The gorilla glanced suspiciously at the eight approaching humans, then turned her back on them to protect what she was holding. A male as heavy as two shot-putters emerged from the bushes, sat down about 10 paces from the visitors, and let out a deep growl.
The treasure these two rare giants were guarding was a pair of baby twins — only the third ever recorded.
We froze in our tracks. Among the world's endangered mountain gorillas, the 37-member Susa troop is the largest that's accustomed to tourists, but any sudden movement could still provoke them.
Our Rwandan guide responded with a growl to assure the gorillas we were friendly. Everyone, gorillas and humans, relaxed. And soon enough, a young gorilla was helping Nyabitondore, the mother, groom the babies — a bonding ritual among the gentle, playful titans of the Karisimbi Mountain.
'Every birth is crucial'
There are no mountain gorillas in captivity, and all of them — just 380 at last count — live in central Africa, the adults eating up to 70 pounds a day of bamboo shoots, wild celery, nettles and ants.
Since the middle of last year, gorilla troops have produced 14 other babies, and the birth of the twins in May has delighted conservation experts.
"Every birth is crucial to the genetic viability of the mountain gorillas, and the birth of twins is an exceptional event," said Fidelle Ruzigandekwa, head of the Rwanda Wildlife Agency. "It is like a miracle because the primates are threatened with extinction."
But the joy is tinged with concern. "The mother is doing fine up to now. She manages to feed and handle them quite well," Ruzigandekwa explained. "But caring for the babies will become more difficult as the twins grow older, heavier, stronger and more active."
Until 12-year-old Nyabitondore weans them at age 4, she will be fiercely protective, carrying them on her arms or back partly to protect them from wild dogs, hyenas and other forest predators.
Other twins died
In 1986, the first recorded pair of twins died after just nine days.
Of the second pair, born in 1991, one infant died within a month. The other survived to adulthood, only to be killed by poachers attempting to steal a baby gorilla in 2002. Gorilla troops are ferociously protective of their young and poachers often have to kill mothers and other adults to steal babies.
Nyabitondore appeared blissfully unaware of the odds against her as she held the babies with one hand and fed herself with the other.
Not far from the mother, a baby gorilla climbed up a tree, looked our way and thumped his chest, trying to produce the sharp clapping sound made by agitated mature gorillas, who have chests of tough, bare skin.
But the youngster's chest was still covered with hair and he could produce only a faint thud. As though embarrassed, he hid his face against a tree trunk.
Volcanoes National Park, Africa's first, was established by Rwanda's Belgian colonial rulers in 1925 after Carl Akeley of the American Museum of Natural History made a plea to protect the gorillas.
It lies on the Rwandan side of a mountain range that straddles the borders of Rwanda, Congo and Uganda. Adjacent parks in Congo and Uganda are both known as Virunga National Park. The three parks are home to the world's entire mountain gorilla population.
A census conducted late last year found that the number was up 17 percent since the last count 15 years ago.
"To see the mother and twins makes you realize that although these animals are extremely powerful, they also have the nurturing side like humans," said Robert Harrison, a tourist from Los Angeles.
"Tracking this group was much more difficult than I anticipated, but the sight also exceeded my expectations," he said. To be so close to a gorilla, he said, "makes you feel extraordinarily alive."