Jake Gyllenhaal is used to showing off his body for Hollywood films and magazine photo shoots ... but not in the sub-zero temperatures of Iceland!

The actor appeared on the season premiere of "Man vs. Wild" last night, where he ventured to a remote area of the very very cold country with host Bear Grylls.

The two ate worms, slept together in a small cave (which Bear said reminded him of "Brokeback Mountain") and, at one point, had to strip off their clothes to keep them dry as they crossed a freezing cold river.

Check out the chilly river crossing above!

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