Josi, part Siamese, part Stegosaurus?

My almost-grown part-Simmy, Josiah, is one of the most playful
cats I've ever known. He's a white furry ball of energy, leaping
into the air, dashing in a blur across the carpet, chasing my feet
under the covers whenever I move in bed... My sister lovingly calls
him "our demon spawn." She brought to my attention a strange
characteristic about Josi: when he's playful (which is almost all
the time he's awake and/or not eating), a thin ridge of fur down his
spine stands at attention. He's not frightened or anything, he's
just being a "demon spawn", crazy with energy and play, but I'm
curious to know if anyone's ever seen this Stegosaurus trait in
other cats. Thanks.

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