Dean Potter is no stranger to going big.

He climbs free solo – without ropes. He’s set speed records climbing Yosemite’s El Capitan. He strolls across slacklines strung thousands of feet up without a safety harness. He flies off mountains in a custom-made wingsuit. He combines the most perilous activities in the world, such as free soloing Switzerland’s famed Eiger, wearing only his parachute for safety.

So when it comes to the timeless motto of dog-parenting – “Never leave your hound behind” – it’s no surprise that Potter has taken that to the next level.

Whisper, his 5-year-old, 22-pound miniature Australian cattle dog, rides in a customized pack on Potter’s back as he reaches 120 mph in his wingsuit. Potter calls it “the ultimate dog walk.”

Potter admits he is addicted to overly-adrenalized pursuits, which he calls “the world’s most dangerous arts.” His craft is remarkably deadly. Adding his best friend to the mix rattled the 42-year-old adventurer.

“Something almost made me crumble when I started putting my dog on my back,” he says in his first film, “When Dogs Fly.”

Potter is a sort of mythological figure in extreme sports. He travels between Europe and Southern California, climbing and piloting his wingsuit. He’s the guy whose videos go viral – quick clips of him on a slackline or sheer cliff that spike heart rates of desktop adventurers across the world. (Potter’s feats prodded Clif Bar to drop him – and a few other free-soloing climbers – from its athlete roster, saying the controversial decision “came down to a sense of responsibility to our own story, what we endorse and the activities that we encourage.”)

Potter seems fearless, undaunted by such pedestrian dread as falling off a mountain.

But Whisper changed that unflinching courage. Well, a goggle-wearing Whisper, in a backpack wedged between him and his parachute, did.

“It wasn’t until I started having to think through the likelihood of something happening to Whisper that I finally got it,” Potter said earlier this week during a break from solo climbing in Yosemite. “This is really serious stuff that we do, and what would I do if I killed Whisper.”

Potter adopted Whisper in Oklahoma when she was 9 weeks old, and she’s been part of his adventures ever since. She grew up in a backpack. The problem was when Potter leaped off a summit, he couldn’t bring his pal.

“It all came about from me going on eight-hour hikes and not being able to take Whisper because of the final three, four minutes of the hike,” Potter said of his lengthy ascents that end in speedy flights down Switzerland’s Bernese Alps.

“I was feeling like a bad owner because she’s not getting enough exercise. So, I thought, ‘How can I take my dog on these ultimate dog walks when really it’s just the final few minutes that seem impossible?’ Then, I just problem-solved.”

It wasn’t that Potter never had pondered the perils of his work. As he was making his 22-minute film in 2013, 24 people died BASE jumping, which is the super-extreme practice of canopy riding that involves leaping from bridges, buildings and cliffs. All of those 2013 deaths – as well as all 22 fatalities in 2014 – came after canopy pilots leaped from cliffs and mountains, almost all of them in the European Alps.

Filmed in Switzerland at the base of the Eiger, Potter’s introspective documentary includes a scene where he is strolling past a graveyard, telling his pal “that’s not going to be us Whisper.”

Twelve BASE jumpers died leaping from cliffs in the Swiss Alps when he was making the film in 2013 and 2014.

“We are not going to be the dead dudes,” Potter says in slow-cadenced narration of his film.

The trailer for the film, which Potter screened at a few mountain film festivals last year but released online three weeks ago, has been viewed more than 1.7 million times in three weeks.

Potter spent almost two years innovating a canopy-capable backpack that included room for Whisper, finally settling on a third prototype.

Whisper wears a burly Ruffwear harness that is tethered to the pack at three points. She is nestled in jackets and inflated airbags that create a cocoon. She wears custom goggles. In the movie, she moves her head from side-to-side as Potter flies down a mountain, looking around like she’s riding shotgun in a truck.

Potter says it’s not uncommon for Whisper to doze off while on his back, even snoring as he counts down his leap.

Military dogs have a history of parachuting from airplanes. Mine-sniffing “paradogs” were used by the British in World War II. Marines parachuted with dogs in Vietnam.

But Whisper isn’t parachuting out of an airplane. She’s BASE jumping, which increases the risk. And then she’s flying, upping an already perilous game.

While the chance Whisper could slip from the pack virtually is none, Potter – who never has suffered an injury in nearly 30 years on the edge – is piloting a fabric wingsuit at about 120 mph in close proximity to cliff walls.

“No harm could happen to Whisper unless there’s some catastrophic flying accident on my part,” he said.

And the part he left out: All accidents while wingsuit flying are catastrophic. So, Potter has to be perfect. Every time.

Not a problem, he says. Conservative decision-making is what keeps him not only with his family – Whisper and girlfriend, writer Jen Rapp – but in the mountains and the skies. Taking his pal along for the ride has intensified his decision process and the meticulous study behind his work, he said.

Potter got Whisper in 2010, shortly after his divorce from professional wingsuit flyer and rock climber Steph Davis.

Whisper was a therapeutic companion during a rough patch.

“Whisper is my best pal, my family,” he said.

Paula “PJ” Wilderman has rescued more than 2,600 cattle dogs and heelers in the last decade with her New Hope Cattle Dog Rescue in Denver. She saw the film’s trailer. She called her board members, asking “What do we think about this?”

In a 20-minute conversation with The Denver Post, she said, “I really don’t know,” at least a dozen times.

Wilderman sees the good in it: an owner and his dog exploring and adventuring. She sees the danger: Any mistake could injure or kill Whisper, who, Wilderman said, is sure she’s nestled in the safest place possible atop the back of her owner.

“At some point you can say ‘Wow what a lucky dog.’ And then there’s another part of me that thinks just because you can do something doesn’t mean you should,” Wilderman said. “I don’t know what to think.”

Potter is, in many ways, an outlaw in the fringe world of professional rock climbing – an outsider with a unique interpretation of his gravity-defying arts. He doesn’t shy from controversy.

While he certainly didn’t start flying with Whisper for publicity or to elevate his image, he heard from plenty of viewers when he screened the film last year at film festivals. And some were not pleased.

“In many ways, Dean is an artist. He takes a really personal, creative approach to his adventures and this is just another example of that,” said Nick Rosen, the Boulder filmmaker whose Sender Films has worked with Potter for years. “Whisper is like his kid. He takes her everywhere, including places where dogs aren’t allowed, telling people he has a hearing problem and she’s his therapy dog. He stirred some controversy with this film, but ... he’s more concerned about this dog’s safety than his own. So I see this as a very good thing for Dean to keep himself safe. He’s got a whole other life to take care of up there and that makes him more on it and careful. And Dean is already very calculated and careful.”

Potter bristles at the idea – often expressed in the online comments section beneath the trailer for “When Dogs Fly” – that he is abusing Whisper. (The American Society for the Prevention of Cruelty to Animals declined to comment.)

Whisper makes it pretty clear when she’s not happy about something, Potter said. She was so scared she pooped on her first and only helicopter ride. It takes her a half hour to submit to a nail clipping.

But when it’s time to go climbing or wingsuit flying, he said, “Whisper is right there, ready to go.”

“When I leave Whisper behind for an hour, that’s leaving her behind for seven hours,” Potter said. “The idea here is never leave your dog behind. I’m not saying take your dog wingsuit flying. But if we can take Whisper BASE jumping or climbing, maybe you can take your dog places you didn’t consider. Just find better ways to take your dog with you. They just love to be with their people and their pack.”

Or, in Whisper’s case, in her pack.