Another young male crashes through the branches, performs a perfect gymnast's roll and scurries after his gargantuan father. A potbellied mature female waddles past, barely glancing at us. Serundori leads me nearer to Rugendo, who sits by a cluster of small trees munching on fistfuls of leaves.
The mountain gorillas are swathed in the shaggy black fur that keeps them warm in their high-altitude habitat, between 7,300 and 14,000 feet above sea level. Mountain gorillas, a subspecies of eastern gorilla, are herbivores, apart from the occasional feast of ants. A silverback has to eat up to 75 pounds of vegetation a day to maintain his great bulk. Females, who weigh about half as much, leave their natal groups between ages 6 and 9 to search for a mate and bear their first offspring around age 10. Young males are called blackbacks. Once they begin to show silver, at about age 12, most leave or are forced out of the group, but some stay and wait for a chance at the dominant male's spot.
A dominant male will lead the group, usually around ten gorillas strong, for about ten years before being overthrown. Silverbacks are devoted fathers. If a mother dies or abandons her infant, the silverback will take over its raising, Serundori says, adding: "I've seen it many times." A silverback keeps his family under his watchful eye at all times. He wakes them about 5 a.m. by beating his chest and charging at them. Then he leads them to their first feeding spot for the day. "The family forages for about two miles a day, eating plants, leaves, wild celery, fruits and even thistles," says Serundori. "They play a lot, and take midmorning and midafternoon naps. Around 6 p.m. the silverback chooses a place for them to sleep for the night."
As if on cue, Rugendo rolls onto his side for a mid-afternoon nap, sated by his bulky snack. He became the master of this group in 2001, when his father was killed by crossfire between the Congolese military and the Interahamwe. Rugendo's easy acceptance of our presence allows the rangers to keep watch over him and his family. But it also allows poachers and soldiers to get dangerously close.
I edge closer, impressed by his brawny arms, many times thicker than a weight lifter's, and salami-size fingers. His massive, furry-crested head holds enormous jaw muscles. While the big chief dozes, Noel and two other sons tussle in mock combat, a favorite gorilla pastime, tumbling, growling, slapping and tugging. The fur on Kongomani and Mukunda, 10- and 12-year-old males, is still black. Noel is especially aggressive, baring his teeth as he repeatedly bangs his fists on the ground and charges his brothers. He leaps on them, pulls at their fur, bites their arms and legs and whacks them on the head. They soon tire of Noel's antics. Now, each time he attacks, one of the brothers grabs him with an arm and tosses him back into the bushes. After a few such tosses, Noel turns to peer at the pale-skinned stranger. Up close his dark brown eyes shimmer.
Along the hike to meet Humba's family a few days later, Serundori points to several circular patches of flattened and bent grass spread around a spray of tall bamboo. "The gorillas slept here last night," he says. A year earlier, I had been with some chimpanzee-like bonobos in Congo about 500 miles to the west. The lithe bonobos live high in trees and construct elaborate nests by weaving branches together. Adult gorillas build nests on the ground that Dian Fossey described as "oval, leafy bathtubs."
After Humba has peered at us through the thicket and resumed feeding, we settle in to watch his clan. The females and youngsters climb out of the undergrowth, stare at us for a few moments, then begin stuffing vine leaves into their mouths. An infant female jumps onto the back of her much bigger brother and whacks him repeatedly on the head, growling in pleasure, until he scurries away. Now and then Humba stops feeding and sits with one hand cupped under his chin, the other resting on an elbow. With his bulging belly, he looks like a sumo wrestler imitating the pose of Rodin's sculpture The Thinker.
Each time the clan moves, a 12-year-old blackback named Nyakamwe plops down between us and his family members, keeping us under watch until they have all disappeared up the slope. He then ambles after them. "He's the sentry," Serundori tells me. "He's there to see them safely away, and to raise the alarm if he thinks we pose a danger." Again and again, Nyakamwe takes up his post, blocking our path until he sees that the others are out of sight.
I listen carefully for the 20 or so sounds that make up gorilla "vocabulary"—growls, grumbles and grunts. WildlifeDirect's de Merode, who has studied gorillas for eight years, interprets for me. A single grunt or a resounding belch means all is right with the world. But watch out, de Merode says, if a gorilla emits what researchers call a pig grunt—part growl and part grunt uttered with bared teeth. It means he is annoyed.
At hour's end, I reluctantly head down the slope. When we encounter the Rwandan Army patrol that had crossed into Congolese territory, I heed de Merode's warning not to take any photographs. But I approach the most important-looking soldier and offer a handshake. He seems unsure what to do and fingers the butt of his machine gun. After a few tense moments, my wide smile and "G'Day mate, howya goin'?" evoke a cautious smile. As the soldier tentatively shakes my hand, de Merode says, "We'd better leave before anything bad happens."