Chimpanzees are capable of inventing new, more efficient tools and, once they have done so, of abandoning their old tools in favour of new ones. This is what Noémie Lamon, a doctorate in biology from the University of Neuchâtel, has shown in a study conducted with researchers from the universities of Geneva (UNIGE) and St Andrews (UK), published today in the prestigious journal Proceedings of the Royal Society: Biological Sciences.
It has long been suggested that the cumulative evolution of cultural behaviour is unique to humans. Now, this study shows that chimpanzees are also capable of evolving, suggesting that this ability comes from our common ancestor. In particular, this study, conducted with Sonso chimpanzees in Uganda, has shown that a new technique for retrieving fluids has spread within the species, probably because it is more efficient than its ancestral version.
Two previous studies ( Hobaiter et al. 2014, Plos Biology ; Lamon et al. 2017, Science Advances ), had documented since 2011 the emergence and social learning in the Sonso community of a new behaviour, called "moss-sponging", i.e. making vegetable sponges using moss. In this new study, the researchers looked at why this behaviour spread through the community, whereas many other innovations do not take hold in the group.
The first objective of the UniNE and UNIGE researchers was to demonstrate that sponges made of moss are more efficient than leaf sponges. They absorb more water when the chimpanzee-made tools are tested individually. They are also faster to make and use. Thus, evidence has been provided that a cultural innovation in chimpanzees can be more effective than its ancestral version. But do chimpanzees realise this and voluntarily favour this new, more advantageous form?
To answer this question, 20 chimpanzees were tested in a field experiment. During the dry season, Noémie Lamon offered the chimpanzees the possibility of using tools to extract water - boiled beforehand to avoid any contamination - from a log. She also provided the materials that the chimpanzees use naturally to make the tools, namely moss and leaves.
Of the 20 chimpanzees, only 9 (the "moss-spongers") were aware of moss as a potential sponge-making material. The other 11 individuals (the leaf-spongers) had never made one. 7 of the 9 moss-spongers preferred moss to make sponges, while only 3 of the 11 leaf-spongers chose moss, preferring leaves. These results show that the chimpanzees’ choices in this experiment are based on their cultural knowledge. Chimpanzees who knew the improved technique before the experiment chose moss preferentially, while chimpanzees who had never tried the technique did not consider moss as a material that could be used to remove water from the log. Thus, knowledge of moss, and potentially its properties, is required to use it, demonstrating the implication of chimpanzees’ cultural knowledge in tool selection.
Nevertheless, although the behaviour has spread to some individuals in the community, the old technique (leaf-sponging) is still predominant. Noémie Lamon therefore sought to understand why, if moss is so effective, this behaviour has not spread more widely in the community and imposed itself against leaf-sponges, which are still manufactured in large quantities and mostly by chimpanzees.
The answer is ecological: moss is found in large quantities in the swampy parts of the forest, where the initial innovation took place, but in smaller quantities in the drier areas. This crucial difference means that only chimpanzees in swampy areas use and propagate this technique within their group, limiting its spread and use in drier contexts. However, the researcher has recently observed chimpanzees using this technique in other areas of the forest, highlighting that the behaviour is becoming more established for this community. Cultural change is apparently underway among the Sonso chimpanzees!