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If you’ve ever been scratching a dog and seen them do the kicky leg thing, it’s truly adorable. When this happens, we usually assume that we’ve hit some sort of magic scratching spot.

But turns out, this isn’t necessarily a feel-good thing. It’s actually an unconscious reflex, like when someone taps your knee with a hammer, and you straighten your leg. And knowing more about why it exists can help us keep dogs healthy, and also teach us a thing or two about evolution.

Now, this reflex works because when a dog gets scratched, the signal travels from sensors in their skin to their spine. But from there, the signal doesn’t just pass information along to the brain like usual. Instead, a network of nerve cells in the spinal cord itself reacts, and it generates signals to the muscles without the brain’s involvement.

That means your dog isn’t choosing to kick their leg all over the place. It just kind of… happens. That doesn’t necessarily mean they don’t enjoy it, but that might depend on the dog.

So if they don’t seem to like it, probably give them a break. Regardless, the key thing here is that not just any scritches will trigger this reflex. Pricking, scratching, and tickling all seem to work, but deeper, stronger touches don’t.

That may be because the nerves that seem to drive all of this are only a few millimeters from the surface of the skin, and found near hairs. It may also be related to why this reflex exists. See, some experts have suggested it evolved as kind of an anti-parasite defense.

The scratching motion might help dislodge or shake off a tick or flea, or just stop it from biting. And these little parasites wouldn’t feel like deep, strong touches. They’d be prickly and light, just like the type of scritches that trigger this reflex.

And there is some evidence to back up this line of thinking, as well. Like, dogs do the scratch reflex more easily when they’re already itchy! Dogs aren’t the only animals to have this response, either.

It turns out cats may have something similar, as well as frogs and turtles. And besides being fascinating, this actually comes in handy for vets! Today, veterinarians can use a type of scratch reflex to help diagnose scabies and mange in dogs, two super itchy, parasitic skin conditions.

The reflex they use is called the pinnal-pedal scratch reflex and involves rubbing the ears rather than the belly or back. But the same kicky leg thing happens sometimes. And it happens more easily in dogs with scabies.

I mean, it’s not a definite diagnosis, and these conditions are more definitely diagnosed with things like skin scrapings. But it can be a good first test to get a hint as to what’s going on. And testing reflexes, including scratching reflexes, can help vets diagnose nervous system problems, as well.

So, in the end, that scratch reflex may have more to do with evolution and defense than overwhelming excitement. But, it’s still pretty adorable. Thank you for watching this episode of SciShow!

And thanks to our patron Ms. Brock for asking us about this! If you have a science question for us, we’d love to hear it.

You can leave it in the comments below, or if you’re a patron, you can guarantee we’ll see your question by leaving it in our QQ inbox at Patreon.com/SciShow. [♪ OUTRO].