Bear Grylls is the kind of guy who flings himself out of airplanes, seeks shelter in the bloody carcasses of camels and drinks water squeezed from elephant dung.

Anyone who has watched “Man vs. Wild” — the Discovery Channel show that turned Grylls’s name into a brand synonymous with extreme adventure and scrappy survival tactics — already knows this. What they might not know about the globetrotting TV star is his back story. How did the son of a conservative member of the British Parliament become, arguably, the most famous outdoorsman in the world, a mountaineering superhero who strongly resembles actor Christian Bale but dons REI apparel instead of Batman’s cape?

Grylls’s autobiography, “Mud, Sweat, and Tears,” is an attempt to recount that transformation. Although compelling in spots, the book may disappoint fans craving new details about every dangerous trek he has undertaken during six seasons of “Man vs. Wild.” Indeed, the “Wild” portion of Grylls’s career gets short shrift; the author doesn’t mention it in a significant way until page 378 in a 408-page book.

But at least Grylls gets off to what he might call a “cracking” good start, with a focus on the explorer’s forebears. We learn about his great-grandfather, Sir Walter Smiles, another M.P., who died when the ferryboat Princess Victoria sank off the coast of Northern Ireland in 1953; about his grandmother, Patsie Ford, who assumed her father’s seat in Parliament after his death and later divorced Grylls’s grandfather after finding romance with a fellow politician; and about Grylls’s father, the aforementioned conservative M.P., and his somewhat eccentric mother. There’s enough heartbreaking tragedy and family drama in all this to justify at least three sweeping novels. But none of it seems to have deterred a young Grylls — dubbed “Bear” from infancy by his older sister — from a life of riskiness.

“Mum, still to this day, says that growing up I seemed destined to be a mix of Robin Hood, Harry Houdini, John the Baptist, and an assassin,” he writes. “I took it as a great compliment.”

Indeed, for those wondering what sets someone like Grylls apart from meeker mortals — those of us who, say, have to be coaxed down inflatable slides at children’s birthday parties by reassuring 5-year-olds — the answer appears to be DNA. Over and over again, whether climbing the dome of the library at Eton College, the distinguished British boarding school he attended, or gritting his way through grueling tryouts to become a Special Forces soldier, Grylls’s need for physical achievement comes across as innate.

“I call it ‘the fire’, ” he writes. That fire burned brightest when he ascended Mount Everest, a potentially deadly climb he completed less than two years after suffering a crippling back injury during an airplane jump. In fact, the book’s Everest section — with its evocative descriptions of oxygen deprivation, altitude sickness and the discovery of previous climbers’ snow-covered corpses — is the climactic, life-altering highlight of “Mud, Sweat, and Tears.”

Unfortunately, it’s marred by the realization that a few of its passages were ripped verbatim from Grylls’s previous book, “The Kid Who Climbed Everest.”Reusing one’s own words is hardly plagiarism, but it does smack of laziness, especially considering how much additional life material Grylls opts to leave largely untouched. For example, he rebuts only briefly the controversial charge that he doesn’t actually put himself in the precarious situations that “Man vs. Wild” implies. He notes one incident in which he went to a lodge to see his wife and family instead of camping overnight in the mountains. “It was an error,” Grylls writes, “and it opened a can of worms for the papers to feed on.”

Clearly, the “Wild” man who parlayed his derring-do into deodorant endorsements is trying to save some of his narrative for a future book. In light of the recent news that Discovery had severed its relationship with Grylls because of a reported contract dispute, that’s probably a savvy move. And Bear Grylls is definitely savvy. As he’s demonstrated on many occasions, he’s smart enough to know that to survive, a man has to keep some key provisions in reserve.

Chaney writes about celebrities and pop culture for The Post.