When the “hunting bark” resounds through the forest, colobus monkeys have to deal with a horde of bloodthirsty chimpanzees: Our closest relatives in the animal kingdom also coordinate joint actions through communication sounds, a study shows. With a specific reputation, they recruit group members for the hunt and thereby increase the chances of success of the raids. The study also sheds light on the roots of cooperative communication in human tribal history, say the researchers.
A lot of plant-based food is on their menu – but chimpanzees (Pan troglodytes) are by no means pure vegetarians. Their appetite for meat even makes them extremely ferocious predators. They mainly target colobus monkeys (Procolobus tephrosceles). In order to catch these nimble prey in the treetops, cooperation during the hunt is advantageous: the colobus monkeys are chased by some animals into the claws of others. The booty is then eaten together. It was already known that chimpanzees make special sounds as part of this hunting behavior. The scientists led by Joseph Mine from the University of Zurich have now devoted a more detailed study to this so-called hunting bark.
Audio: This is how the chimpanzee hunt call sounds. © Claudia Wilke, PhD
On the trail of the hunting bark effect
For their study, the behavioral researchers analyzed data from around 230 hunting events recorded over the past 25 years in a free-living chimpanzee community in Kanywara, Uganda. These were mainly records of call behavior, the formation and size of the hunting groups, and the later success of the community actions. Using special evaluation methods, the scientists were able to link this information to one another. As they report, the considerable importance of the special communication sounds in the context of the colobus monkey hunt became apparent. The hunting bark catalyzes the cooperative behavior in the difficult environment of dense vegetation: group members are mobilized for the hunt and the chances of success increase with a lot of barking.
“Chimpanzees who utter it communicate to those around them that they are motivated to hunt. This information can convince reluctant individuals to join, increasing the chance of success at loot for all involved,” explains Mine. “It is striking that after the hunting bark, more hunters join, the hunt begins faster and the first catch takes less time,” reports the behavioral researcher. The scientists also considered a range of other factors that can affect the outcome of a hunt, including the presence of skilled, experienced hunters or potential distractions. However, the hunt barking always played the key role in the system, according to the evaluations.
Animal behavior with an anthropological dimension
However, the exact meaning of the hunting bark remains a mystery. Through further investigations, Mine and his colleagues now want to uncover the effects in more detail: “It is currently still unclear whether the calls only announce the decision of individual animals to hunt in order to recruit more, or whether they are made specifically to indicate the exact actions in the animal group,” says senior author Simon Townsend from the University of Zurich.
In addition to providing insights into the communication skills of chimpanzees, the research results also have important implications for anthropology, say the scientists. Because it is assumed that the special talents for communication and cooperation in humans have developed in dependence on each other: over time, if one became more complex, the other also developed further, so that a feedback loop was created that influenced the development of language and of cooperation. However, it is unclear when this process began in the history of development. “Our findings are now the first evidence that vocal communication also facilitates group cooperation in our closest living relatives,” says Townsend. Mine adds: “So this suggests that this relationship is very old. The connection seems to have existed for at least seven million years, since our last common ancestor with the chimpanzees.”
Source: University of Zurich, specialist article: Science Advances, doi: 10.1126/sciadv.abo5553