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A Lesson in Anatomy

Pupil size changes based on lighting conditions

Pupil size changes based on lighting conditions

Your dog's pupils are those dark, black circles that you see in the middle of the colored portion of your dog's eyes (the iris). Interestingly though, your dog's pupils are not really black. In reality that's just an optical illusion!

In reality, your dog's pupils are holes that appear black simply because light rays which enter them end up being absorbed by the tissues inside the eye. 

What are pupils needed for? The primary function of your dog's pupils is to control the amount of light that enters your dog's eye. In low light conditions, your dog's pupils get bigger (dilate) so to let in more light, while in bright light conditions, your dog's pupils get smaller (constrict) so to let in less light.

The technical term for when a dog's pupils constrict is "miosis," whereas, the technical term for when the pupils dilate is "mydriasis".  The physiological response of the pupils dilating or constricting is known as "pupillary response."

A Matter of Emotions 

Emotions such as fear or surprise are known to cause dilated pupils in dogs

Emotions such as fear or surprise are known to cause dilated pupils in dogs

While your dog's pupils dilate and constrict based on whether it's dark or there is plenty of light, sometimes they may also dilate without any changes in light conditions. For example, dog pupils are also known to dilate in response to emotions. 

For example, a dog's eye pupils may dilate in response to fear. There is belief that this happens as part of a survival tactic known as the dog's fight and flight response. The goal in this case may be to allow more light to enter the eyes so that the dog's brain can process information quicker.

 Taking in more visual information indeed can come handy at a time when the dog's life may be in peril and every second counts. This is something that occurs at a physiological level and isn't under a dog's control. In other words, dogs cannot dilate or constrict their pupils when they want to and on command, it's more of an automatic, physiological response.

According to Scientific American, the autonomic nervous system is responsible for the fight and flight response known to trigger the pupils to dilate when under stress, whereas the parasympathetic system, responsible for "rest and digest" functions, causes the pupils to return to their normal size once the threat is over.  

On top of the pupils dilating from fear, many dog owners witness their dog's pupils dilating as well when other emotions are at play and when the dog is playing. So the next question is: Why do dog pupils get big when playing?

Why Do Dog Pupils Get Big When Playing?

Notice the size of the pupils in this dog

Notice the size of the pupils in this dog

When dogs are put into the position of chasing a toy and playing, they tend to go into hunting mode. Their predatory drive is turned on and a surge in adrenaline takes place, although in this case, the adrenaline is there not to help save the dog's life, but to increase his chances of catching dinner. 

In such a case, more than the negative stress known as "distress", the adrenaline surge is associated with a form of positive stress,  known as eustress. Both terms were coined by Hungarian endocrinologist Hans Seyle. The Greek prefix "eu" in eustress means good, so it ultimately means "good stress."

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Compared to the negative type of stress experienced upon feeling threatened, eustress is also known for causing several physiological changes causing similar physiological effects. 

With the adrenaline pumping, more oxygen-rich blood reaches the brain and is sent  to the muscles of the dog's legs so that he can quickly spring into action. At the same time, the pupils dilate to let in more light and allow better visual clarity. 

The eustress, in the case of dogs who are playing, aims to increase the acuity of the dog's senses, and overall performance, so that their eyes can capture the slightest movements and their ears are ready to detect the faintest sounds as their bodies quiver in anticipation.

Playing Versus Hunting 

Dog play adopts many moves and behaviors seen in hunting

Dog play adopts many moves and behaviors seen in hunting

While playing is not really hunting, the physiological response remains the same. Your dog sees that stuffed animal tossed in the air as prey to catch, and in order to be successful, he must use his body effectively along with his senses.

For sake of comparison, something similar may occur when humans are playing video games. Although, you aren't really chasing a car, the feelings experienced are very similar to the real thing. "The excitement from video games can trigger an adrenaline response," explains Dr. Carlo Carandang a psychiatrist, anxiety and depression expert.

 This adrenaline response causes your heart rate and blood pressure to increase, your pupils to dilate, your sweat glands to perspire, your muscles to tense up, and your gastrointestinal system to slow down, almost as if you were really dealing with something occurring in real life.

Dogs may therefore be playing with a bouncing ball, but their bodies and minds are acting as if they were really getting ready to pound on a rabbit. 

Eye Use in Playing Dogs

Many people are surprised when they learn that dogs upon tracking prey often use their eyes and ears first, and then finally, as needed, their strongest sense: their sense of smell, explains Clarissa von Reinhardt in the book "Chase, Managing Your Dog's Predatory Instincts."

"The reason for this is that dogs always try to get to their prey using the method that requires the least amount of work because, in the wild, it is necessary to utilize their energy in the most efficient way possible," she explains. 

Spotting prey using the eyes is the least strenuous thing to do and using the ears to listen closely for any rustling sounds caused by an animal walking on dry leaves, does not require any major effort.

Sniffing, on the other hand, requires the dog to concentrate (it's quite a tiring activity!) especially if the dog must carry the head to the ground sniffing and following a scent that is a few hours or days leading through terrain that is challenging to navigate through.

Now That You Know...

As seen, pupils enlarging when dogs play is a normal, physiological response. Now that you know why dog pupils get bigger when playing, here are some things to keep into consideration. 

  • As with all things in life, consider that too much of a good thing can lead to harm. In humans, excessive playing of video games has been associated with negative stress due to the adrenaline response being chronically activated.  Something similar may happen to dogs when they are taken every single day to day care to play with other dogs with little downtime from one day to another. This can lead to chronic stress and being stuck in a state of hyperarousal  in the long term. It helps in such cases to cut down on overly arousing activities replacing them with calmer activities and mental stimulation.
  • Consider that dilated eyes in a dog when not playing can be indicative of a health problem. For example, head trauma, the ingestion of toxins ingestion, use of cheap flea products known to cause side effects, nerve inflammation, brain lesions and retina problems can all cause dilated pupils, explains veterinarian Dr. Scott. If your dog has dilated eyes for a prolonged period of time or out of the playing context without a plausible explanation, please have your dog see the vet at your earliest convenience. 

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