I wonder what their jokes are (outside of tickling) it’s hard to imagine a rat punchline.
I doubt any of the laughs are provoked by verbal ‘jokes.’ But rats are very smart, and play (wrestle) around with each other. It’s probably not too different from kids giggling as they engage in rough and tumble play.
One of my rats loved to nibble pencil erasers. She would find all sorts of creative ways to get the eraser from me. With rats, who need stimulation, it's good to have arms wars. I don't put the eraser in an impossible spot, rather a harder-to-get-to spot.
She figured out that if she distracts me while doing homework, I'll turn my attention to the distraction. She could then run around from the other side to scoop up my eraser and take it somewhere to nibble. I began to catch her in the act and call her name as if I'm frustrated (but laughing) when she does this, like I do with my children today when they put a clothespin on my shirt then run off. One time, she picked up the eraser and ran off. But I was too busy to play along right away, I just let her scoot off. So she sat there, with the eraser, waiting for me to finish and then chase her down. Apparently she was no longer stealing the eraser to nibble, rather, it had become a joke between us.