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Why Does Dog Hair Stand Up? Raised Hackles Or Piloerection In Dogs Explained

why does dog hair stand up

Raised hackles, or when the fur between the shoulders and along a dog’s spine stands up, is called piloerection and it is a very important part of how dogs communicate and regulate their body temperature. So why does dog hair stand up and what do we need to know about it?

Of course, whenever we talk about a dog’s coat, we need to mention the importance of keeping it brushed and clean with a dog wash for sensitive skin. Keeping them clean and brushed is about more than when they roll in dirt. Matted or dirty hair interferes with a dog’s ability to raise their hair to communicate or to warm themselves up, so never neglect their grooming needs.

If you are having aggression in your dog, and wondering if it’s related to nutrition, you should also see our article on if raw meat makes dogs aggressive. But let’s look at what behavior scientists have to say about this part of canine body language.

What makes dog hair stand up?

Piloerection, also known as “raised hackles,” refers to the phenomenon in which the hairs on a dog’s body stand up, making the dog’s fur look thicker and more rigid. This occurs due to the contraction of tiny muscles at the base of each hair follicle, called arrector pili muscles. 

Now, it’s important to remember that you won’t always see a dog’s hair raised. Some dog breeds have such thick, long hair that it’s too heavy for the tiny muscles to lift up. 

Piloerection that happens because the dog is in a state of arousal (excited, afraid, aggressive, etc.) happens because the dog’s sympathetic nervous system is activated. So it’s a part of their natural flight, fight, or freeze survival responses. The parasympathetic nervous system is the “rest and digest” system that helps mammals like dogs calm down. When this is activated, their hair should lay flat on their backs.

Piloerection can also be a response to cold temperatures. In this case, the contraction of the arrector pili muscles serves to trap air between the hairs, creating an insulating layer that helps to keep the dog warm.

Overall, piloerection is a natural and normal response for dogs that serves various functions, from communication to thermoregulation.

Are Raised Hackles Always A Sign of Aggression? 6 Reasons That Dog Hair Stands Up

Raised hackles are not always a sign of aggression in dogs. This phenomenon is caused by a variety of factors, including genetics, temperature, and emotional state. Let’s take a look at the many different reasons a dog may have raised fur.

1. The dog is cold

One factor that can cause a dog’s hair to stand up is the temperature. When a dog is cold, the muscles around the hair follicles contract to trap heat and keep the dog warm. This causes the hair to stand up, creating an insulating layer of air that helps keep the dog’s body heat in.

2. To communicate aggression

Of course, most people associate a dog’s hair standing up with aggression, but true aggression is actually quite rare in dogs. Nevertheless, aggression is the most common emotional state we link to raised hackles. 

Dr. Barbara Sherman, PhD, DVM, writes that “Piloerection is an autonomic response to

arousal that has become ritualized as part of a canine aggression display.” This means that raised hackles is an autonomic nervous reaction that has become part of how dogs display aggression to other dogs (or humans).

In truly aggressive dogs, you will also see:

  • A stiff straight or upright tail
  • A puffed up, tense body
  • Ears positioned forward, locked onto their trigger or target
  • A closed tense mouth, or snarling, growling, or barking

3. To show fear

Raised hair in dogs is also common in dogs that are extremely fearful, but still want to defend themselves. This is because the sympathetic nervous system is activated with all the same stress hormones as when they are aggressive, like adrenaline and cortisol. It is also a display meant to communicate that “I am afraid, so I will fight to defend myself.”

In a fearful dog with raised hackles, other signs you may see include:

  • Tail usually hanging lower (but is sometimes raised)
  • Growling, showing teeth, often excessive barking
  • The dog will try to withdraw or shrink back, often leaning on the owner if they are on a leash, or fighting to get off the leash
  • Ears will be pinned back

4. To communicate dominance or to challenge another dog

It happens quite often that when a dog meets another dog for the first time, they will raise their hackles and lift their tail in the air, often trying to stand over the other dog. This doesn’t always lead to a fight, but it often does. The display is meant to intimidate the other dog. 

In some cases, the other dog may try to defuse the situation by showing submissive signals like rolling on their back. But this kind of intimidating behavior is a sign that the dominant dog should be removed immediately before the situation escalates.

5. To show possession (warning others)

Dogs who are resource guarding their food, favorite toy, bed, or person will also raise their hackles to warn others to stay away from their “thing.” Raising their fur in resource guarding looks similar to when a fearful dog does, as both are symptoms of anxiety rather than real aggression.  The major difference is that the hair raising will only take place when they have the high-value object they are guarding. Like when a dog covers their food

6. Piloerection in seizures

It does happen that dogs have piloerection during some kinds of seizures due to neurological disorders. It can be a symptom of autonomic dysfunction in non-focal seizures. If you suspect your dog is having seizures because the hair raising seems to happen randomly and is accompanied by other sudden and abnormal behaviors, then see a vet immediately. 

Can dogs raise their hackles when they are playing or just excited?

Can dogs raise their hackles when they are playing or just excited?

It is possible that when dogs are playing or very excited, they may raise their hackles. This is more common in very intense play, such as a competitive game of tug. However, raised hair is linked to a dog’s sympathetic nervous system, so it happens when dogs get a sudden rush of adrenaline.

A calmly playing dog is a happy dog and will not raise their hackles. So if dogs are playing and one starts to raise their hackles, it may be a sign that something has shifted. What started off as play may now be a more serious situation where a dog feels challenged or threatened. Perhaps they want to take possession of a toy and no longer share it. Even if a dog does raise their hair out of pure excitement, it’s not good that they do this around other dogs since it’s seen as  an aggressive display amongst dogs. 

Frequently Asked Questions

What dog breeds naturally have hair standing up on their backs?

Thai Ridgebacks and Rhodesian Ridgebacks are two dog breeds that naturally have hair along their spine growing in the opposite direction of the rest of their fur. This causes a natural ridge of upright or upward facing hair on their backs. This is because they have a specific genetic mutation that causes their dorsal hair to grow in the opposite direction.

Why does my dog’s hair stand up while sleeping?

If your dog’s hair stands up while sleeping, they are likely having a bad dream of something that is scaring them or making them feel aggressive. The average dog has about 2.9 hours a day of rapid eye movement (REM) sleep, in which they dream like we do. It follows that they can have nightmares which can cause raised hackles

Dog fur standing up in patches?

Hair standing up in patches on the back usually means fear or aggression. Hair that seems to stand up in patches elsewhere on the body may be the result of the dog’s naturally wiry coat, where some long, stiff guard hairs protrude at certain places. In some cases, it could indicate a damaged coat.

Final Thoughts

Dogs raise their hair naturally to insulate themselves against the cold. However, it is also a response from their nervous system when they feel threatened, so it could indicate fear or aggression. Understanding your dog’s body language is key to avoiding accidents, as raised hackles is common warning from a dog that those around them should back off.


Tamsin De La Harpe


Tamsin de la Harpe has nearly two decades of experience with dogs in rescue, training, and behavior modification with fearful and aggressive dogs. She has worked closely with veterinarians and various kennels, building up extensive medical knowledge and an understanding of canine health and physiology. She also spent two years in the animal sciences as a canine nutrition researcher, focusing on longevity and holistic healthcare for our four-legged companions.

Tamsin currently keeps a busy homestead with an assortment of rescue dogs and three Bullmastiffs.

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