Scientists Discover Chimps Use Stones as Long-Distance Communication

Chimpanzees are our closest living relatives, and this adds to the fascination of learning how they behave and interact with each other. We know that they, like humans, live in social groups and that communication will, therefore, be an essential part of their behavior. We are still learning about the complex world of chimpanzee communication. A new study conducted by an international team of researchers has shown that chimps use throwing stones against trees as a method of communication. This reveals further intriguing information about how these amazing primates live.

Chimpanzee Communication in Context

Before we dive further into chimpanzee communication, we need to look at the environment within which they live and their lifestyle. Both of these factors will influence the methods of communication available to them. Chimpanzees (Pan troglodytes) live in the tropical rainforests of central Africa. However, they can also be found in forest-savanna habitats, and some even live in primarily savanna habitats. Here, they can have a lifespan of up to 60 years. In such widespread habitats, the chimps need to communicate with each other over long distances. There are also a lot of trees available to use as communication devices!

Chimps are omnivores. Their diet consists of ripe fruits and young leaves supplemented with stems, buds, seeds, insects, and some vertebrates. They spend most of their time feeding or looking for more food. Sometimes they will hunt a larger animal, such as a colobus monkey, on their own, but at other times they need to act as a team. For this, communication is vital.

Typical Chimpanzee Behavior

Chimps are social animals that travel along the ground but spend a lot of time in the trees. In their chosen sites amongst the branches, they feed and build nests made of plant material. Their social structure is known as a fission-fusion society. They live in a group that occupies a home territory, which can split into smaller subgroups with fluid membership. Individual chimps may also move around alone. Males tend to be more social than females and move around in larger groups. Some chimps are more social than others, and the sociability of an individual can vary across their lifespan.

The characteristics of social relationships between chimpanzees are as varied as those among humans. These relationships range from coalitions and loose friendships to sworn enemies. Relationships change over time and are influenced by births, deaths, and immigration. Aggression can break out, especially when two individuals are competing for a resource. Adult males spend a lot of time together, and there is a clear dominance hierarchy.

Communication Between Chimps

As a highly social species, communication is important for chimps. They have been observed using visual communication with gestures, postures, and facial expressions. If you see a chimp raise its arms, slap the ground, or stare directly at something, it is displaying a threat. Courtship, on the other hand, often involves shaking branches and tapping feet. If a chimp wants another to groom it, it will signal this with a long, hard stare and loud arm scratching.

You can also tell how a chimp is feeling from its vocalizations, and chimps use sound to let others know what is going on. Specific food grunts are used to let the group know that a food source has been discovered. They use a “pant hoot” call to locate each other when they are out of visual range. Their vocal abilities also include grunts, barks, and screams.

Tactile communication is used to comfort distressed group members and to reduce aggression. Frequent bouts of social grooming help to cement relationships. Chimps have even been seen playfully tickling each other! Conversely, aggression is communicated through biting, slapping, and kicking. Experts believe that chimps use smell to learn more about each other, but less is known about this form of communication.

Using Vibration and Drumming for Communication

Many animal species drum or hit their body parts against a surface to communicate. For example, rabbits thump the ground to let others know that danger is nearby. Amongst primates, Tibetan macaques (Macaca thibetana) knock on dead tree trunks and, as mentioned above, chimps slap the ground. In both species, these actions are usually associated with intimidation or are displays of dominance. Studies of chimp populations in Africa have observed them hitting tree trunks with their hands or feet, often accompanied by their pant hoot call. Drumming sounds typically have a low frequency and are audible over longer distances, making them an ideal choice for long-distance communication.

Combining Tools With Drumming

Chimpanzees are known to use tools in many different situations. They use sticks to lever ants out from the ground and stones to crack nuts. Recently, they have been seen banging and throwing rocks at hollow trees so frequently that rocks accumulate at the base of the tree. This behavior, known as ‘accumulative stone throwing,’ has only been observed in Western chimpanzees (a subspecies) at four research sites. Scientists wanted to learn whether this behavior was a type of long-distance communication. Were the chimps elevating tree drumming by incorporating the use of tools? To explore this hypothesis, the scientists conducted a study in a forest and savanna area in the southeast of the Republic of Guinea-Bissau, Africa, where the chimpanzees live in generally undisturbed forests. Drumming tree locations were selected, and camera traps were set up.

What Does Stone Drumming Mean?

In most cases, the chimps picked up stones that were already at the tree rather than finding them elsewhere. The activity occurred when the chimps were highly aroused and took place both alone and alongside other long-distance communication methods and male signaling. However, the behavioral dynamics surrounding stone drumming indicate that it is more than just a modified form of tree drumming. Immediately before stone drumming, the chimps behaved differently than they did before hand drumming. They swayed less and pant-hooted more. Also, afterward, they panted less. This suggests that the motivation for stone drumming is different and that the activity has its own unique meaning. Different drumming patterns may even convey information about the individual making the noise.

Male Display Behaviors and Cultural Learning

Interestingly, it was mainly adult males who performed the stone drumming, and it was often combined with other male display behaviors. Males may be demonstrating their strength and virility by showing that they are big enough and strong enough to pick up a rock and bash it against a tree. This display would be heard over longer distances because the noise made by a rock against the tree is louder and lower in frequency than that produced by hands and feet alone. This sound carries for longer distances.

How chimps use tools and communicate varies between locations. Scientists have explained this by referring to the concepts of culture and social learning. Basically, it means that chimps learn to do this as a know-how copying activity. They observe other chimps around them to learn how to do it. Chimps who have not been exposed to others performing this behavior would therefore not do it. This tells us a lot about how skills and behaviors spread through chimp populations. The scientists conducting this study also observed younger chimps appearing to copy the stone-assisted drumming they had seen in older members of their group. These are significant findings and pave the way for further studies into culturally evolved animal communication behaviors in other primates and even other species.

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