1For the last two decades, a persistent impression in the literature has been that Ugandan chimpanzee (Pan troglodytes schweinfurthii) cultures were limited compared to those of other African chimpanzee populations (e.g. McGrew, 2010). The three long-term habituated Western Ugandan chimpanzee communities (the Sonso community of Budongo Forest, and the Kanyawara and Ngogo communities of Kibale Forest) display roughly half as many tools as found in other chimpanzee communities, both within East African chimpanzees and across sub-species (Whiten et al., 1999), with particularly limited use of tools for subsistence (Gruber et al., 2016). This picture has been reinforced by further studies of the Sonso and neighbouring Budongo communities, which strongly suggest the absence of stick use throughout the Budongo Forest (Gruber et al., 2009; Gruber et al., 2012). The three articles in this special issue by McLennan et al. (2019a), McLennan et al. (2019b) and Oxley and Jovan (2019) are of great interest as they reveal an unsuspected diversity in tool use in Western Uganda, particularly with respect to sticks. These findings shatter the idea that Ugandan chimpanzees are necessarily limited in their extractive tool use, making the puzzle of community-specific patterns of tool absence, and its impact on the debate on chimpanzee culture, all the more interesting. In this commentary, I first address the elephant in the room: regional variation in Western Uganda; before engaging with the findings described in this issue, particularly in comparison to Central African chimpanzees (Pan troglodytes troglodytes), and what these results mean for chimpanzee culture.

Regional variation in Uganda: the unusual Western Ugandan chimpanzees?

2Until recently, there were four long-term habituated chimpanzee projects in Uganda (the three previously cited in the Budongo Forest Reserve and Kibale National Park, and the M-group of the Kalinzu Forest Reserve in the South-West of Uganda), with the Western Ugandan chimpanzees of Budongo and Kibale forming a cluster of communities that rarely engage in tool use activities for subsistence (Watts, 2008). Despite some behaviour being present and practiced at the community-level (e.g. fluid-dipping for honey in the Kanyawara chimpanzees, Whiten et al., 2001), extractive tool use has been referred to as ‘rare’ (Watts, 2008, p.83) or ‘absent’ (Gruber, 2013, p.8) in the tool kits of Western Ugandan chimpanzee communities, contributing to the idea that they do not use many tools. However, these findings have been refined over the years. On the one hand, while the Sonso chimpanzees of Budongo Forest have been repeatedly confirmed to have little appetite for sticks (Gruber et al., 2011), they have recently developed their leaf-based culture by adding a new behaviour, moss-sponging (Hobaiter et al., 2014; Lamon et al., 2017), to their tool kit. The neighbouring Waibira chimpanzees consume ants with leaf tools rather than sticks (Mugisha et al., 2016), likely confirming that the wider Budongo culture truly lacks any kind of stick use. On the other hand and in striking contrast, chimpanzee communities in riverine forest fragments just a few kilometres from the current Budongo forest border use stick tools to extract honey (Kasokwa, Reynolds, 2005; Bulindi, McLennan, 2011), particularly during periods of fruit scarcity (McLennan, 2015).

3The new findings reported by McLennan et al. (2019a), McLennan et al. (2019b) and Oxley and Jovan (2019) establish a strong picture of widespread evidence for an extensive stick tool use culture in the fragments that surround the main Budongo forest block. McLennan et al. (2019b) report that stick use in the digging context can be classified as ‘customary’ in Bulindi (following Whiten et al. (1999)’s classification), with most chimpanzees of all classes of age and sex engaging in the behaviour. The other studies suggest that this stick culture is also present in previously unreported forest fragments close to Budongo (Kasongoire, Oxley & Jovan, 2019) and Bugoma forests (Mairirwe, McLennan et al., 2019a) respectively. The chimpanzees’ cultures of Bugoma Forest, around 80 km south of Budongo Forest (Figure 1 in McLennan et al., 2019a), remain a mystery, with long-term research only very recently established in this area. Beyond confirming that the Budongo chimpanzee communities remain an outlier as compared to the other stick-using Ugandan chimpanzees, the new findings suggest that the most likely explanation for the Budongo chimpanzees’ lack of stick tool use is ecological. While widespread in local populations, the Budongo chimpanzees may have not needed to use sticks during foraging because of large food supplies, leading to the extinction of the behaviour (Grund et al., 2019). The finding that stick use is particularly expressed in periods of necessity by the Bulindi chimpanzees (McLennan, 2015) supports this, and further investigation of whether this pattern also holds in the newly described forest fragments of Kasongoire and Mairirwe would make a substantial contribution to the argument. In addition, more detailed comparison with the rates of tool use already described in Kibale (Watts, 2008) and Kalinzu (Hashimoto et al., 2000) would also be welcome to establish how the Bulindi, Kasongoire, and Mairirwe chimpanzees compare with other stick users in Uganda and beyond. Finally, these findings highlight the importance of research in the other numerous forest fragments in the area (McCarthy et al., 2015) as well as in the large but relatively unstudied Bugoma Forest Reserve, particularly as its vegetation appears closer to that of Kibale, where stick use is present (Berger et al., 2019). Only then will we have a complete picture of the Western Ugandan chimpanzee cultures and their connection with the broader chimpanzee cultural realms across the East African chimpanzee subspecies, including those in the nearby Democratic Republic of Congo (Hicks et al., 2019).

How digging in Western Uganda compares and informs on chimpanzee cultures

4The fine grained analysis provided by McLennan et al. (2019b) is particularly noteworthy because it also allows comparisons with other chimpanzee communities outside Uganda. McLennan et al. (2019a) provide a welcome comparison of stick tool use between the Ugandan communities and both Central African (P. t. troglodytes) and Nigeria-Cameroon (P. t. ellioti) chimpanzees. Their findings show that the tools manufactured by the Ugandan chimpanzees are strikingly similar to the ones made by other schweinfurthii chimpanzees (Hicks et al., 2019), suggestive of behavioural continuity, but also resemble those of ellioti chimpanzees (e.g. Dutton & Chapman, 2015). In contrast, Central chimpanzee tools appear to be longer (e.g. Boesch et al., 2009), although the small sample size in the latter case impacts current comparisons. Here, future work will need to assess whether the length of the tools in various areas may also be influenced by the availability of the raw material, with the possibility that a more degraded environment may offer less suitable twigs or a smaller set of suitable tree species for tool manufacture. In this respect, work in forest fragments that are highly impacted by human activities may offer a unique opportunity to study the balance between opportunity and necessity (Grund et al., 2019) for chimpanzees to produce their tools. Comparisons with other Central and East African chimpanzee communities are also possible with respect to tool sharing as McLennan et al. (2019b) analyse tool use facilitation in Bulindi using the recent nomenclature developed by Musgrave et al. (2020). Musgrave and collaborators (2020) describe active transfer from a possessor towards another individual in the Goualougou community of the Nouabalé-Ndoki National Park in the Republic of Congo, in contrast to the tolerance of their tools being taken but no active transfer in the Gombe chimpanzees in Tanzania. Similarly, McLennan and collaborators (2019b) observed at most tolerated taking of tools, but no begging for tools or active transfers. Still, experience of studying tool use in Ugandan chimpanzees should make us cautious about claiming absence. And as the current data remain limited by poor visibility, it is impossible to assert whether the Bulindi chimpanzees are similar to the Gombe chimpanzees, who additionally display a high rate of refusal, or constitute an intermediate step with respect to social facilitation of tool use. Indeed, the Gombe community often appears at the less social end of a behavioural continuum possibly stretching across all Panins (Gruber & Clay, 2016), making it likely that not all East African communities display such a high refusal rate for tool transfer.

5Finally, the finding of tool-assisted digging in Western Uganda is also interesting with respect to the still fierce debate on chimpanzee cultural variation. Along with chimpanzee stone tool-using, chimpanzee honey-gathering is often hailed as one of the most complex tool use behaviour, involving the sequential use of several tools with different functions to reach the final goal of extracting honey (Boesch et al., 2009; Sanz & Morgan, 2009). As such, it is a prime candidate for the possibility of the existence of cumulative culture, that is the ability to build upon on existing cultural behaviour, in wild chimpanzees (Davis et al., 2016). Until recently it was believed that these complex sequences of behaviour displayed during honey-gathering were limited to Central chimpanzees. But, the findings of McLennan et al. (2019a) and McLennan et al. (2019b), in addition to those of Hicks et al. (2019), raise several questions. On the one hand, honey-digging, as proposed by Hicks et al. (2019), may be part of a chimpanzee mega-culture that would stretch from Gabon to Western Uganda. Yet, variations between communities (as described above) and the absence of the behaviour in some areas remain to be explained. Data from other communities within and neighbouring the Western Ugandan population will be absolutely crucial in this respect. For example, one reason for investigating stick use in the Kaniyo-Pabidi community located in the eastern side of Budongo Forest was anecdotal reports of “digging chimpanzees”. While experimental work did not allow us to find evidence of stick use there (Gruber et al., 2012), and subsequent work continues to find no evidence for stick use in at least five communities within Budongo Forest, more data points are needed with more communities to confirm the absence of stick use north of Kasongoire and Kasokwa, which appear to represent a northern boundary in stick tool use (Oxley & Jovan, 2019). In particular, research must establish whether there is a pattern of particular ecological correlates in these forest fragments, absent from the main forest blocks. At the same time, we also must consider that chimpanzees may invent or re-invent behaviour naturally (Gruber et al., 2015). This ability to invent or re-invent a given behaviour may be used to argue against the existence of cumulative culture in chimpanzees (Bandini & Tennie, 2017). If the main argument for cumulative culture is that a particular behaviour appears to be too complex to have appeared in a single innovation, it is necessarily weakened by evidence that multiple sites stretched over thousands of kilometres show similar evidence of a seemingly complex behaviour necessitating the use of multiple tools with different functions, as is apparent in McLennan et al. (2019a)’s findings. Several independent innovators would have to have come up with the same cumulative series of steps building on the behaviour across multiple communities. A possible way out of the conundrum is to accept that cumulative culture is still present in chimpanzees in embryonic form by considering that the core definition of cumulative culture is that it builds on an already present socially transmitted behaviour, rather than the cumulative innovation having to occur outside of the lifetime of a given individual (Lamon et al., 2018; Mesoudi & Thornton, 2018).

Conclusion

6For all the reasons outlined above, the findings of stick tool use in seemingly ‘insignificant’ small forest fragments scattered around main forest blocks in Western Uganda are of particular importance. They are not only important to understand the evolution of chimpanzee cultures at the regional level, but also to bring unexpected data points to the wider debate on culture at the species level. Beyond scientific interests, these findings are also crucial for the survival of these threatened chimpanzee populations. In a recent open letter to Science, Hockings and McLennan (2019) stressed the need to study and conserve chimpanzee populations and their cultures in these small forest fragments, which are subject to particularly intense threat from human activities. While the impact of anthropogenic activities on cultural evolution in chimpanzees remains unclear (Gruber et al., 2019), the findings by McLennan et al. (2019a) that chimpanzees must compete with humans for the very spots where they are likely to exhibit these cultures underlines the importance of supporting projects such as the Bulindi Chimpanzee and Community Project. Studying and demonstrating the unique cultures of these unknown communities, provides opportunities to foster conservation collaborations with local human communities and the wider public based on the uniqueness of each chimpanzee community, a step recently taken by the United Nations in recognizing chimpanzee stone tool use as a unique cultural behaviour. It is incumbent upon researchers and conservationists to make sure that all remaining chimpanzee cultural behaviours also make the list, before they are lost forever.

Acknowledgements

7I was supported by the Swiss National Science Foundation (Grants CR13I1_162720 / 1 and PCEFP1_186832) during the writing of this commentary. I thank Cat Hobaiter and Brigitte Senut for comments and corrections on this manuscript.