Nav: Home

Why do chimpanzees throw stones at trees?

March 03, 2016

Chimpanzees often use tools to extract or consume food. Which tools they choose for which purpose, however, can differ depending on the region where they live. Researchers of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, have thus initiated the 'Pan African Programme: The Cultured Chimpanzee' and, since 2010, have collected data on chimpanzee behavior, demography and resource availability across Africa following a standardized protocol. This is how the researchers encountered a thus far unknown behavior: In West Africa chimpanzees throw stones at trees resulting in conspicuous accumulations at these sites. Why exactly the animals do this the researchers do not yet know, yet the behavior appears to have some cultural elements.

Chimpanzees have been studied for almost 60 years at a few long-term field sites, which are mainly located in East and West Africa. They are proficient tool-users, using sticks to fish for termites, to dip for ants, to extract honey, and even using stone or wooden hammers to crack open nuts. Outside the foraging context male chimpanzees sometimes throw branches and stones during displays, or leaf-clip to solicit sex from females. This research has therefore been fundamental for providing insights into natural chimpanzee behavior and most importantly into the differences between populations. However, at the same time it has also become clear that chimpanzee behavior observed at such a small number of sites is unlikely to be representative of other chimpanzee populations.

In an effort to overcome this limitation, researchers of the Department of Primatology at the Max Planck Institute for Evolutionary Anthropology have initiated the 'Pan African Programme: The Cultured Chimpanzee' (PanAf) to better understand the ecological and evolutionary drivers of behavioral diversification in chimpanzees. Following a unique standardized protocol, data on chimpanzee behavior, demography and resource availability have been collected since 2010 at 39 different temporary research sites across Africa. "The PanAf project represents a new approach to studying chimpanzees and will provide many interesting insights into chimpanzee demography and social structure, genetics, behavior and culture", says Hjalmar Kuehl of the Max Planck Institute for Evolutionary Anthropology and the German Centre for Integrative Biodiversity Research. "The Pan Af is only possible due to the numerous collaborations with chimpanzee researchers, field workers and national wildlife authorities in 14 countries across Africa". Since chimpanzees were not habituated to human presence at these sites, the researchers rely on a wide spectrum of non-invasive sampling methods, including remote camera traps.

After discovering conspicuous piles of stones next to trees at four research sites in West Africa, the field teams placed camera traps next to them. For instance, at the site of the Chimbo Foundation in Guinea Bissau some impressive videos were recorded which confirmed the researchers' suspicion that chimpanzees were responsible for these stone piles and were regularly visiting these trees. "The PanAf cameras filmed individual chimpanzees picking up stones from beside, or inside trees, and then throwing them at these trees while emitting a long-distance pant hoot vocalization", says Ammie Kalan of the Max Planck Institute for Evolutionary Anthropology. Importantly, the behavior results in accumulations of rocks at these locations. Whereas it is mainly the adult males practicing this behavior in the context of ritualized displays, some camera traps also revealed females or juveniles doing it. The behavior has only been observed in West Africa and appears to be independent of any foraging context, in which the majority of tool-use behaviors were previously described in chimpanzees.

"This study reports a new chimpanzee behavior not known previously and highlights the potential of the PanAf project to uncover unknown facets of the life of chimpanzees, our closest living relative", says Christophe Boesch, director of the Department of Primatology at the Max Planck Institute for Evolutionary Anthropology. "As the stone accumulation behavior does not seem to be linked to either the abundance of stones or the availability of suitable trees in an area, it is likely that it has some cultural elements."

Chimpanzees are often used as a model for the evolution of early hominins. Due to the conspicuous accumulations of stones associated with this newly discovered behavior, it raises questions regarding the interpretation of stone accumulation sites in archaeology. Intriguingly, the authors also suggest that this behavior could shed some light on the origin of ritual sites in hominin evolution.
-end-
Citizen science project

Further analyses of videos and other data collected from the PanAf are currently underway. The public can participate in watching and annotating the PanAf videos using the online citizen science website http://www.chimpandsee.org. At Chimp&See everyone can watch the over 1 million video clips the PanAf has recorded from across Africa of chimpanzees, gorillas, elephants, buffalo, leopards and many more species! Visit http://www.chimpandsee.org for more details on how to become a citizen scientist!

Original publication:

Hjalmar S. Kühl et al.
Chimpanzee accumulative stone throwing
Scientific Reports; 29 February, 2016 (DOI: 10.1038/srep22219)

Max-Planck-Gesellschaft

Related Chimpanzees Articles:

Chimpanzees modify grooming behavior when near higher ranking members
Chimpanzees modify grooming behavior when near higher ranking members.
Chimpanzees adapt their foraging behavior to avoid human contact
Research by PhD candidate Nicola Bryson-Morrison from the University of Kent's School of Anthropology and Conservation (SAC) suggests chimpanzees are aware of the risks of foraging too close to humans.
Genetic opposites attract when chimpanzees choose a mate
Duke University researchers find that chimpanzees are more likely to reproduce with mates whose genetic makeup most differs from their own.
Chimpanzees are 'indifferent' when it comes to altruism
New research into chimpanzees suggests that, when it comes to altruistically helping a fellow chimpanzee, they are 'indifferent.'
New study: Male chimpanzees can be players and good fathers
New research suggests that male chimpanzees are more invested in protecting their own offspring than previously thought.
Genome sequencing reveals ancient interbreeding between chimpanzees and bonobos
For the first time, scientists have revealed ancient gene mixing between chimpanzees and bonobos, mankind's closest relatives, showing parallels with Neanderthal mixing in human ancestry.
Female chimpanzees don't fight for 'queen bee' status
Male and female chimpanzees achieve social status in dramatically different ways, says a new study by Duke University primatologists.
Chimpanzees choose cooperation over competition
Tasks that require chimpanzees to work together preferred five-fold, despite opportunities for competition, aggression and freeloading.
Chimpanzees: Travel fosters tool use
Chimpanzees traveling far and for longer time periods use tools more frequently to obtain food.
Why do chimpanzees throw stones at trees?
Newly discovered stone tool-use behavior and accumulation sites in wild chimpanzees are reminiscent to human cairns.

Related Chimpanzees Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Jumpstarting Creativity
Our greatest breakthroughs and triumphs have one thing in common: creativity. But how do you ignite it? And how do you rekindle it? This hour, TED speakers explore ideas on jumpstarting creativity. Guests include economist Tim Harford, producer Helen Marriage, artificial intelligence researcher Steve Engels, and behavioral scientist Marily Oppezzo.
Now Playing: Science for the People

#524 The Human Network
What does a network of humans look like and how does it work? How does information spread? How do decisions and opinions spread? What gets distorted as it moves through the network and why? This week we dig into the ins and outs of human networks with Matthew Jackson, Professor of Economics at Stanford University and author of the book "The Human Network: How Your Social Position Determines Your Power, Beliefs, and Behaviours".