A troop of wild mountain gorillas recently turned a typical ecotourism encounter upside down when the animals paid a visit to a tented camp in Uganda's Bwindi Impenetrable National Park.

Video footage, now gone viral, shows baby and adult gorillas approaching U.S. tourist John J. King II, sitting with him, and even grooming him as he sits in quiet amazement—right next to a giant male silverback gorilla.

"These gorillas were interacting with me just like I was one of their own, and it happened completely naturally," King said. "Who knows why it happened?"

Local rangers were also stumped, telling King that, while baby gorillas sometimes interact with humans, the rangers had never known adult animals to take such an interest.

Mountain gorillas are listed as endangered by the International Union for Conservation of Nature. Only about 800 are thought to exist in the wild, though recent studies suggest their numbers may be growing. (Related: "'Spectacular' Gorilla Growth in Congo, Despite War.")

A major population of some 300 animals can be found in Bwindi, where King's astonishing encounter took place this month.

"A Total Gorilla High"

Visiting the gorillas in Bwindi is a serious endeavor that requires permits issued by the Uganda Wildlife Authority and the help of local rangers and trackers who can find wild gorillas that are habituated to human presence.

Permits run $500 (U.S.) a day—with no guarantee of anything more than a "nice spirited hike of one to eight hours," said King, a wildlife photographer and conservationist. (Visit King's blog for more details about his gorilla experience and other travels.)

But on two successive days, King's group found and observed gorillas eating wild celery, playing, nursing, and resting.

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On the second day, the tourists met a gorilla troop called the Rusheguras—the same group that would later return the favor with an unexpected visit to the tent camp the next morning.

"It was a total gorilla high going back to camp the evening after observing the Rusheguras group, and we were ready to go home the next day," said King, who's from Massachusetts. But "around 6 o'clock the next morning, one of the expedition members in the tent next to ours said, Hey John get up, you won't believe it."

King said that he wasn't particularly scared to see the gorillas in the camp, because the same animals had been so gentle the day before. He merely sat down by the path and prepared for some very close-up photography.

Close Gorilla Encounter

Mountain gorillas live in troops or bands that can include up to 30 animals. Each group is led by a dominant male called a silverback, because of the male's identifying streak of silvery fur.

The animals are typically calm and nonaggressive, but they do pack incredible physical power, which makes any close encounter potentially dangerous.

"The trackers told us, sometimes babies might approach you, and so just basically sit there in a docile position and they will usually just move away," King said.

At first, "this baby kind of grabbed my arm in a very gentle way. I just can't tell you how gentle it was. It was like a young child touching your arm in a way that's very endearing.

"Then it just escalated," King continued. "Instead of moving away, it went behind me and started to touch me on the shoulder and on the head in what was obviously grooming.

"When the silverback walked up, I was transfixed and really set upon not engaging his eyes and trying to be docile. I had no idea he had moved right behind me until my friend [Jonathan Rossouw, director and expedition leader for the Seattle-based ecotravel company Zegrahm Expeditions, who also filmed the encounter] told me," King said.

"From their touch to this sort of sweet, musty, wild smell, I felt like I was being caressed by wild creatures. It was really more surprise than fear and mostly just exhilaration."

Fears for the Gorillas

Craig Sholley of the African Wildlife Foundation once directed the Mountain Gorilla Project in Rwanda and has spent thousands of hours in the field with the animals.

He's also visited the Bwindi camp many times and said that, while gorillas are often seen moving around its perimeters—the camp is smack dab in gorilla country, after all—it's very rare to see them marching through camp on the sidewalks. Sholley hopes that remains the case.

"This kind of encounter looks delightful on film, but it can be dangerous on a number of levels," he said.

In general, most mountain gorillas are habituated to humans and are very tolerant of sharing space with them, Sholley said. So while gorillas certainly could inflict harm, it's not very likely.

"Frankly I'm more worried about the danger to gorillas," he said.

One concern is that gorillas that get too accustomed to humans could change their behavior for the worse.

Virunga National Park spokesperson LuAnne Cadd said changes in gorilla behavior around humans has already happened with several of her park's animals.

"Two of our gorilla families in Virunga National Park often come out of the forest to eat from the cultivated fields, causing major issues with the local farmers," Cadd said.

Another major problem is that gorillas are greatly susceptible to human disease.

"The introduction of human diseases into a gorilla population could be population threatening," said AWF's Sholley. "And considering that half of the world's gorillas live in this one forest, it could be a disaster."

Following "Gorilla Etiquette"

That's one reason gorilla-tourism standards mandate that people should stay at least 20 feet (7 meters) away from the animals and sometimes wear respiratory masks as well. However, when it's the gorillas who initiate close contact, as they did with King, there's little that people can do.

"I think anybody in their situation would have done the same thing," Sholley said. "I think it's amazing that John was as poised as he was and handled himself so well considering the situation. I really have to commend him for adhering to 'gorilla etiquette.'"

While gorilla tourism raises some risks, it just might be the animals' best chance for survival.

For instance, revenue produced by the practice may be the reason that gorillas survived the violence that gripped Rwanda during the 1990s, Sholley said. And the animals' economic value continues to be a great asset to gorillas and their neighbors—both animal and human.

"Gorilla visits are a high-revenue, low-impact tourism, as small groups paying relatively large sums visit the gorillas," said Cai Tjeenk Willink, tourism director at Virunga National Park.

"This is ideal, as impact on the environment is minimal, whilst park benefits, reinvested in conservation, are quite high. In Congo, 30 percent of the revenues go directly to community projects whereby neighboring communities of the park benefit from the park's revenues. Twenty percent goes to park costs, and 50 percent is directed toward the conservation of the other parks in the DRC, which have lower tourism revenues."

But even if tourists hoping for the opportunity to observe gorillas up close can ultimately help African animals and humans, encounters like John King's should remain extremely rare, experts say.

"It was one of those experiences that the more you think about it, the more you begin to absorb just how special it was," King said.