Western lowland gorillas have much more complex social dynamics than we have previously recognized, with bonds stretching far beyond their immediate family. The tiered nature of these relations resembles our own, while being very unlike those of chimpanzees. The discovery raises the possibility that our ancestors have been maintaining extended networks far longer than previously imagined.
Gorillas spend most of their time in family units composed of a single dominant male, several adult females, and their offspring. Males usually live as isolated bachelors after childhood before they've established silverback status. Similar patterns are seen in many other animals – including those we have studied in more depth – and it has been thought this is all there is to gorilla society.
However, Cambridge University's Dr Robin Morrison has used 20 years of observations from the Wildlife Conservation Society's study site at Mbeli Bai clearing in the Republic of the Congo to show gorillas maintain connections over many years with those outside their immediate family.
In Proceedings of the Royal Society B Morrison categorizes these relationships into tiers. She calls their family group the first tier. The second tier is an average of a dozen related gorillas referred to as the “dispersed extended family”, comparable to human cousins. Beyond this are almost 40 members of the “aggregated group” with whom a gorilla will spend time harmoniously. "An analogy to early human populations might be a tribe or small settlement, like a village," Morrison said in a statement.
Being closely related increases the chances gorillas will be part of each other's aggregated group, but it is far from essential. Morrison notes females move between family groups over the course of their lives, taking their younger offspring with them. As a result, unrelated males may be raised together, a little like Robert Baratheon and Ned Stark, and similarly, form a bond that lasts a lifetime.
Sometimes teenage males reinforce these links by joining “bachelor groups” – a phenomenon recently reported in Indian elephants, but there reflecting a response to human disturbance.
Morrison attributes our failure to notice this before to the difficulties of studying gorillas in dense forests. However, fruit harvests bring large populations of western gorillas together, just as nomadic human societies hold regular gatherings of the tribes. These provide an opportunity to resume old friendships, and also benefit animals that can work cooperatively to find abundant but short-lived food sources.
Species as distant from us as dolphins and elephants also have tiered networks, so we know the phenomenon has evolved independently. Nevertheless, the paper argues tiered relations are most likely a feature of great ape evolution that emerged during the time of a common ancestor of humans and gorillas, and atrophied in the much more territorial chimpanzees. Finding it in mountain gorillas, whose environments are somewhat different, would strengthen the case.
Either way, these extended bonds allowed the spread of ideas and cooperative hunting practices and later let us live together in large groups, making humans what we are today.