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Gorilla Mafia? Groups Ruled by Related Males, Study Says

John Pickrell in England
for National Geographic News
March 31, 2004
 
New genetic findings reveal that ruling dynasties may monopolize leadership of many neighboring communities of Africa's western gorillas—like a primate version of the Mafia.

Paternity tests reveal that leaders of adjacent western lowland gorilla territories in Africa are closely related as fathers, sons, and brothers.

The results, detailed in the current issue of the science journal Current Biology, may help to explain curiously peaceful interactions among neighboring social groups. The groups were observed in new behavioral studies of the western lowland gorilla (Gorilla gorilla gorilla).

The study could also provide clues about the role and development of kinship in early human society, say researchers behind the work.


Despite being the most numerous kind of gorilla, the western lowland gorilla species is the shyest and least understood. Up to a hundred thousand western lowland gorillas are thought to inhabit the forests of central Africa.

Aggressive Interactions

Most knowledge of gorilla behavior comes from studies of the eastern mountain gorilla (Gorilla beringei beringei), found mostly in Rwanda and Uganda. In that species several reproductively active mature males (silverbacks) may remain within the group in which they were born for life. And among the eastern mountain gorillas, it is the females that migrate when mature.

Competition among eastern mountain gorilla groups for females can therefore be fierce, and levels of aggression among males in neighboring mountain gorilla groups can be extremely high.

Studies have revealed that male mountain gorillas engage in displays of aggression over 90 percent of the time when neighboring groups come in close contact, said geneticist Brenda Bradley at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. These spectacular male-to-male displays often involve chest beating, charging, and hooting. One in five displays may culminate in physical violence, she said.

"That's why recent behavioral studies of western gorillas were really surprising," said Bradley, who is lead author of the new study.

Co-author Diane Doran-Sheehy, of the State University of New York in Stony Brook, has carried out pioneering studies of one of the very first groups of western gorillas that was habituated to human presence.

Studying lowland gorillas has proved more difficult than studying their mountain counterparts. The dense, flat forests that lowland gorillas call home do not provide them the long-range visibility that allows mountain gorillas to spot researchers from afar and slowly get used to their presence.

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 another—the 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.

Grueling Journey

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.

Important Step

"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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