Ask A Vet: Why Does My Dog's Hair Suddenly Stand On End?

Have you ever been out on a walk with your dog when they pause for a moment and suddenly, the hair along their back shoots straight up like a tiny mohawk? As a veterinarian, I get asked about this all the time. This curious little reaction is called piloerection, and it involves minute muscles in the skin called arrector pili. You and I have them too—only when ours contract, we call it “getting goosebumps.”

Most people associate goosebumps with feeling cold or having an emotional moment, and dogs experience something similar. But there’s one important difference: while humans don’t have much control over goosebumps, dogs have none. Piloerection is entirely automatic. Your dog couldn’t stop it even if they wanted to. Their nervous system handles it behind the scenes.

Why Do Dogs Get Piloerection?

Piloerection can happen for several different reasons, and understanding the context can tell you a lot about what your dog is feeling in the moment.

1. To Stay Warm

Let’s start with the simplest explanation: temperature regulation. When a dog is cold, the arrector pili muscles contract and lift the hair to trap a layer of warm air next to the skin. This creates insulation—much like how we zip up a puffy winter coat. It’s clever biology at work.

I see this most often in short-haired breeds or dogs with thin body condition. If it’s a chilly morning and your dog’s hair stands up briefly, they may simply be trying to capture a little extra warmth. In those cases, a comfortable dog coat can make a big difference, especially for seniors or small dogs who lose heat quickly.

2. Emotional Arousal: Fear, Stress, or Excitement

Far more often, piloerection happens because your dog is emotionally charged—not necessarily aggressive, but definitely activated.

When something startles or concerns a dog, their nervous system releases adrenaline, which triggers the arrector pili muscles. This physical response makes the hair stand up, giving the dog a slightly larger silhouette. In the wild, appearing bigger could discourage a predator or rival. Even though your dog is more likely facing the neighbor cat than a wolf, their body doesn’t know the difference.

This emotional form of piloerection usually appears in one of a few patterns:

• Just over the shoulders – often mild alertness or interest
• Over the shoulders and down the spine – more intense emotional arousal
• Only near the tail – excitement or uncertainty
• From shoulders to tail in one continuous strip – high-stress or high-alert moment

Not all dogs express fear or excitement the same way, so raised hackles don’t automatically mean aggression. I know many wonderfully sweet dogs who get a vertical mohawk the moment something surprises them!

3. A Sign of Uncertainty or Overstimulation

Raised hackles can also indicate that a dog is simply feeling conflicted—curious but cautious, excited but unsure. For example:

• Meeting a new dog with unclear intentions
• Seeing a fast-moving animal like a squirrel
• Hearing a loud, unexpected noise
• Feeling overstimulated at a busy dog park

Think of it like their internal alarm system: “I don’t know what this is yet, but I’m paying close attention.”

What Should You Do When Your Dog’s Hackles Go Up?

Piloerection itself doesn’t hurt and isn’t dangerous, but it’s an important clue for you as their advocate.

Here are a few things I tell my clients:

✔ Give your dog space. A dog with raised hackles is emotionally heightened. Touching them or pulling them forward can add pressure and make them feel trapped.

✔ Don’t scold them. They aren’t choosing to do this—it’s a reflex. Correcting them can increase anxiety.

✔ Observe the rest of their body language. Hackles alone don’t tell the whole story. Check their ears, tail, mouth, and posture to determine whether they’re fearful, excited, or overstimulated.

✔ If needed, calmly leave the situation. If your dog is reacting to another dog, person, or environmental trigger, moving away allows adrenaline levels to drop and gives your dog time to reset.

✔ Practice positive reinforcement. When your dog recovers from a moment of arousal, reward calm behavior. This helps build confidence over time.

When Should You Be Concerned?

Most of the time, raised hackles are normal and harmless. However, if you notice:

• Frequent piloerection without an obvious trigger
• Accompanying signs of fear-based aggression
• Increasing reactivity on walks
• General anxiety or tension

…it might be worth talking to your veterinarian or a qualified behavior professional. Chronic stress isn’t healthy for dogs (or humans).

A Peek Into Your Dog’s Inner World

Piloerection may be just one small physical change, but it can open a window into your dog’s emotional state. Whether they’re cold, nervous, excited, or just processing something new, those little raised hairs are one of the many ways dogs communicate with us.

Dogs don’t have words, but they’re constantly “talking” with their bodies. When we understand these subtle cues, we can support them better, keep them safe, and strengthen our bond.

So the next time your dog’s hair stands on end, take a moment to observe the situation through their eyes. Their body may be whispering something important, and now you’ll know how to listen.