According to new research, gorillas have complex social structures, from lifetime bonds forged between distant relations, to social tiers with striking parallels to traditional human societies. The results, published in the July 3 issue of the Proceedings of the Royal Society B, indicate that the hierarchical social organization observed in humans may have evolved far earlier than previously asserted and may not be a product of the social brain evolution unique to the hominin lineage.
A research team led by University of Cambridge’s Dr. Robin Morrison used over six years of data from two research sites in the Republic of Congo, where scientists documented the social exchanges of hundreds of western lowland gorillas (Gorilla gorilla gorilla).
“Gorillas live in small family units — a dominant male and several females with offspring — or as solitary male ‘bachelors’,” they explained.
“We used statistical algorithms to reveal patterns of interaction between family groups and individuals in the datasets.”
“By analyzing the frequency and length of ‘associations,’ found hitherto undetermined social layers.”
Beyond immediate family, there was a tier of regular interaction — an average of 13 gorillas — that maps closely to ‘dispersed extended family’ in traditional human societies e.g. aunts; grandparents; cousins.
Beyond that, a further tier of association involved an average of 39 gorillas, similar to an ‘aggregated group’ that spends time together without necessarily being closely related.
“An analogy to early human populations might be a tribe or small settlement, like a village,” Dr. Morrison noted.
Where dominant males (silverbacks) were half-siblings they were more likely to be in the same ‘tribe.’ But over 80% of the close associations detected were between more distantly related — or even apparently unrelated — silverbacks.
“Females spend time in multiple groups throughout their lives, making it possible for males not closely related to grow up in the same natal group, similar to step-brothers. The bonds that form may lead to these associations we see as adults,” Dr. Morrison said.
Occasionally, when lots of young males ‘disperse’ from their families at the same time but are not yet ready to strike out on their own, they form ‘all-male bachelor groups’ for a while.
“This could be another bond-forming period,” the study authors said.
They uncovered hints of an even higher social tier of ‘periodic aggregations,’ similar to an annual gathering or festival based around ‘fruiting events,’ although these are too infrequent to detect with certainty from this study’s data.
In fact, sporadic fruiting schedules of the gorillas’ preferred foods may be one reason why they evolved this ‘hierarchical social modularity.’
“Western gorillas often move many miles a day to feed from a diverse range of plants that rarely and unpredictably produce fruit. This food is easier to find if they collaborate when foraging,” Dr. Morrison said.
“Gorillas spend a lot of their early life in the family group, helping to train them for foraging. Other long-term social bonds and networks would further aid cooperation and collective memory for tracking down food that’s hard to find.”
A small number of mammal species — baboons, toothed whales and elephants — have a similar social structure to humans. These species also rely on ‘idiosyncratic’ food sources and all have spatial memory centers in their brain to rival those of humans.
Before now, the species on this short list were evolutionarily distant from humans. Our closest relatives, chimpanzees, live in small territorial groups with fluctuating alliances that are highly aggressive — often violent — with neighbors.
As such, one theory for human society is that it required the evolution of a particularly large and sophisticated ‘social brain’ unique to the hominin lineage.
However, the addition of gorillas to this list suggests the simplest explanation may be that our social complexity evolved much earlier, and is instead merely absent from the chimpanzee lineage.
“Our findings provide yet more evidence that these endangered animals are deeply intelligent and sophisticated, and that we humans are perhaps not quite as special as we might like to think,” Dr. Morrison said.
Robin E. Morrison et al. 2019. Hierarchical social modularity in gorillas. Proc. R. Soc. B 286 (1906); doi: 10.1098/rspb.2019.0681