If you have a dog, you've probably stumbled onto a ticklish spot or two, and you've probably also gotten a lot of amusement out of scratching that spot and watching your dog kick his back leg. This seems to be about as universal among dog owners as that involuntary kicking reaction is among dogs. But what causes it? Is it just that your dog is ticklish, or is there something deeper going on?
All dogs have an ingrained scratch reflex. This is an involuntary reaction to something that causes irritation, like a bug bite, or a flea crawling through their fur, or being incessantly tickled on their tummy by mom or dad. When you scratch or tickle your dog in a sensitive spot, according to Animal Planet, it activates a nerve under the skin that relays a message along the spinal cord and tells the back legs to start scratching to make the irritant go away.
The scratch reflex is typically noted in an area called the saddle region. This area includes the belly, the back and both flanks. Which leg is activated by the scratch reflex, and how hard a dog kicks, depends on the individual dog, as well as on where in the saddle region you scratch and how hard.
The scratch reflex is so universal among dogs that veterinarians have found it useful in helping to diagnose neurological problems or nerve damage. It can serve much the same purpose as the test your doctor gives you when tapping your knee with a hammer to test your knee-jerk reflex.
While the scratch reflex is designed to deal with irritation, it doesn't necessarily mean you're irritating your dog when you scratch him in his sensitive spots. Generally, dogs like having their tummies rubbed and scratched, and will move or roll over if they are no longer enjoying it. If you're concerned that your dog doesn't like it when you make him kick his leg, just move to another spot that doesn't make him kick and be confident that your dog is getting as much enjoyment from his tummy rub as you are.