Bangor Daily News Outdoors contributor Aislinn Sarnacki's dog Juno trots along a trail in Acadia National Park on Sept. 4, on Mount Desert Island. (Courtesy of Aislinn Sarnacki)

I had a scary experience a few days ago. I was hiking up a mountain in Acadia National Park when my dog Juno started to act strange.

Juno, a boxer-husky mix, accompanies me on many of my hikes. When she was a puppy, I introduced her to the activity with short, easy trails. Now nearly 3 years old, Juno is quite the climber. Just last winter, she hiked to Cadillac Mountain with me, a 7 1/2-mile trek through snow. And she wanted to play fetch afterward.

Usually, she leads the way, often tugging on her leash in an effort to go faster. But during our recent hike, as we traversed an open ridge of Penobscot Mountain in the sun, she began walking beside me, then lagging behind.

She seemed distracted, wandering over to bushes and boulders. Juno likes to sniff things, as all dogs do, but this behavior seemed different. I watched her closely and realized she was actually seeking out shade.

The day was humid and hazy. The temperature was climbing toward 80 degrees, and we were walking over open bedrock in the dazzling sun. Juno was overheating.

I stopped near a large boulder, and she immediately laid down in its shadow. Right then, I decided that we needed to turn around. The mountain wasn’t going anywhere. We could hike it again, on a cooler day.

Juno rests in the shadows. (Courtesy of Aislinn Sarnacki)

I sat with her a while, just letting her rest. I poured her some water, which she drank. I also poured some over her head. She didn’t appreciate that.

The day was only getting hotter, and we were out in the open (aside from the boulder’s shadow). So when her panting calmed down a bit, I encouraged her to follow me down the mountain. As we descended, I watched her closely and repeated encouraging words — though I’m not sure who I was reassuring more, her or me.

In a wilderness first aid course, I’d learned all about heat exhaustion and heat stroke. While spending time outdoors, these conditions can sneak up on you. And if untreated, they can be fatal.

The frustrating thing about dogs is that they can’t tell you how they’re feeling. You have to suss it out by watching their behavior. And to do this, you need to know how they usually act.

Juno, for example, always leads the way when hiking. She also rarely lies down during breaks. And I know what “normal” panting looks like for her.

Once we reached a forested area with an abundance of shade, I stopped and she immediately laid down, then rolled onto her side. Her long tongue lolled out of the side of her mouth. I tried to hide my alarm as I sat down beside her.

I got her to drink more water. I considered taking off my shirt, drenching it in water and holding it against her less hairy areas (armpits, belly, etc.). But then she sat up and started to look a bit brighter.

As an experiment, I unearthed a bag of crackers from my pack and started snacking on them. She seemed interested, so I offered her one. She ate it. That reassured me that she wasn’t too distressed. I fed her a few more, then we continued our slow descent.

Fortunately, most of the remainder of the hike was in the shade. She was hiking ahead of me again, pulling on the leash from time to time.

We completed our truncated hike where we began, at the shore of Jordan Pond. The pond, one of the most scenic spots in Acadia, is surrounded by dramatic granite mountains — Penobscot, Pemetic and The Bubbles.

Swimming isn’t allowed, so I led Juno down a short side trail to Jordan Stream, where she could splash around.

When we reached the car, I cranked up the AC and we drove straight home. No more exercise for the day.

The experience taught me Juno’s limit for heat. With a dense undercoat, she’s never seemed to enjoy hot days. She thrives in the winter, in the snow. But I’d never seen her react to heat quite like that.

Our route that day, up and over Penobscot Mountain, was selected specifically for her. Looking at the contour lines of a trail map, I selected the most gradual trails. Steep trails can be difficult, if not impossible, for dogs to traverse, especially if they include big steps, ladders or rungs.

Juno stands at an overlook while hiking Penobscot Mountain in Acadia National Park on Sept. 4, on Mount Desert Island. (Courtesy of Aislinn Sarnacki)

The Acadia National Park website actually has a list of trails that are off limits to dogs, as well as trails that are not suggested for dogs. The park also requires dogs to be on leash (no longer than six feet). I always follow those guidelines.

Dogs can make the best outdoor companions, but it’s important to pay close attention to their behavior so you can gauge whether they’re comfortable. Each dog is different in terms of what they can handle. Their breed, age and fitness level are all factors.

My former dog, Oreo, would have been fine to hike on that sunny September day. He didn’t have much hair, so could tolerate the heat. But he would easily become cold during winter treks. So I’d bundle him up in fleece coats and watch for shivering or whining. And on especially cold days, I left him indoors.

It’s a challenge to know your dog’s limits, but it’s crucial if you want them to join you on your outdoor adventures. I want hiking to enrich Juno’s life, not cause injury or distress. Fortunately, colder weather is on the horizon.

Aislinn Sarnacki is a Maine outdoors writer and the author of three Maine hiking guidebooks including “Family Friendly Hikes in Maine.” Find her on Twitter and Facebook @1minhikegirl. You can also...