Pancaking is usually accompanied by uncontrolled drooling... and by digging behavior. It may seem bazaar that a rat that's overheated starts digging into his or her litter, where they will only get hotter faster, but that's what rats will do in hopes of reaching someplace cool.... I'm thinking it's a survival mechanism that takes over after their rational brain is already shutting down.
Shoulder rat handlers should always keep a spray bottle of water along with them in summer. Drooling, drooling,pancaking and digging comes on very quickly when outside temps go over 83 degrees F. One minute your rat will be frolicking around in the grass, the next it will pancake flat and stop moving... IMMEDIATELY, move your rat into the shade, even better if you can find a breeze and soak it. Or make a fast dash for A/C. As soon as your rat cools down, it will recover and begin to act normally, if you didn't wait too long.
You won't necessarily notice the drooling unless you pick your rat up, rats don't stick their tongues out like dogs.... but their mouth, neck and belly will be wet.
Dark rats are affected more quickly and more severely than white rats in direct sunlight.
As to what it looks like... it really does look like your rat has gone flat or deflated... it's hard to describe, but you will know it when you see it, if you know what you are looking for.
Also beware that some rats like to ride in cars under your seats. You might have your windows open and you feel cool, but under the seats it can get very hot. And rats dig down when they overheat. You may feel fine, but your rat can be cooking beneath you and trying to dig through your carpeting instead of coming up for cool.
For all of the wonderful design features that brown rats have, they absolutely can't deal with high temperatures. In nature they burrow into the ground and stay there until it cools off. If you live in an area where it gets over 83 degrees make sure you have A/C or a cool basement or consider choosing a black rat (R. rattus) as a pet. I don't know how much heat they can tolerate, but they are generally known to be more heat tolerant and are common in tropical areas.
Perhaps a black rat owner or someone who lives in a tropical area with wild R. rattus nearby can add something from their personal experience.
Zeeky does this too. He likes to lay flat on the carpeted cat trees we have and on the couch were we have a soft blanket like that. When he moves he'll drag his belly along the surface. I assume he just likes the feel of the carpet/blanket on his belly. (or his junk )
I suspected as much, but... This is the time of year I always post my heat warning and I was just too lazy to start a new thread today.
Several years ago, I got blindsided when we took Fuzzy Rat to the 4th of July handicapped picnic for the first time, it was nearly 100 degrees outside and Fuzzy Rat was very busy entertaining the handicapped children that just couldn't get enough of her... First she was all wet underneath (from drool) and then suddenly she went flat like a pancake... We rushed her home and into A/C and she recovered in time for the fireworks. If some people don't recognize the dangers of heat, things can and will go badly fast.... I mean Fuzzy Rat just dropped from a full gait into a puddle of furry mush with no warning except the drool and we didn't recognize the symptom until it was nearly too late.
Oddly enough I used to have a couple of cuddle bug girls that loved to get real comfy like this. Funny that my boys couldn't be bothered to sit still long enough for more than a few kisses. Gender stereotype swap for me I guess! Cute guy though