used to have Leopard Geckos, and their tails would shake when they were about to strike at (and eat) a cricket.
I’ve worked with a few different lizard and other reptile species, I’ve only noticed them run away and freeze when scared. As most lizards can’t run long distances without a break (as they can’t breathe and run at the same time), there’s not so much point in keeping warmed up for a long sprint, even if shivering did help their muscles stay warm. They’re probably all better off staying still and hoping they get passed over, if shelter is out of running distance.
Monitor lizards can run and breathe, but I never noticed ours shivering at all.
There’s not going to be any point warming up for a long run if you’re a tortoise either…
I did notice pythons ‘vibrating’ on occasion, but it didn’t appear to be a stress/nervous response, as they were just lying out in the open apparently relaxing, and not showing any other signs of stress. Never figured out what that was about, they didn’t do it often.