With the multitude of golden retrievers, Labs, Bernese Mountain dogs, and various types of doodles you see strutting around Aspen, you may think you’ve got this one figured out. But it’s not so clear-cut, after all.

“It depends on your purpose,” says Seth Sachson, director of the Aspen Animal Shelter. “A Bernese is a great dog, but they’re not incredibly agile and athletic and don’t have endurance. So it might be a good dog if you want to walk around town and meet girls or to après-ski with you and hang out at The Little Nell.”

As it turns out, one of the best dogs you can have for hiking, biking, and running in the high country is—apologies, designer breeds—a medium-size mutt.

“Going up and down mountains, and all that force on elbows, is hard on big dogs,” says veterinarian Anne Cooley of the Aspen Animal Hospital. “We see a lot of arthritis on bigger dogs, and huge, bulky breeds are predisposed to joint problems.”

A 40- to 50-pound canine, on the other hand, has a lighter bone structure. “A lot of our shelter mixes are great mountain dogs,” says Cooley. Other top picks include Aussie shepherds, collies, and Lab and retriever mixes. Even some active smaller breeds, like Jack Russells and border terriers, can be good hikers.

It all comes down to what you expect of a canine companion. “Think about how you want to spend time with your dog,” Cooley advises. “Pick the dog for what you want to be doing.”

“If you’re an active person looking for an athletic companion in the mountains,” advises Sachson, “pick a dog with speed and endurance.” He cites German short-hair pointer mixes as a perfect example.

With eight dogs of his own—shelter pups he’s taken in over the years—Sachson’s idea of the ultimate one changes daily. “Whatever activity I’m doing—hiking up the mountain or skinning up and skiing down—I select from my dogs,” he says, joking that he’s like a car aficionado with a fleet of rigs for different kinds of driving.

And if you really just want a Chihuahua to tote around in your bag? “Then that’s your perfect mountain dog for taking to lunch,” says Sachson.