The group is not on a hunt; it is securing the borders of its territory. While out on patrol, they occasionally intrude on their neighbours’ habitat. And if they do come across their neighbours, all hell breaks loose: when two groups of chimpanzees cross paths or when one group encounters a single male from another tribe, sparks will usually fly. Sometimes the altercations are so violent that one of the males ends up dead.
The members of the different groups recognise each from quite far away by their calls, which are known as “pant hoots” – slowly swelling calls that can be heard one kilometre away, even in dense jungle.
Scientists from the Max Planck Institute for Evolutionary Anthropology in Leipzig conducted a playback study to find out how the chimpanzees living in Taï National Park in Ivory Coast react to the pant hoots of neighbours, strangers and familiar group members. It turns out that the calls of neighbours and unknown individuals evoke a broader range of gestures than the vocal expressions of familiar group members. While the chimpanzees respond to members from their own group with pant hoots, they react to neighbours and strangers by screaming. Male chimpanzees always react to the calls more strongly than females. The results of the study show that chimpanzees are able to distinguish between the calls of members from their own group and calls of neighbours and strangers.
To date, deadly clashes between chimpanzees have been observed in many chimpanzee habitats. Yet there appear to be regional differences. , Scientists from the Max Planck Institute for Evolutionary Anthropology headed by Christophe Boesch have compiled their notes recording almost 500 encounters over a period of 23 years and, for the first time ever, documented deadly attacks carried out by the Taï chimpanzees. Their findings show, however, that the acts of aggression there are often fatal less than in other regions. Furthermore, there were fewer incidences of males killing the offspring of females from other groups – a common practice carried out not only by many chimpanzee populations, but by lions and other species as well.
The researchers also observed that the Taï chimpanzees rush to each other’s aid more often. Support from their own ranks discourages or drives away enemies before there are any injuries. The Max Planck scientists believe this could be due to the poorer visibility in the Taï forest, so that the animals can never really be sure how many enemies they are actually facing. Moreover, the researchers repeatedly witnessed Taï females also becoming involved in the altercations. Sometimes, the females even initiate the conflicts. In other regions, on the other hand, it is almost exclusively the males that participate in the fights.
Clashes between different groups of chimpanzees are often compared to human warfare. Both actually bear significant similarities, ranging from the participation of large male coalitions and the systematic control of territorial borders over many years to deadly attacks on isolated individuals from other groups. Like humans, chimpanzee groups can even completely annihilate each other, as scientists have observed. However, it was long believed that – unlike humans – neighbouring groups of chimpanzees never interact peacefully.
Yet thanks to their long-term data, the Max Planck researchers were able to disprove this assumption: they documented not only deadly attacks, but also observed sexually mature, adult females from different groups interacting with each other over many years and mothers with offspring peacefully visiting neighbouring groups.
Chimpanzees therefore live in a world that is sometimes peaceful and sometimes hostile – just like us humans. The researchers have yet to discover why the animals behave so aggressively towards each other, and are even willing to take the risk of suffering fatal injuries. The Leipzig-based scientists suspect that chimpanzees fight to increase their chances of successful reproduction, as females that leave their group after reaching sexual maturity prefer to join large groups of chimpanzees which are able to defend themselves. Perhaps even small groups with only a few males can impress others with particularly aggressive behaviour and induce the adolescent females of a larger group to “switch sides”. In any case, demographic factors, such as the group size and number of adult males, influence the aggressive behaviour.
Image: Sonja Metzger, MPI for Evolutionary Anthropology