What Does It Mean When a Dog’s Eyes are Fully Dilated

Were you playing with your dog and saw his pupils dilating? Did you wonder why that happens?

If your answer is yes, then you’re at the right place because, in this article, we will tell you everything a dog parent would want to know about.

Your dog’s eyes reflect what emotions he’s going through. For example, if he’s threatened or scared, you’ll probably see his pupils dilating. If he’s playing or hunting, the adrenaline will produce the same reaction in his pupils. Other reasons for your dog’s fully dilated eyes might include light changes or some medical conditions.

To know more about these points, keep reading, and if you stick to the end, you might stumble upon some interesting facts about your dog’s eyes.

Here are 4 reasons why your dog’s eyes are fully dilated:

1. Playing

Their hunting mode kicks in when your dog is in a playful mood. Whether chasing a toy or harassing other dogs, the predatory drive in dogs comes alive.

As you might already know how most dogs are instinctive hunters, the adrenaline responds to this hunting urge and creates physiological responses in dogs. One of these changes is pupil dilation.

This is how adrenaline changes the pupil size:

There are 2 types of stress in dogs, good stress and bad stress. Bad stress is when they feel threatened and fearful. Good stress is when they’re positively challenged, usually physically.

When dogs are playing, they’re under good stress. It’s very interesting how good stress affects a dog’s body. The adrenaline from all the good stress enhances a dog’s senses by regulating the brain and muscles with oxygen-rich blood. This helps your dog to quickly act, and that’s why your dog might look like ‘the flash’ while playing.

While all of this is happening, your dog’s pupils are dilating as well to allow more light in, improving his visual clarity.

You see your dog giddy and excited, but a lot is happening inside them to enable them to act this way. Good stress deserves all the credit for increasing a dog’s overall performance, from hearing even the faintest sounds to detecting even the slightest movements.

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2. Fear

If you’ve ever tried scaring anyone or were a victim of such a prank, then you probably know what reaction everyone waits for – Eyes wide, jaw drop, or maybe even a few screams.

Dogs are no different.

The link between fear and pupil dilation can be explained using a theory. When a dog gets scared, their fight or flight response is activated, owing to its survival instincts.  

A dog’s body helps them to tackle this situation by dilating its pupils.

Are you thinking:

But how would dilated pupils help in a fight or flight situation?

The dilated pupils allow more light in the eye, so your dog’s brain will process visual information better and allow your dog to determine what their next step should be.

A fight or flight situation is where the automatic dilation of a dog’s eyes is its most valuable possession.

According to Scientific American, “Stimulation of the autonomic nervous system’s sympathetic branch, known for triggering “fight or flight” responses when the body is under stress, induces pupil dilation. Whereas stimulation of the parasympathetic system, known for “rest and digest” functions, causes constriction.”

This also explains the first point in the article about how a dog’s pupils dilate not only when the dog’s scared but also when they’re excited.

3. Changes in brightness

Like us, dogs’ pupils dilate with a change in light. You might notice your dog’s pupils dilate in the dark or less light. This is because, in the dark, the eyes try to absorb more light by expanding the pupils. 

The bigger the pupil, the more room for light for the eyes.

Similarly, when your dog is in a bright place, like when you take him for a morning run, you might notice his pupils getting smaller. Sometimes you might only find a teeny tiny black dot in his eyes. This is because the eyes are already getting too much light, so the eyes block extra light by contracting the pupils.

If you’ve ever taken your dog for a checkup, you must’ve noticed the vet checking the dog’s eyes by shining a torch in their eyes. It might look like a stupid way of checking, but as they say, ‘trust your doctor’ because they know what they’re doing.

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The vet is actually checking whether your dog’s pupils constrict with light. This determines if your doggy has a healthy “pupillary response.”

4. Medical Reasons

Ever heard the saying, “The eyes are the mirrors of the soul?”

Well, it’s true because just by seeing the pupil size in a dog’s eye, we can determine whether something is wrong with the dog’s health.

We will mention some health conditions that might be the underlying reasons for the changes in a dog’s pupil size. Of course, we advise you to base only some of your assumptions on this article and go to a professional vet for a proper diagnosis.

Eye Conditions

1. Glaucoma

This is a disease where the pressure inside a dog’s eye is increased and therefore causes the pupil to dilate. So, if you see this sign, remember to get your dog’s eye pressure checked.


 Pupil dilation often is a sign of a tumor related to a dog’s brain, retina, or optic nerve.

3. Iris atrophy

If you own an elderly dog, pupil dilation could result from a condition where a muscle responsible for constructing the pupil fails to work. The pupil may look out of place or distorted.

4. Blindness

If enlarged pupils remain the same and the size doesn’t go back to normal, the dog might be at risk of blindness.

5 Interesting Facts About Your Dog’s Eyes

  • Doctors use the abbreviation “PERRLA” to identify healthy pupils. It means “Pupils, Equal, Round, Reactive to Light and Accommodate.”
  • Dogs are dichromats. That means they only see the world in red and green colors.
  • Dogs see better in the dark because of their history of being crepuscular hunters.
  • The blue eyes of Huskies are just an optical illusion. There’s no blue pigment in their eyes, just light entering and exiting so that the eyes look blue.
  • Research shows that dogs dedicate their left eyebrow movement to their owners. Their left eyebrow moves a little when they see their owners.
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Nadine Oraby

My name is Nadine; I am a passionate writer and a pet lover. People usually call me by the nickname “Joy” because they think that I am a positive and joyful person who is a child at heart. My love for animals triggered me to create this blog. Articles are written by vets, pet experts, and me. Thanks for visiting. Your friend, Nadine!

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