BRIEF REPORT

Tool Use in Insect Foraging by the Chimpanzees ofNgogo, Kibale National Park, Uganda

HOGAN M. SHERROWn

Department of Anthropology, Yale University, New Haven, Connecticut

Chimpanzee tool use for resource acquisition has been reported atnumerous research sites. The chimpanzees of the Kibale Forest, westernUganda, have not previously been observed to use tools in foraging forinsects. Here I report the first observation of tool use by the chimpanzeesof the Ngogo community of Kibale National Park, Uganda, in insectforaging. Three adult females, one adolescent male, and one juvenile malewere observed making and using tools to probe into a fallen dead tree tocollect insect and wood pieces. I discuss the importance of thisobservation, and the behavioral similarities with chimpanzees fromother sites. Am. J. Primatol. 65:377–383, 2005. r 2005 Wiley-Liss, Inc.

Key words: tool; chimpanzee; Ngogo; insectivory

INTRODUCTION

Chimpanzees (Pan troglodytes) have the most extensive tool kit of anynonhuman primate species, and tool use to acquire resources is widespreadamong communities of wild chimpanzees [reviewed in McGrew, 1992; Whitenet al., 2001]. The tools used by chimpanzees are made mostly of organic matterthat decomposes quickly after the tools are discarded. The stone tools used in nut-cracking by the chimpanzee communities at Tai [Boesch & Boesch-Achermann,2000] and Bossou [Inoue-Nakamura & Matsuzawa, 1997] are exceptions to thisgeneralization. The fragile nature of the majority of tools used by chimpanzeesmakes it difficult to infer tool use from the evidence left behind; instead thebehavior usually must be observed directly (but see McGrew et al. [2003]). Thetypes of tools used by chimpanzees vary across populations and have beenproposed to indicate differences in socially learned behaviors between populations[Whiten et al., 1999].

Contract grant sponsor: L.S.B. Leakey Foundation; Contract grant sponsor: American Society ofPrimatologists; Contract grant sponsor: Sigma Xi Foundation; Contract grant sponsor: John F.Enders Foundation.

nCorrespondence to: Hogan M. Sherrow, Department of Anthropology, Yale University, PO Box208277, New Haven, CT 06520. E-mail: hogan.sherrow@yale.edu

Received 15 June 2004; revised 30 September 2004; revision accepted 4 November 2004

DOI 10.1002/ajp.20122Published online in Wiley InterScience (www.interscience.wiley.com).

r 2005 Wiley-Liss, Inc.

American Journal of Primatology 65:377–383 (2005)

The chimpanzees of the Kibale Forest have been studied continuously atKanyawara and Ngogo for 17 and 9 years, respectively. Ngogo and Kanyawara areseparated by 12 km and at least one other chimpanzee community, and the twochimpanzee communities never come into contact with each other. Tool use hasrarely been observed in either of the Kibale communities (Watts, personalcommunication; Wrangham, personal communication). Most observations of tooluse in these communities have involved the use of leaves to gather water or toclean body parts [Whiten et al., 2001]. It was thought that the chimpanzees of theKibale Forest did not use tools to forage for insects [McGrew, 1992, 2001; Whitenet al., 2001]. In addition, there have been no reports of tool use for insect foragingby the chimpanzees of the Budongo Forest in northern Uganda [Whiten et al.,2001]. This is the first report of chimpanzees from the Ngogo community, in theKibale Forest in western Uganda, using tools to forage for insects.

MATERIALS AND METHODS

The Ngogo study area is in the center of the Kibale National Park in westernUganda. The study area is approximately 1,350m above sea level and is comprisedof primary, secondary, and regenerating forest [Struhsaker, 1997]. Rainfallaverages about 1,600 mm per year and usually occurs during two rainy seasons(March–May and September–December) [Struhsaker, 1997]. The Ngogo chim-panzee community is the largest on record and has approximately 140–150members, including 21 adult males, 15 adolescent males, and 40–50 adult females.In 1995, Watts and Mitani began an ongoing study of the Ngogo chimpanzeecommunity, and research has been continuous.

All of the males and several of the females of the community are wellhabituated and can be followed on the ground at distances of 3–5 m. I conductedbehavioral observations between July and December 2003 during all-day followsof mixed-age and -sex groups.

RESULTS

On 5 September 2003, two field assistants and I were following a large partyof chimpanzees in the northern part of the Ngogo study area. September is therainy season in Uganda and there was an abundance of fruit, allowing for largeparties to form. At least 28 individuals were in the party, though there wereprobably many more than that. It was impossible to obtain an exact count of allparty members because several of them were out of visual range. We identified atleast 15 adult males, seven adolescent males, five adult females, and oneadolescent female in the party. Three of the five adult females had dependentoffspring.

At 1556 hr we entered a Phoenix reclinata swamp, following the party alongthe northeast edge of a large Cyperus papyrus swamp. After being brieflyseparated from most of the individuals for several minutes, we caught up to theparty and found five adult males at a fallen dead tree. The males were using theircanine teeth to remove and eat pieces of dead wood. Several of the wood chunksdetached by the males contained what appeared to be pieces of insects. At about1624 hr, after the adult males moved away from the fallen dead tree, anadolescent male (CA) and three of the adult females and their dependent offspringapproached the location.

The adult females and the adolescent male each pulled a flexible stem fromthe ground from within 2m of the dead tree. The stems (from the genus

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Leptaspis) were 18–42 cm long and approximately 5–6 mm in diameter. Leptaspisis a type of grass that is abundant in the swamp areas of the Ngogo Forest. Tomodify the vegetation, the chimpanzees put one end of the stem into their mouthsand, using their molars, crushed the vegetation. They then used their canines andincisors to further modify the stems until they resembled a paintbrush (Fig. 1a).The brush end of the tool was approximately 4–7 cm long. The four chimpanzeesmade at least five tools during the observation.

Once the chimpanzees finished manipulating the reeds, they inserted the‘‘brush’’ ends into holes along the main trunk of the fallen dead tree, presseddown firmly into the holes, manipulated the tools in a circular motion, and thenremoved them. The chimpanzees then used their lips to pluck insects, insectpieces, and wood particles from the ends of the stems. This process was repeatedfive to 10 times by at least three different adult females, the adolescent male, andat least one juvenile (Fig. 1b). When a tool was bent or the bristles appeared to betoo worn, the tool was discarded on the ground and another reed was picked andmodified.

At 1634 hr three of the adult males returned to the fallen dead tree andsupplanted the adolescent male, the adult females, and their offspring. The adultmales resumed eating deadwood, using their canine teeth. I observed at least fivediscarded tools on the ground, and one rested on the fallen tree, yet none was usedby the adult males. One of the adult males (DO) picked up a discarded tool andcarried it for at least 5 m before he dropped it; however, he did not use it to probeinto the fallen dead tree.

Fig. 1. a: ‘‘Brush-stick’’ tool next to the observer’s binoculars for scale. b: Adolescent male using atool to probe for insects. c: Juvenile male unsuccessfully using a tool to probe for insects.

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At 1642 hr the adult males moved away from the fallen dead tree. Thefemales, their offspring, and the adolescent male (CA) returned to the tree andresumed the tool-use behavior. At one point an adult female with a young infantprobed into one of the holes in the fallen dead tree while her infant intentlywatched the process. Moments later a juvenile male recovered a discarded tooland tried to use it as a probe (Fig. 1c). However, she failed to accomplish the task,dropped the tool, and instead used his teeth in an unsuccessful attempt to pulldead pieces of wood from the tree. At 1701 hr CA and the adult females left thefallen dead tree. None of the tool users took their tools with them.

DISCUSSION

At least five members of the Ngogo chimpanzee community in KibaleNational Park, Uganda, were observed to use tools to extract insect and woodpieces from a fallen dead tree. The tools were made of locally abundant, flexiblereeds, and were 18–42 cm long and approximately 5–6mm in diameter. Themodified ‘‘brush’’ ends of the tools were approximately 4–7 cm long. McGrew(personal communication) has suggested that this tool should be called a ‘‘brush-stick,’’ because it is similar to tools mentioned in previous reports [reviewed inWhiten et al., 1999].

Unfortunately, I was unable to determine the species of insect being preyedupon during the observation. After viewing video recordings of the behavior,other researchers suggested that the chimpanzees might have been harvestingwood-boring beetles (McGrew and Marchant, personal communication). Althoughit is unknown how many species of wood-boring beetles are present in the Ngogoarea of the Kibale Forest, they are very common in Kibale (Quicke, personalcommunication). No confirmed observations of chimpanzees consuming beetleshave been recorded. However, a single case of a chimpanzee extracting beetles atGombe has been reported (McGrew and Marchant, personal communication). AtFongoli, Senegal, local research assistants claim that wild chimpanzees eat beetlelarvae extracted from dead wood, but this remains unconfirmed (McGrew,personal communication). More data are needed before we can assess whether theNgogo chimpanzees were consuming wood-boring beetles or not.

The adult males did not probe for insects, but used their teeth and jawstrength to remove large chunks of wood to harvest this resource. The onlyindependent male that took part in the tool construction and use was an early-adolescent male that may not have possessed the jaw strength necessary toremove large chunks of wood. In contrast, multiple adult females used the probesto forage for insect and wood pieces, and did not use their teeth. Whether thisindicates a real sex difference in tool use during foraging remains to bedetermined. Sex differences in tool use during foraging are common at other sites[Boesch & Boesch-Achermann, 2000; Goodall, 1986], and there is no reason tothink that they do not exist at Ngogo, if this behavior is habitual within thecommunity.

The infants and juveniles at Ngogo showed interest in the tool-use behaviorsof their mothers, like chimpanzee infants and juveniles elsewhere [e.g., Boesch &Boesch-Achermann, 2000; Hayashi & Matsuzawa, 2003], but seemed to lack thenecessary experience or dexterity. This is consistent with reports of tool use inother populations [e.g., Goodall, 1986; Hayashi & Matsuzawa, 2003], andindicates that insect probing at Ngogo is a complex behavior that requires yearsof practice and observation before proficiency can be achieved.

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This is the first observation of tool use in insect foraging by the chimpanzeesof either the Kanyawara or the Ngogo community. At Ngogo this may be becausestudies have been conducted for only 9 years, with most of the research focusingon the adult males of the community (who were not observed using tools).However, this does not explain the absence of observations of tool use in insectforaging at Kanyawara, where research has been conducted for a longer period oftime, and where females are observed more often than at Ngogo. Therefore, theremust be other explanations for the difference between the two sites.

There are three possible reasons why the Ngogo and Kanyawara chimpan-zees differ in the behaviors they employ in insect foraging: 1) The beetle speciesthe Ngogo chimpanzees were exploiting may not be present at Kanyawara. Whilethis remains undetermined and will be difficult to assess, Kanyawara and Ngogodiffer in terms of ecology and have different tree species that are available aspotential resources for wood-boring beetles [Struhsaker, 1997]. 2) Insect probingmay be a new behavioral pattern, and this may have been the first time the Ngogochimpanzees used tools to forage for insects. This is unlikely, however, since thechimpanzees were neither hesitant nor unskilled in their actions, indicatingfamiliarity with this form of foraging. 3) The behavior of probing for insects maybe restricted within the Kibale Forest to the Ngogo community. This can only beassessed through continued studies of the adult females and immature animals ofthe Kanyawara and Ngogo communities, and observations of other communitiesin the Kibale Forest.

CONCLUSIONS

This study reports the first observed case of chimpanzees from the KibaleForest using tools to forage for insects. This is also the first time that wildchimpanzees have been seen to make tools referred to as ‘‘brush-sticks,’’ and thento use them for a specific task. It is unlikely that this was the first time that thechimpanzees from Ngogo used tools for insect probing, since several of theindividuals involved appeared to be quite adept at the behavior.

There are several similarities between the insect probing observed in theNgogo chimpanzee community and the use of tools for extractive insect foragingobserved at other research sites:

1. The tools used were selected from nearby vegetation and were nottransported over a large distance.2. Adult females and their young were the most active tool users.3. Adult males did not take part in using the tools.4. The tools were modified several times before and during use.5. The tools were discarded after they were used, and were not taken away fromthe location.

Tool use in insect foraging has not been observed at the other research site in theKibale Forest, Kanyawara. Though the reasons for the absence of this behavior atKanyawara are unclear, continued studies of both the Ngogo and Kanywaracommunities, and the habituation and study of other communities in Kibale couldshed light on this issue.

ACKNOWLEDGMENTS

I thank the Uganda Wildlife Authority and Uganda National Council forScience and Technology for permission to conduct research in Kibale. I am

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grateful to Professor Basuta and Professor Kasenene, directors of the MakerereUniversity Biological Field Station, for their sponsorship and support. I amindebted to Professor David Watts, who provided me with the chance to work atNgogo and over the years has been a tremendous source of guidance andknowledge. I also thank Professor John Mitani, who has taught me aboutwatching chimpanzees in the wild, and so much more. I am grateful to Dr.Jeremiah Lwanga, who is not only a good friend but also a great advisor andcompanion in the field. A. Magoba, G. Mbabazi, L. Ndagizi, and A. Tumusiimeprovided invaluable assistance in the field. I thank Professors Bill McGrew andLinda Marchant for discussing my observations with me and viewing footage ofthe tool use. Professor McGrew also provided comments on a draft of themanuscript. Two anonymous reviewers contributed to the improvement of thisreport.

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