By Helen Briggs
BBC News Online science reporter
The western gorilla lives peacefully in human-like social groups, a study shows.
Western gorillas are threatened by poaching and disease
Only the mountain gorilla, which is known for its aggressive behaviour, chest-beating and fighting, has been widely observed in the wild until now.
But the new research suggests the western gorilla, another gorilla species, interacts peacefully when it comes into contact with other apes.
The study, published in the journal Current Biology, may give an insight into the social world of early humans.
It suggests that some aspects of gorilla behaviour are shared with chimpanzees and humans.
The evidence is based on field studies of western lowland gorillas (Gorilla gorilla gorilla) living near the Mondika Research Station on the border of Central African Republic and the Democratic Republic of the Congo.
Researchers in Germany and the United States collected hair and faecal samples from nests used by different social groups to analyse the animals' DNA.
Paternity tests indicated that neighbouring social groups of western gorillas were led by genetically related males.
When a mature male, known as a silverback, encountered a tribe led by its brother or cousin, the encounter was almost always a friendly one.
Gorilla age was estimated from dung size; (l-r) infant, juvenile, adult female or blackback male, and silverback
"Usually in mountain gorillas, when groups come across each other in the forest, the male-male interactions are quite aggressive, involving chest-beating displays and sometimes even physical violence," Brenda Bradley, of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, told BBC News Online.
"In western gorillas, [researchers] have observed interactions where two groups have come together and members have co-mingled and interacted very peacefully and this has been very puzzling to researchers because they're used to observing interactions between gorillas as being something highly aggressive."
The researchers believe the behaviour is due to the existence of an extended family of males - a male kin network.
When a male gorilla leaves the group it was born into and starts its own family, it tends to stay in the same area of the forest and continues to interact with groups led by its father and brothers.
The same has been seen in chimpanzees, our closest animal relatives, suggesting that early human societies evolved in the same way.
"As early human society evolved, it would have been relations beyond the very close male-female offspring network - between male-male networks - that would have formed the basis for larger and larger groups coalescing and forming the basis of society eventually," said co-author Linda Vigilant, also at the Max Planck Institute for Evolutionary Anthropology.
Gorillas are the largest of the great apes, and among the world's most endangered species.
Their precise taxonomic hierarchy has been hotly debated but they can be viewed as two species with four sub-species.
Are there clues here to the formation of early human communities?
The two species would be the western, including the Cross River gorilla (Gorilla gorilla diehli) and the western lowland gorilla (Gorilla gorilla gorilla); and the eastern, including the eastern lowland gorilla (Gorilla berengei graueri) and the mountain gorilla (Gorilla berengei berengei).
Mountain gorillas have been widely studied in the wild since the 1950s. Only about 700 are left.
The western lowland gorilla is much more common, with a population size of about 94,000.
However, recent surveys show they are declining rapidly due to poaching and disease.