It's one of those curious behaviors dogs do: you give them a nice belly rub and they start kicking or shaking their rear leg, what gives? Many dog owners have been wondering what is going on when their dogs start moving their leg upon being scratched. While the possible reason why this happens has been extensively studied for many years, despite decades of research, certain key aspects of this leg-kicking behavior still remain unknown. Today we're discovering what's likely behind this mysterious behavior, but before we do that, here's our trivia question of the day!
Why do dogs start kicking their legs when you pet them?
A: It's a way for the dog to push you away so to make you stop
B: It's an involuntary reflex that activates when you pet certain areas
C: It's a way for the dog to exercise his rear leg muscles
D: It's a voluntary action. The dog is trying to take over the task of scratching.
The correct answer is: drum roll please........
The correct answer is: B, it's a reflex that is activated when you pet certain areas.
The Dog's Scratch Reflex
So why do dogs pump their legs when you rub their tummies? Turns out it is a reflex, basically an involuntary mechanism that occurs as a response to a stimulus, and thus, happens without conscious thought. In layman terms, your dog can't control it, he just gotta do it. One of the first people to be intrigued by this reflex was a certain Sir Charles Sherrington, an English neurophysiologist who decided to study this reflex more closely.
Sherrington was very interested in anything that involved reflexes and neurological activity and his many studies helped us attain a better understanding of the central nervous system. Thank you, Sir Sherrington!
Location, Location, Location
One main question to ask is why does this reflex activate only when the dog is scratched over a certain area?
Those back legs after all do not seem to give any signs of moving or kicking when the dog is scratched on the top of the head or in the chest area!
All the surface of the dog's skin is covered by many sensory neurons that are activated by stimulation, but there is a specific area in particular that seems to activate the scratch reflex.
According to J. E. R. Staddon, author of the book "Adaptive Behavior and Learning,"this specific saddle-shaped area is called the"receptive field of the reflex" and touching a spot within this area may therefore elicit the dog's involuntary scratch reflex.
Why Just There?
Why is only this area triggering this behavior compared to other areas of the body? Here's one possible explanation.
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Do Dogs Fall Off Cliffs?
Yes, dogs fall off cliffs and these accidents aren't even uncommon. As we hike with our dogs, we may sometimes overestimate our dog's senses. We may take for granted that dogs naturally know what areas to avoid to prevent falls. However, the number of dogs who fall off from cliffs each year, proves to us that it makes perfect sense to protect them from a potentially life threatening fall.
The scratch reflex may have an adaptive purpose of ridding the animal from fleas or other dangerous parasites in an area (the flanks) that isn't highly mobile as say, the tail or the dog's front or hind legs.
The scratch reflex therefore may allow the dog to quickly attempt to remove the stimulus by using his rear leg.
Anther theory comes from the book "Why Do Dogs Drink Out of the Toilet?" by Dr. Marty Becker and Gina Spadafori. According to the book, the upper part of the dog's flanks or on the belly are areas where fleas tend to congregate, so those leg kicks are likely an effective means for pest control. Mother Nature's version of an automatic fly swatter.
A Matter of Timing
As seen, the scratch reflex is one of those fascinating things dogs do that are quite interesting to discover! Why though would it be a reflex and not a conscious, voluntary movement? Dr. Lore Haug, a board-certified veterinary behaviorists thinks it's a matter of urgency.
In an article for Popular Science, she compares it to our reflex of withdrawing our hand from a hot stove.
" Let’s say you touch a hot stove, and before your brain recognizes it’s painful, the spinal cord recognizes the pain, and you involuntarily jerk your hand back. If you had to wait until your conscious brain recognized something was in danger, your delay in reaction time could cause an injury or even death in some cases.” ~Dr. Lore Haug
Some Interesting Findings
Sherrington's studies, have revealed some quite interesting findings about the scratch reflex. Following are some interesting phenomena he has observed in his studies.
The Latency Period. When you touch a spot in the dog's receptive field of reflex, you may notice how the leg kicking action may not start immediately. Why is that?
Even though the reflex might be faster than voluntary movement, there's still a small delay.
According to Sherrington's studies, this slight delay tends to happen because it may take a teeny bit of time for the nervous signal to travel from the superficial skin receptors all the way to the muscles of the dog's leg responsible for start the leg-kicking action. Sherrington decided to call this brief delay "latency."
The Warm Up. Sherrington also noticed how the scratch reflex in dogs involves also a warm-up period. He mentions that when the leg kicking action begins, it doesn't reach a maximum level of intensity right away, but rather, it builds up in intensity gradually. The first leg kicks therefore consist of slight sweeping motions compared to the broader ones occurring a bit later.
The After Discharge. Just as there is a latency period, a brief delay before the kicking-leg action takes place, there is an after discharge moment, where the leg licking still occurs despite you stop scratching your dog. Basically, upon removing your hand, you may noticed that your dog keeps moving a leg for a little bit.
Fatigue. Reflexes may be prone to a phenomenon known as fatigue. If a stimulus applied by the dog's flank is not removed, after a few leg-kicking movements, the behavior slowly dies off until the dog stops moving the leg. This is sign that something in the system is clearly fatigued and that it can take a bit of recovery time to elicit the leg moving action again. This can occur because the leg muscles involved in leg -kicking action are simply exhausted or the sense organs are tired out and they no longer detect the stimulus, suggests Peter J.B. Slater in the book "Essentials of Animal Behaviour."
Intrigued by all this? Watch the Scratch Reflex in Action! Did you notice the warm-up period?
- Essentials of Animal Behaviour, By Peter J. B. Slater, Cambridge University Press; 1 edition (May 28, 1999)
- Why Do Dogs Drink Out of the Toilet? By Marty Becker, D.V.M., Gina Spadafori, HCI (September 15, 2006)
- Adaptive Behavior and LearningHardcover– November 25, 1983 by J. E. R. Staddon, Cambridge University Press (November 25, 1983)
- The Integrative Action of the Nervous System, By Sir Charles Scott Sherrington, Andesite Press (August 8, 2015)
- Popular Science, Why Does My Dog Scratch When I Scratch his Belly, by Loren Grush, retrieved from the web on May 10th, 2016