1Every studied community of chimpanzees has been reported to incorporate insects into their diet to some degree, in many cases involving the use of a variety of processed tools for extractive foraging (McGrew, 1992; Whiten et al., 1999; Wondra et al., 2016). However, eastern chimpanzees (Pan troglodytes schweinfurthii) in western Uganda have small foraging toolkits as compared with western chimpanzees (P. t. verus), central chimpanzees (P. t. troglodytes), Nigeria-Cameroon chimpanzees (P. t. ellioti) or those in other eastern chimpanzee study sites (Fowler & Sommer, 2007; Gruber et al., 2012; Hicks et al., 2019; Sanz et al., 2004; Whiten et al., 1999). From observations of eastern chimpanzees, foraging with stick tools for insects (including honey bees, Apis spp.; stingless bees, Meliponini tribe; termites, Macrotermes spp.; arboreal carpenter ants, Camponotus spp.; terrestrial driver ants, Dorylus spp.) has been recorded in all long-term field sites in Tanzania (Goodall, 1964; McGrew, 1974; McGrew & Collins, 1985; Nishida & Hiraiwa, 1982; Nishie, 2011; Stewart & Piel, 2014; Wondra et al., 2016). In Uganda, chimpanzees appear to have a more limited use of tools, with reports of extractive foraging mostly limited to sites situated in the west and southwest of the country, and only for honey (Stanford et al., 2000; Watts, 2008; Webster et al., 2014) and driver ants (Hashimoto et al., 2000).
2Towards the north-easterly end of the eastern chimpanzees’ geographic range, the Sonso chimpanzees of Budongo Forest have famously never been observed to use stick tools, despite being studied since 1990 (Gruber et al., 2009; Mugisha et al., 2016; Reynolds, 2005; Whiten et al., 1999). Chimpanzees in the Sonso community habitually use leaf tools (e.g. using leaf-sponges for drinking, Quiatt, 2006), but extractive foraging with sticks appears to be absent even after chimpanzees were experimentally offered honey (Gruber et al., 2009; Grund et al., 2019). Insects are rarely consumed (Hedges & McGrew, 2012; Mugisha et al., 2016), despite the availability of suitable prey species (for example, Apis spp., Camponotus spp. and Dorylus spp.: Hedges & McGrew, 2012).
3More recently, since the beginning of chimpanzee research in heavily human-modified environments, chimpanzees in the forest fragments within the ‘corridor’ landscape separating the Budongo and Bugoma Forest Reserves have been reported to use stick tools to forage on honey and stingless bee nests in Bulindi (McLennan, 2011, 2015) and, on one occasion, honey bees in Kasokwa (Reynolds, 2005). The Budongo-Bugoma corridor is comprised of a network of fragmented riverine forests situated in an agricultural landscape mosaic between the two large forest reserves (Figure 1). While there are several common tree species found in both Budongo Forest and the nearby fragments, the corridor forests are more heterogeneous in habitat types than the larger forest blocks and similarity in species composition is low (McLennan & Plumptre, 2012; Oxley, 2019).
4Previously, low habituation levels of chimpanzees in forest fragments had typically resulted in few, or no, direct observations of chimpanzees using stick tools for extractive foraging. However, the first study to systematically analyse the diets of fragment chimpanzees using faecal analyses found that insectivory is a habitual foraging activity for chimpanzees at Bulindi (McLennan, 2014). Chimpanzees there forage primarily on subterranean stingless bees’ nests and weaver ants, the former of which are accessed with stick tools (McLennan, 2011, 2015). Data from Bulindi suggest that levels of tool-assisted foraging by chimpanzees in other forest fragments in this area could be higher than those reported in Budongo Forest, highlighting the need for further information from other fragment communities. While current data available from Budongo suggest the complete absence of stick tool use, we expected foraging with stick tools to be present in a chimpanzee community in a nearby forest fragment, given both the slightly closer geographical proximity and similarity of the habitat in forest fragments. Here, we report preliminary observations of stick tool use at Kasongoire, a newly studied site within the riverine forest fragments in the Budongo-Bugoma corridor.
5Kasongoire (1°34’0.18”N, 31°32’56.08”E) lies ca. 15 km south of Budongo Forest Reserve (Figure 1) and is one of several locations that chimpanzees are found in unprotected riverine forest fragments in the Budongo-Bugoma corridor (McCarthy et al., 2015; McLennan, 2008). The Kasongoire chimpanzee community ranges within a forest-farm landscape mosaic comprising privately owned forests and experience high exposure to anthropogenic disturbances, including subsistence agriculture, a large-scale commercial sugarcane plantation, roads and human settlements (Oxley, 2019). The forest (~4.5 km2) has been severely fragmented over the past few decades, measuring only 350 m wide at its widest point. Common tree species include Pheonix reclinata, Pseudospondias microcarpa and Macaranga schweinfurthii.
Figure 1
Map of the Budongo-Bugoma Forest corridor in Uganda, showing the study site Kasongoire and the locations of other nearby chimpanzee study communities. Modified from McCarthy et al. (2015).
Carte du corridor forestier entre Budongo et Bugoma en Ouganda, montrant le site d’étude Kasongoire ainsi que les localisations des autres communautés de chimpanzés se trouvant aux alentours. Modifiée à partir de McCarthy et al. (2015).
6We followed the chimpanzees at Kasongoire from 6:30am until 6:30pm or until lost over 13 months from July 2015-July 2016 (Oxley, 2019). During this time we collected behavioural data using instantaneous scan samples every 15 minutes (Altmann, 1974), resulting in 625 hours of direct observations. Prior to this study, the Kasongoire community had not been the focus of detailed behavioural research, but had been partially habituated for a tourism project which was no longer running, meaning that it was possible to make direct behavioural observations. The chimpanzee community composition ranged from 36-40 individuals over the course of the study, varying as a result of deaths and births, and comprised 10 adult males and 13 adult females and their offspring. We collected all occurrence ad libitum data to describe all observed novel or unusual behaviours, recording video data and making detailed notes.
7On two occasions, Kasongoire chimpanzees were observed preparing and using stick tools to feed on what appeared to be arboreal ants. Video data were available from the first of these occasions (see Supplementary Video), allowing for a thorough assessment of the foraging behaviour, which is described in detail below.
8A party of five adult females and their offspring (total 12 individuals) was feeding on ripe fruits in a Parkia filcoidea tree, including an adult female, SYL, and her three offspring, GER, STO and SAB. When all adult chimpanzees in the party had finished feeding, most individuals began grooming. During this time SAB (infant male, ~3 years old) was observed holding a twig, which we estimated to measure ~10 cm long and 1 cm wide, and stripping the bark with his teeth. He then brushed the stick on the underside of the branch of the P. filcoidea tree, put it in his mouth and drew it away. From direct observations and the video data it was clear that there were ants, likely Camponotus sp., on the tree. He inserted the stick in a gap between the bark and the wood of the tree branch in a dried-out crevice where the ants were swarming from, brought it out and put the stick in his mouth, inter-changing between inserting the stick and his finger. This behaviour was sustained for five minutes until SAB dropped the stick and moved away.
9After SAB moved away, his brother GER (subadult male, ~10 years old) approached the same spot on the tree, broke off a stick which we estimated to measure ~15 cm long, <1 cm wide, stripped it of the bark and proceeded to perform the same behaviour as SAB. He inserted the stick and wiggled it between the bark and wood of the tree before withdrawing it and placing it in his mouth, either by using a rapid swiping motion or by simply holding the stick with his mouth (Figure 2a). After first withdrawing the stick, he sniffed it and briefly tasted it. He interchanged between using the same stick and his finger to disturb large ants visible between the bark and the wood (Figure 2b) and withdrew his finger sharply, possibly having been bitten by the insects. When using his finger, he either held the stick in his mouth or in the same hand he used to disturb the ants. Between using the stick or his finger he disturbed ants along the branch by gently and quickly brushing the branch, and flicked ants off of his hair and self-groomed, most likely to get rid of biting ants (Figure 2c). SAB returned and watched over the shoulder of GER (Figure 2d). After sustaining this behaviour for six minutes, GER dropped the stick and continued to self-groom before moving away. Both individuals moved away, until SAB returned five minutes later, prepared another stick and continued to use the stick for both probing and play for a further five minutes. Although ants were clearly visible, we were unable to confirm if the chimpanzees ingested the ants or whether they were targeting other insect prey such as grubs or larvae under the bark.
Figure 2
Video stills of a subadult male chimpanzee (GER) a) probing a stick tool into a dry tree crevice where ants are swarming from; b) inserting his finger into the crevice; c) picking off ants while holding the stick tool in his mouth; d) with his infant brother (SAB) watching over his shoulder as he inserts the probe into the crevice.
Captures d’images d’un mâle subadulte chimpanzé (GER) a) plaçant un bâton dans une fissure d’un arbre sec d’où des fourmis s’échappent ; b) insérant son doigt dans la fissure ; c) récupérant des fourmis tout en tenant l’outil dans sa bouche ; d) avec son petit frère (SAB) surveillant par-dessus son épaule pendant qu'il insère l'outil dans la crevasse.
10On the other occasion, a juvenile female from the same family (STO, ~6.5 years old) was observed brushing a twig which we estimated to measure ~10 cm long, <1 cm wide on the underside of a dry branch of a Sapium ellipticum tree. She inserted the stick into a hollow crevice in the branch and put the stick in her mouth several times before dropping the stick and moving away. It was not clear whether she ingested any insect prey or products since visibility was more obscured on this occasion and video recordings did not provide further information.
11Here we report new evidence of probing stick tool use in a chimpanzee community inhabiting riverine forest fragments close to Budongo Forest, apparently in a foraging context. The method of stick tool use described here corroborates with descriptions of ant-fishing from elsewhere, whereby stick tools are inserted into and withdrawn from a tree to forage on ants, usually Camponotus spp. (Mahale, Tanzania: Nishida & Hiraiwa, 1982; Nishie, 2011; Bossou, Guinea: Yamamoto et al., 2008). Although ants were apparently not ingested and the chimpanzees were perhaps targeting other insect prey between the bark and trunk of the tree, e.g. grubs or larvae, the observations from Kasongoire provide further evidence of stick tool use in the forest fragments near Budongo Forest, where stick tool use is seemingly absent.
12It is possible that these observations, which involved only immature chimpanzees, could be examples of one-off play or exploratory behaviours. Rare instances of occasional “invention” of extractive tool-use foraging have been reported elsewhere: a single juvenile male chimpanzee in Bossou, Guinea, was witnessed ant-fishing in trees using a similar method as described here, yet ant-fishing in trees was never previously reported there in 27 years of chimpanzee observations (Yamamoto et al., 2008). In immature chimpanzees, object manipulation (including sticks) is particularly linked with play in younger individuals, with goal-directed behaviours (e.g. for tool use) developing with age (Koops et al., 2015a; Lamon et al., 2018). However, even though stick tool use at Kasongoire was observed directly only by three immature individuals on two occasions, the mounting evidence of habitual stick tool use (i.e. for probing and digging) from other nearby fragment chimpanzee communities suggests that stick tool use is likely to be present to some extent in Kasongoire (Bulindi: McLennan, 2011; McLennan et al., 2019a; Mairirwe: McLennan et al., 2019b).
13A detailed genetic analysis of chimpanzee dispersal within the Budongo-Bugoma corridor found that females manage to disperse in this fragmented landscape, despite a lack of connectivity between most forest fragments (McCarthy et al., 2018). In that study, it was confirmed that a female immigrated to the Kasongoire community from the main Budongo Forest block, which is separated by only 12 km when measured from the nearest points. If tool use is a result of cultural knowledge (Gruber et al., 2009), then the transfer of females between forest patches in a relatively small area makes it curious as to why stick tool use appears to be completely absent from Budongo chimpanzees.
14Interestingly, the individuals we observed using the stick tools were all offspring of the same adult female, suggesting the behaviour could have been socially learnt from the mother: in Bossou, Guinea, infants whose mothers spent more time ant-dipping than others developed more advanced tool use skills and at an earlier age (Humle et al., 2009). If learnt from the mother, it is possible that this behaviour was transferred from another fragment community (although we did not witness the mother interacting with any stick tools). However, it has been previously reported that even neighbouring chimpanzee communities exhibit differences in the type and size of tools used despite ecological similarities (Koops et al., 2015b; Luncz et al., 2012; Pascual-Garrido, 2019), and regardless of the presence of immigrant females from neighbouring communities (Luncz & Boesch, 2014). This may also be the case among chimpanzees inhabiting the forest fragments in the Budongo-Bugoma corridor where reports suggest that the types of tool used and prey species targeted may differ between sites (c.f. McLennan et al., 2019a, 2019b: this issue). It still remains unclear why there appears to be a distinct geographical cut off point in extractive foraging with stick tools in Budongo chimpanzees.
15With the current limited data from Kasongoire, it is impossible to draw inferences about the extent of stick tool use in the foraging strategies of chimpanzees there. Kasongoire chimpanzees may not exhibit the high levels of insectivory recorded in nearby Bulindi, with or without the use of tools, but since insectivory among chimpanzees is best determined through faecal analyses rather than direct observation (McLennan, 2014), further research is needed to reveal the full extent of these foraging behaviours in Kasongoire.
16We sincerely thank Oxford Brookes University, the Rufford Foundation, Columbus Zoos and Aquariums, IDEAWILD and the Primate Society of Great Britain for funding support; Uganda National Council for Science and Technology, Uganda Wildlife Authority and Budongo Conservation Field Station for permission to work in Kasongoire; Matthew McLennan for comments which improved the manuscript; Marie Cibot for providing translations into French and Kimberley Hockings, Matthew McLennan and Tatyana Humle for useful comments on the video data.
17The authors have no conflict of interest to declare.
18The supplementary video for this article is available in the online version of the article on the website of the Revue de Primatologie (Varia 10 | 2019).
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