Human impacts erode behavioral diversity in chimpanzees

Compared to other animals, chimpanzees show tremendous variation across groups in their behavior - from the types of tools they use in their feeding behavior to the specific gestures they use in communication. Research in captivity suggests that chimpanzees acquire information socially, learning to do things in a certain way based on how it is done by other members of their group. Thus, much of the variation in the behavior among wild chimpanzee groups may be akin to "cultural" variation in humans.

As part of an international team led by researchers at the Max Planck Institute for Evolutionary Anthropology and the German Centre for Integrative Biodiversity Research, Arizona State University researchers Kevin Langergraber and Kevin Lee were coauthors in a study examining the relationship between human impact and behavioral diversity in chimpanzees. This paper will be published online by the journal Science on Thursday, March 7, 2019.

Unfortunately, chimpanzee populations are declining across their range in equatorial Africa due to deforestation, hunting and diseases, many of which originate in humans. Much of the empirical work and debate surrounding the loss of biodiversity has focused on declines in genetic diversity or population size.

However, behavioral diversity is also a facet of biodiversity, but has not been considered as an additional concern until recently. Several international conservation organizations, such as the Convention on Biological Diversity or the Convention on the Conservation of Migratory Species of Wild Animals, of the United Nations Environment Programme, now call for the protection of biological diversity in its entirety, including behavioral diversity of culturally rich wildlife.

Due to limited empirical data, until now it had been unclear whether behavioral diversity would similarly be negatively affected by human impact.

"Chimpanzees are highly intelligent and adaptable creatures," says Lee, "and there have been various reports of chimpanzees, both in captivity and in free-living conditions, that have relatively more frequent contact with humans exhibiting some 'novel' behaviors not observed in more remote populations, but it was not clear how overall behavioral diversity would be affected."

The international research team compiled an unprecedented dataset on 31 chimpanzee behaviors across 144 social groups or communities, located throughout the entire geographic range of wild chimpanzees. Though part of this information was already available in the scientific literature, the team also conducted extensive fieldwork at 46 locations, including on a group near Langergraber's long-term research site in Kibale National Park, Uganda, while Lee led a separate team in Bwindi Impenetrable National Park, Uganda. The particular set of behaviors considered in this study included the extraction and consumption of termites, ants, algae, nuts and honey; the use of tools for hunting or digging for tubers; and the use of stones, pools and caves among several others.

The occurrence of behaviors at a given site was investigated with respect to an aggregate measure of human impact.

"We measured multiple levels of human impact, including human population density, roads, rivers and forest cover," says Langergraber. "The analysis revealed a strong and robust pattern - chimpanzee behavioral diversity was reduced by 88 percent when human impact was highest compared to locations with the least human impact."

There are a number of potential mechanisms that may explain the loss of behaviors observed. As is known for humans, population size plays a major role in maintaining cultural traits and a similar mechanism may function in chimpanzees. Chimpanzees may also avoid conspicuous behaviors that inform hunters about their presence, such as nut cracking. Habitat degradation and resource depletion may also reduce opportunities for social learning and thus prevent the transfer of local traditions from one generation to the next. Lastly, climate change may also be important, as it may influence the production of important food resources and make their availability unpredictable. Very likely a combination of these potential mechanisms has caused the observed reduction in chimpanzee behavioral diversity.