Why is it when I scratch my cat on the very end of his spine, where the tail connects, he seems to enter throes of cat ecstasy? And why does it only seem to work with male cats?
-- Curiosity and the Cat, the net
This one sent the elves out to test your theories in the field. Luckily they turned in their domestic-animal data before they fell into the fish tank. Grandma Alice is drying them off now, and preliminary results indicate that guppies don't respond. We had much better luck with cats and dogs, all of whom reacted more or less ecstatically, then followed the elves home and are now all piled on the back porch waiting for them to reappear. Breed and gender didn't seem to figure into it, in our experience.
The sacral region of cats and dogs (like humans too) is thick with sensory and motor nerves, all attached to various anatomical fun zones. If you get a good buzz going by stimulating the peripheral nerves, most dogs and cats will respond as you might expect. The main nerve pathways are well buried, so some animals might not react as enthusiastically. It's also vulnerable anatomy, so there's sometimes an element of danger mixed in, which accounts for the heightened alertness. And don't confuse this with an animal's scratch reflex -- that goofy thing your dog does with his back foot when you tickle his side. That's an involuntary response controlled by the spinal cord meant to help rid the skin of insects and irritants. Your dog knows you're already scratching him (creating the "itch" and getting rid of it too), but he can't stop waving that leg anyway.