A 72-year-old hunter lost in the woods for 18 days used a garbage bag and leaves to stay warm in the freezing weather and ate lizards, squirrels and frogs to stay alive, officials and his family said.

Gene Penaflor, of San Francisco, was rescued Saturday in the Mendocino National Forest in northwest California by hunters who heard him calling for help from the bottom of a canyon, according to authorities, who had feared the worst due to the extreme weather and treacherous terrain.

The hunters, who were not identified by police, used their coats and poles they had cut down to rig a makeshift stretcher and carry Penaflor to rescue teams, who had been slowed down for hours due to the forest's terrain.

"It's extraordinary that people can survive this type of environment. I mean he's fit and he made the right choices," Lieutenant Shannon Barney of Mendocino County Sheriff's Office told ABCNews.com.

Penaflor was reported missing Sept. 25 after he failed to reunite with his hunting partner at a camp within the forest. The Mendocino County Sheriff's Office activated more than 10 search teams, including canine units and helicopters, Barney said.

But the search was called off after four days "due to no clues being found to suggest the whereabouts of Gene and due to a significant incoming storm," Barney said.

"He ate squirrels, whatever he could catch," Penaflor's son, Jeremy Penaflor, told local ABC affiliate KGO. "He ate lizards, he ate a couple of frogs, he ate snakes, and survival mode kicked in. When you have no food for 18 days, whatever protein you can get, he did."

Penaflor also told authorities he started a fire and used dry leaves and a heavy-duty garbage bag to stay warm in the snow and fluctuating temperatures, which ranged from 25 to 70 degrees, according to the Mendocino County Sheriff's Office.

"On days when it rained or snowed he was able to crawl under a large log to stay dry," Barney said, noting Penaflor had access to plenty of water nearby.

On Saturday, the 911 call came in from a group of hunters who found Penaflor, and, using GPS coordinates from the caller's cell phone, authorities tracked down the location.

Penaflor was airlifted to Ukiah Valley Medical Center for medical treatment and a reunion with his family, who had also assisted in the search.

Penaflor, who was found less than four miles from his original location, told authorities he travelled further away from the road than planned on his hunting excursion and then fell and hit his head, leaving him temporarily unconscious and disoriented when he awoke.

"When we found him he was weak and dehydrated," said Barney. "From talking to his family, we learned that he had studied basic survival techniques."

Penaflor has been discharged from the Ukiah Valley Medical Center, hospital officials confirmed today to ABCNews.com. Attempts to reach Penaflor and his son, Jeremy, for comment were not successful.