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Some dogs aren't happy with just some small random leg movements and will actually start kicking their leg when you scratch their belly. It's almost as if dogs had a special "sweet spot" that activates when you scratch them on certain parts only. 

The spot tends to vary from one dog to another, but one thing is for sure, that dog kicking behavior when pet sure yields many chuckles from dog owners. So why do dogs kick or shake their legs when you pet them? Here are some interesting findings!

A Matter of Reflex

Turns out, that dog scratching motion is the result of an involuntary reflex, better known as the "scratch reflex." Consider that dogs have several sensory neurons along their skin that activate by stimulation, whether it's your hand touching that sweet spot, an irritant or some pesky parasite crawling between his hairs. 

One of the earliest persons to study this reflex was Sir Charles Sherrington, an English neurophysiologist who was intrigued by this reflex. And he sure had a good reason!

The integrative action of the nervous system, by Sherrington, Charles Scott, Sir,

The integrative action of the nervous system, by Sherrington, Charles Scott, Sir, 1906.

A Receptive Field

The dog scratch reflex is typically not elicited by scratching a dog on the top of his head or in the chest area. Rather, there appears to be a certain region of the dog's body that is more likely to elicit this reflexive response.

 According to J. E. R. Staddon, author of the book "Adaptive Behavior and Learning" there is a specific region of the skin known as the "receptive field of the reflex" within which a stimulus can elicit a response. 

The scratch reflex in dogs can therefore be elicited by touch anywhere within this saddle-shaped area, shown in the picture above. 

The integrative action of the nervous system, by Sherrington, Charles Scott, Sir, 1906.

The integrative action of the nervous system, by Sherrington, Charles Scott, Sir, 1906.

A Short Latency

According to Sherrington's studies, the slight delay of response between the moment you touch your dog's "sweet spot" and the time your dog starts kicking, occurs because it takes time for the nervous signal to travel all the way from the skin receptors to the muscles involved in the scratching action. 

Sherrington referred to this delay as "latency."

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The Warm Up

Interestingly, after the dog starts scratching, Sherrington also found that there is a special warm-up period, during which the behavior of scratching doesn't reach maximum levels of intensity right away. Rather, the first strokes appear to have slighter sweeps compared to the ones occurring later.

The After Discharge

The same short delay as seen in the latency period can also be seen the moment you stop scratching. You'll likely notice it takes a bit for the dog to remove his paw after you stop scratching. Sherrington referred to this phenomenon as "the after discharge."

Post-Stimulus Fatigue

Touching the dog's side with something that cannot be removed by scratching will cause the dog to initially scratch, and after a period of warm up, the behavior will slowly die away and the dog will stop kicking his leg despite the fact the stimulus is still there. 

This denotes that something in the dog's system is "fatigued " and requires some time to recover before the dog starts scratching again, explains Peter James Bramwell SlaterEmeritus Professor School of Biology and author of the book "Essentials of Animal Behavior."

The dog's scratch reflex most likely helped dogs get rid of pesky parasites and bugs

The dog's scratch reflex most likely helped dogs get rid of pesky parasites and bugs

Desired or Dreaded?

One may assume that dogs may feel irritated by the scratching when they move their legs, but many dog owners claim that their dogs are actually enjoying the interaction. So is being scratched in those "sweet spots" something dogs dread or something that dogs desire? 

There's really no clear cut answer to this. Each dog is an individual when it comes to being pet, so it's up to the individual dog to decide if something is pleasurable or not.

When it comes to petting dogs, it's important to remember that, unlike us, animals can't say "thank you very much, now please stop", says Dr. Bonnie Beaver, veterinarian and Professor, Texas A&M University Veterinary Medicine & Biomedical Sciences.

 It's therefore a good idea to pay attention to the animal’s body language, –does he appear relaxed or tense? Listen to your dog. If you stop and your dog starts asking for more, most likely your dog loves having his belly rubbed.

Did you know? Veterinarians may scratch or tickle the margin of the dog's ear and observe whether the dog extends his hind leg to scratch as an aid to diagnosing dogs for canine scabies, explains veterinary dermatologist Valerie A. Fadok. This reflex is called the "pinnal-pedal reflex."

According to a study, the pinnal-pedal reflex is positive in 73% to 90% of affected dogs.

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