In the past few years Doran-Sheehy has documented neighboring groups of western lowland gorillas feeding alongside one another in swamps and other habitats. "Females from one group may move along and sit right by the silverback from anotherthe two groups are totally nonchalant, just hanging out," Bradley said. "You would rarely see this kind of behavior in mountain gorillas."
To complement behavioral observations, Doran-Sheehy, Bradley, and others collected DNA from gorilla hair and dung left in night nests of 12 western gorilla groups. Genetic data is important to help understand the mating system and dispersal patterns of species. The data is also fundamental for conservation efforts, Bradley said.
Collecting the DNA samples was no simple task. The Mondika Research Center, set up by Doran-Sheehy, is in remote jungle straddling the border of the Democratic Republic of the Congo and the Central African Republic (CAR).
A 16-hour drive from the CAR's capital, Bangui, is followed by a 6-hour trip in a dugout canoe and a 13-kilometer (8-mile) hike, Bradley said. The researchers enlist the help of local Pygmy people known as the Baka as trackers to lead them through dangerous and near-impenetrable rain forest.
Paternity tests performed in Leipzig on DNA from silverback hair and dung samples revealed that many western gorillas (especially those in directly neighboring groups) are related as half or full brothers or as fathers and sons. "Our results suggest that males have to leave their group to acquire females, but they don't go far," Bradley said.
Western gorilla groups are small and are led by a lone reproductively active silverback. Other mature males appear to leave home to set up house nearby. In contrast, around 50 per cent of mountain gorilla males remain in the group of their birth, Bradley said.
It makes sense for related western silverbacks to be friendly toward one another, Bradley said. Peaceful interactions may help sons and brothers to set up their own neighboring territories and attract females, and these relatives share a proportion of their genes to be passed on to the next generation.
"Primates typically live in social groups [and] upon reaching sexual maturity, some individuals leave the group in which they were born," commented primate behavioral ecologist John Mitani, who is with the University of Michigan in Ann Arbor.
"Dispersal is an important aspect of primate behavior, yet we have very little information about it for any [nonhuman] primate species," Mitani said.
"The results are fascinating and indicate that male gorillas don't go very far after dispersing," Mitani said. He added that more lengthy studies with fully habituated western gorillas are required to confirm that intergroup interactions are indeed typically peaceful.
Finding that dispersing males stay near home "is an important step in understanding the social systems and intergroup dynamics of gorillas," said Amy Vedder, former mountain gorilla field worker and vice president of the Living Landscapes Program at the New York City-based Wildlife Conservation Society. However, there could be other explanations for peaceful interactions observed between groups, she said.
"Neighboring groups may be more familiar with each other and therefore be more tolerant than they are with strangers," Vedder said. "It turns out that western gorillas frequently interact in clearings in the rain forest where certain foods are abundant. This is not the case for eastern mountain gorillas, who live in forests without [these snack-filled] hot spots."
Nevertheless, Bradley said, "it is interesting that some of the rare instances of peaceful intergroup interactions that have been observed in mountain gorillas were between two groups [with possibly related males] that had split from one group."
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