​ Cultural diversity describes the variety of socially learned, behavioural traditions special to each chimpanzee society. One of the best-known examples is the presence versus absence of tool use behaviours across wild chimpanzee populations/subspecies. For example, nut-cracking has predominantly been observed among Western chimpanzees, while termite-fishing is most prevalent in Eastern and Central subspecies. Differences have also been identified regarding the types of tools and techniques used by termite-fishing chimpanzees[5]. Eastern chimpanzees, such as those living in Gombe (Tanzania), manufacture simple probe tools by removing leaves and stems from twigs. In contrast, Central chimpanzees, such as the Goualougo-Triangle population (Republic of Congo), prepare multiple implements which they use consecutively[8,9]: 1) a sturdy stick to open up holes in the termite mound; 2) a thinner flexible twig with a brush tip to extract the termites from the hole. Another example of sequential tool use comes from a population in Gabon, who use five different types of tools to harvest honey[10]. Together, these tools with distinct characteristics and functions constitute a tool set and represent one of the most complex forms of chimpanzee technological behaviour. Other examples include variation in the length of tools used to harvest ants[11], and differences in the tool materials used for nut-cracking[12]. Nevertheless, tool use constitutes only a small part of chimpanzee cultural diversity. Variations in what different chimpanzee communities eat and how they use different parts of their habitat for their day-to-day activities are customary. For example, only three out of four chimpanzee groups living in Cantanhez (Guinea Bissau), are known to feed in mangrove habitats, consuming the leaves of black mangrove trees and drinking saltwater[13]. Similarly, while several chimpanzee habitats have caves and rockshelters, only chimpanzees living in dry environments such as Fongoli and Dindefelo (Senegal) are known to use them[14]. This is likely a cultural adaptation to the high temperatures that these savanna-dwelling populations experience[15]. Habitat use and feeding are perhaps the two activities that are most sensitive to climate change as it directly affects the types of plants and foods available. Furthermore, for chimpanzee communities sharing landscapes with human populations, these behaviours frequently overlap with human activities, often leading to conflict[16]. For example, in Bossou (Guinea) chimpanzees frequently visit crops to feed on cultivated foods such as bananas, oranges, pineapples, and cassava[17,18]. As such, understanding variations in feeding behaviours and habitat use has the most direct implications for developing conservation strategies and managing human-wildlife coexistence. Behavioural diversity also extends to social behaviours such as grooming styles. In a grooming hand-clasp (or high-arm grooming), two chimpanzees join their hands over their heads while they groom each other with the other hand[18]. This behaviour is present in, for example, Mahale (Tanzania), Kibale (Uganda), and Taï (Côte d’Ivoire), but is absent from other populations such as Gombe (Tanzania), Bossou (Guinea), and Budongo (Uganda). Such differences even exist between neighbouring communities of the same population. While the grooming hand-clasp is common in Mahale chimpanzees, the two residing groups differ in their hand-clasp techniques: the M-group members limit the overhead contact to the wrists, while the now extinct K-group used to connect their palms or their wrists[19]. Communication forms a big part of chimpanzees' social lives and there’s ample cultural diversity here too. Every group has its own vocal dialect, similar to regional accents in humans [20,21,22], and certain gestures may have different meanings to different populations. For example, leaf-clipping, which entails ripping parts of a leaf using the mouth, is associated with play for Bossou chimpanzees (Guinea)[23], while in Taï (Côte d’Ivoire) and Budongo (Uganda) it occurs in multiple contexts including social and sexual displays and travel initiation[21,24].

Much like our own, chimpanzee cultural heritage can be traced back several centuries to millennia through both historical and archaeological records. Written accounts from 16th and 17th Jesuit priests visiting West Africa record naturalistic observations of chimpanzee behaviours that can still be seen in the region today[25]. This includes drumming on tree buttresses, construction of nests, and nut cracking, indicating that these behaviours have been part of the chimpanzee cultural repertoire for at least 400 to 500 years. ​ More recently, archaeological excavations of a chimpanzee nut-cracking site in the Taï forest (Côte d’Ivoire) have revealed that Tai chimpanzees have been cracking nuts for at least 4,300 years[26] – from around the time the pyramids of Giza were being built in ancient Egypt!

So far, most of our knowledge on the emergence of cultural behaviours comes from tool use in foraging contexts. This research has shown that the local environment plays an important role on whether a tool using behaviour arises in a population, but not in others. This role may take many forms, and often it is specific to the population, its habitat and the type of technology. Nevertheless, there are two prevailing, non-mutually exclusive, hypotheses. On the one hand, the environment can stimulate tool use by providing opportunities for chimpanzees to encounter the foods and the materials needed to extract them[27,28]. On the other hand, it can also limit the abundance and availability of food that can be foraged without tools, thereby inducing the development of technological behaviours in order to exploit alternative food types[29]. Chimpanzees learn socially from others. This social learning is one of the key processes by which new behaviours become part of a social group’s cultural makeup, and are passed down from one generation to another. Once a new behaviour emerges, via invention or through introduction by an immigrating individual, it can be acquired by other members of the community along social associations[30] (e.g., among peers) and family ties[31] (e.g., from mother to offspring). The social group is crucial for keeping behaviours in the cultural pool and can sometimes act as a barrier to the introduction of behaviours or cultural variants from neighbouring communities. For example, in the Taï forest females who migrate to a new group adopt the nut-cracking technique of that group even though it may be less efficient than the method they had learned in their natal group[32]. This act of “conformity” is thought to be a strategy used by immigrant females to foster group belonging and avoid rejection from the new community, as well as adapt to the novel environment.

While chimpanzees may be cultural conformists, they are also innovators. In the Budongo Forest Reserve, Uganda, some members of the Sonso chimpanzee group have recently started using moss instead of leaves to make their sponges to extract drinking water[33]. The novel sponges made from moss, were not only able to absorb more water, but were also more efficient, indicating a clear improvement on the original leaf-sponging method. This ability to innovate has also allowed some chimpanzee populations to adapt to changes in their environment caused by human activities and climate change[34,35]. Examples include the use of introduced plant species for nesting[36], and the consumption of cultivated foods[37]. However, evidence also suggests that high levels of anthropogenic disturbance may result in the disappearance of cultural behaviours and the reduction of a population’s cultural diversity[14], on top of other well-known adverse effects such as population decline, displacement, and habitat fragmentation.