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Chimpanzee (Pan troglodytes troglodytes) tool use in the Ngotto Forest Central African Republic.

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American Journal of Primatology 65:221–237 (2005)
RESEARCH ARTICLE
Chimpanzee (Pan troglodytes troglodytes) Tool Use in the
Ngotto Forest, Central African Republic
THURSTON C. HICKS, ROGER S. FOUTS*, and DEBORAH H. FOUTS
1
Chimpanzee and Human Communications Institute, Central Washington University,
Ellensburg, Washington
Over a 7-month period, stick tools constructed by chimpanzees were
collected and measured at the Ngotto Forest site in Central African
Republic. The chimpanzees were found to use tools to dip for ants and to
probe for honey. The basic descriptions of these tools and the contexts in
which they were found are presented. The lengths of two of the tool types
were compared with the use of a t-test for independent groups. It was
found that the lengths of the tools differed significantly depending upon
their function. The location and habitat type of each tool site were plotted
on a map. The tool types were distributed throughout the southern part
of the study area, and with one exception all tool sites were found in the
same type of habitat. Two tool sites with two other types of tools (honey
hammer/club and ant club) were found. The tool types at Ngotto are
compared with those found at other chimpanzee field sites, and the
implications for diversity in chimpanzee material culture are discussed.
Am. J. Primatol. 65:221–237, 2005.
r 2005 Wiley-Liss, Inc.
Key words: Pan troglodytes troglodytes; tool use; Ngotto Forest; tool
standardization; ant dip; honey probe
INTRODUCTION
Free-living chimpanzees (Pan troglodytes) have developed a diverse set of tool
traditions that are considered by some authors to represent culture [Whiten et al.,
2001]. Goodall [1964] first discovered that chimpanzees not only use tools but
modify them prior to their use. Since the time of that discovery, each population
of chimpanzee that has been studied has been found to possess its own unique tool
kit [McGrew, 1998]. Some components of these kits are shared between
populations, while others appear to be unique to particular populations.
Contract grant sponsor: Arcus Foundation; Contract grant sponsor: Friends of Washoe.
n
Correspondence to: Roger S. Fouts, Chimpanzee and Human Communications Institute, Central
Washington University, 400th East University Way, Ellensburg, WA, 98926-7573.
E-mail: foutsr@cwu.edu
Received 18 June 2004; revised 1 September 2004; revision accepted 28 September 2004
DOI 10.1002/ajp.20111
Published online in Wiley InterScience (www.interscience.wiley.com)
r
2005 Wiley-Liss, Inc.
222 / Hicks et al.
Whiten et al. [2001] considered 65 behavioral patterns from nine long-term
chimpanzee study sites as candidates for cultural behavior. To be considered a
cultural variant, a behavior had to be customary or habitual at one or more sites
but absent from others, where the absence of the behavior could not be explained
by environmental differences. Of the 65 candidate behaviors, 39 were accepted as
cultural variants. The majority of these could be considered tools. Of these, 36
involved the use of an external object not including another chimpanzee, and 26
involved the use of an object by a chimpanzee as an active extension of the body
(as opposed to a substrate for an action). The evidence for cultural diversity in
chimpanzees has been accumulating so rapidly that Wrangham et al. [1994]
suggested it would be productive to construct an ethnography of chimpanzee
cultural variants, as is practiced by researchers of human cultures.
Few large-scale population patterns of chimpanzee tool use have been
discerned [McGrew, 1992]. One of these is the cracking of nuts with wood or stone
hammers, which appears to be unique to Western chimpanzees (Pan troglodytes
verus), and occurs in many P. t. verus study sites to the west of the Sassandra-Izo
River, Ivory Coast. Boesch and Boesch-Acherman [2000] found no evidence of
nut-cracking behavior in P. t. verus populations on the east side of the SassandraIzo, despite the availability of hammers and appropriate nut species (but see
Joulian [1995]). The behavior has not been observed in the chimpanzee
populations that have been studied in Eastern and Central Africa [McGrew
et al., 1997]. Another possible large-scale pattern involves the use of brush-sticks
[Sugiyama, 1985] or sticks used as picks to open up termite mounds [McGrew &
Rogers, 1983; Sabater Pi, 1974], both of which have been found only at Central
chimpanzee (Pan troglodytes troglodytes) study sites [McGrew, 1992]. Beyond
these two uses, tool traditions seem to be distributed rather idiosyncratically
among chimpanzee populations, and no clear patterns between the subspecies are
discernible. For example, Eastern chimpanzees (Pan troglodytes schweinfurthii)
at Gombe, Tanzania, commonly use sticks to dip for driver ants (Dorylus sp.)
[Goodall, 1964; McGrew, 1974]. Chimpanzees of the same subspecies at the
nearby long-term research site of Kasoje (Mahale, Tanzania) show no evidence of
this behavior, even though ants are plentiful there [McGrew, 1992]. However, the
chimpanzees use a different technique (ant fishing) to acquire Camponotus ants
[Nishida, 1973]. Likewise, at Okorobikó, Equatorial Guinea, Central chimpanzees
use tools to obtain Macrotermes termites [Jones & Sabater Pi, 1969], while at
Lopé, Gabon (another Central chimpanzee study site), this insect species is
present but is never eaten [Tutin et al., 1995].
Chimpanzee populations at opposite ends of the continent use similar tools to
acquire the same species of insect. For example, Macrotermes are fished at
Gombe, Tanzania, and Assirik, Senegal [McGrew et al., 1979], as well as at other
sites; however, there are often subtle differences in the construction of these tools
between sites. For example, Gombe chimpanzees used stick tools with a mean
length of 66 cm [McGrew, 1974] to dip for ants, whereas the tools Tai
chimpanzees used for the same purpose have a mean length of 24 cm [Boesch
& Boesch, 1990] and achieve inferior results. At Bossou, Guinea [Humle &
Matsuzawa, 2002], chimpanzees use both long and short tools, each of which is
used for different-colored species of driver ants and require different fishing
techniques.
Previously unknown examples of tool use have continued to be discovered,
both at older long-term sites, such as Bossou [Hirata et al., 1998], and in less wellstudied populations, such as at Tenkere, Sierra Leone [Alp, 1997]. Each
population of chimpanzees studied offers the potential to expand the database
Chimpanzee Tool Use in the Ngotto Forest / 223
of chimpanzee cultural variations, both in the description of new tools and in the
documentation of alternative ways of making tools or different uses for tool types
that are shared with other sites.
In addition to culture, tool preparation provides an ideal opportunity to
investigate the flexibility of chimpanzee behavior in selecting and constructing
appropriate tools for the job. Boesch and Boesch-Acherman [2000] described
standardization of stick foraging tools based on tool function by the Tai Forest
chimpanzees. The honey-fishing tools made by these chimpanzees were
significantly longer than their ant-dipping tools, and both of these types of tools
were significantly longer than the nut-emptying and bone marrow-scraping tools.
Similarly, Stanford et al. [2000] reported that Eastern chimpanzees at Bwindi,
Uganda, manufactured short stick tools to obtain honey from stingless-bee
(Meliponula bocandei) nests, and significantly longer, thicker tools to acquire the
honey of African honeybees (Apis mellifera).
A population of chimpanzees (estimated density=0.78–1.06 weaned individuals/km2 (Hicks et al., unpublished results)) inhabits the dense, semideciduous,
moist, lowland Ngotto Forest on the north bank of the Mbaéré River in Central
African Republic (Fig. 1). These chimpanzees belong to the Central African
subspecies Pan troglodytes troglodytes, which remains relatively unstudied
compared to the Eastern and Western subspecies. This paucity of knowledge
extends to their tool-use traditions [McGrew, 1992]. In this paper we provide a
detailed description of the different types of chimpanzee tools found during a 7month survey of the Ngotto Forest to the south of the village of Grima, and
compare the tools with tool types found at other sites. In addition, the lengths of
two of the tool types discovered at Ngotto (ant-dip tools and honey probes) are
compared with one another. We expected that the chimpanzees would construct
stick tools of different lengths depending on whether they were using them for
honey-probing or ant-dipping.
MATERIALS AND METHODS
Participants
The chimpanzees (Pan troglodytes troglodytes) of the Ngotto Forest live in a
semideciduous, moist, lowland tropical forest/swamp environment approximately
8 km south of Grima, the village which served as the project base (41010 N, 171040
E) [Brugiére et al., 1999] (Hicks et al., unpublished results). They share the forest
with western lowland gorillas (Gorilla gorilla gorilla), and their diet consists
primarily of fruits and leaves (personal observation). Roger and Deborah Fouts
initiated the Grima project in 2000 in an effort to document the gestural and
material traditions of an unstudied population of chimpanzees, as well as to
promote the conservation of the chimpanzees and gorillas living in the forest.
Throughout the duration of the project they coordinated the research and
conservation efforts from the United States. The study was conducted between
the dry and early wet seasons, from November 2001 to May 2002. The research
team was made up of Thurston Hicks, Research Director Philip Hoffman, and six
Bofi-speaking assistants from the village of Grima, who were experts in
identifying the flora and fauna of the region. Since this population of chimpanzees
was not habituated to humans, only indirect trace evidence, such as tools and
feeding remains, could be obtained. During the study period the northeast section
of the forest was being selectively logged by the timber company Industrie
Forestière du Batalimo (IFB), and logging transects had recently been cut across
the northern part of the study site. The southern forests adjacent to the Mbaéré
224 / Hicks et al.
Fig. 1. Map of the Ngotto Forest study region between the Lobaye and Mbaéré rivers in Central
African Republic.
River had not yet been surveyed by the timber company, and this is where the
research team made the majority of its ape contacts and found a large number of
ape nests.
Chimpanzee Tool Use in the Ngotto Forest / 225
Materials
The materials used in this study consisted of a steel tape measure with plastic
casing (in units of inches), data notebooks and pens, and a GARMIN 2001 model
(Garmin AT, Inc., Salem, OR) geopositional satellite (GPS) device that recorded
the coordinates of the tool sites. A 35-mm camera was used to photograph the tool
sites. The GPS Utility mapping program (version 4.04.0, copyright Alan Murphy,
1998–2002, licensed to Friends of Washoe, CAR Project, Grima) was used to
construct the tool map shown in the Results section.
Procedure
We found tools in the forest while walking along survey transects or taking
random forest walks in the 87 km2 study area. For the purposes of our study, the
forest types were divided into seven categories, as defined in Table I. In his survey
of 92,829 ha of the Ngotto Forest north of the Mbaéré River (which included our
study area), Bastin [1996] divided the forest into nine types, only seven of which
were present in the study area of the current project. Three of Bastin’s categories
corresponded to our dense, mixed, and open forest areas, and made up 21.6%,
31.2%, and 20.8%, respectively, of the forest area surveyed by his team. In all, 13%
of the forest surveyed by Bastin was swamp forest, and 1.6% and 1.9% were logged
forest and degraded forest, respectively (Bastin’s remaining forest types were rare
or absent in our area). Although we made no exact measures of the relative
proportions of the forest types that we surveyed, we found (as did Bastin) that
mixed forest was the most common vegetation type, followed by dense forest and
then open forest, with swamp forest restricted to 1–2 km on the banks of the
Mbaéré River.
Tool sites were easily recognized by the presence of one or more sticks (or in
two cases, vines). Often these sticks had been ripped from nearby small trees and
were usually stripped of their leaves, and sometimes of their bark, in association
with an underground beehive or ant nest. A stick was classified as a tool if it was
associated closely with an insect colony, had been stripped of its leaves, had been
broken off from nearby trees, and/or showed blunting at the end (presumably
caused by probing). In many cases, clusters of tools were found in or beside the
insect nests in piles, and could be dated to several different time periods. We
assessed a tool’s age by the freshness of the exposed sections that had been
TABLE I. Definition of Forest Types Surveyed in the Ngotto Forest
Forest type
Open forest–closed-canopy, open-understory, dry-ground forest
Mixed forest–intermediate between open and dense dry-ground forest
Dense forest–canopy relatively open with dense, closed understory
Open swamp forest–relatively closed canopy, open understory, seasonally-flooded forest
Closed swamp or river forest–relatively open-canopy, closed understory seasonally
flooded swamp or riparian forest
Swamp-edge tangle–extremely dense strip of vegetation running along edge of swamp
Herb patch (thicket)–open canopy forest with dense covering of Marantaceae herbs on forest
floor
Logging road–8-m wide logging access road, along which trees are felled
Logged forest–dry-ground forest in which tree-cutting operations are underway,
often crisscrossed with access roads
226 / Hicks et al.
stripped of bark, as well as by using other clues, such as fresh ape prints or nests
at the tool site. At three fresh ant sites and four fresh honey sites, chimpanzee
prints and/or dung were found in association with the tools, and at the majority of
sites one or more tools were found inserted in the hive or mound. No sign of
gorillas (e.g., dung, footprints, or food remains) was ever found in association with
any of the tool sites. Because tool use is unknown in free-living gorillas [McGrew,
1992], it is assumed that all tools found in this study were used by chimpanzees.
At the fresher honey sites, wadges (masses of beeswax that have been sucked on
by apes to extract honey) were sometimes found lying beside the open holes, and
occasionally showed ape tooth marks. Tools were found scattered on the ground
beneath trees containing beehives. Ant-dip tools were usually found inside or next
to a subterranean bivouac at the base of a tree, or in association with columns of
ants moving away from the nest. Sticks had not been merely selected by the
chimpanzees from debris on the ground, but had in many cases been broken off
from small trees in the area around the insect colony, which we verified by
matching the breaks in the tools with the ends of nearby severed branches or
saplings.
Depending on whether they were found in association with ant nests or
beehives, the stick tools were classified as ant tools or honey tools. Within each
category, the tool types were further classified as ant-dip tools and ant clubs, and
honey probes and honey hammer/clubs.
We measured all tools found in the field lengthwise in inches to the nearest
half-inch, using the standard steel tape measure. The measures were later
converted to centimeters. The Griman assistants held the tools in place during
the measurements. With the exception of four tools brought back to the United
States, for which measures of diameter were acquired, no measures of tool weight
or diameter were taken.
In addition to the measurements of the tools, the following information was
recorded at each site: the number of tools at the site, the distance of each tool
from an ant or bee hole, the estimated age of the tool based on the freshness of the
sap or wood, the Bofi language name for the species of sapling used, the presence
or absence of a source tree for the tool within a 25-m2 radius, the type of habitat
for the tool site, and any additional evidence of chimpanzee presence (e.g., hairs,
dung, prints, etc.). Bofi names were used to identify the bee and ant species, which
were identified by T. Hicks. For the most part the ants were Dorylus, and the bees
were various stingless species (probably Halictidae and Trigonidae) and, rarely,
stinging bees.
T. Hicks drew detailed maps of the tool sites, made photographs, and
recorded GPS waypoints to provide satellite coordinates for the sites. The
waypoints were used to construct a 1:64,308 map of the location of the tool sites
relative to our camp waypoints. No GPS waypoints with errors of 413 m were
used, and the majority of error readings were r10 m.
The research team made a 1-week trip through the forest at the beginning of
the project to ensure the accuracy of the instruments and establish the methods
used. As a test trial for our methods, tools were identified based on the criteria
mentioned above and measurements were made. Beginning with the second
forest trip, all tools were included as part of the final data set.
RESULTS
As shown in Table II, a total of 102 individual tools were found at 25 ant-dip
and honey-probe tool sites. With the exception of two that were made from vines,
Chimpanzee Tool Use in the Ngotto Forest / 227
TABLE II. Ngotto Tool Sites: Ant Dips and Honey Probes
Tool-types
Ant Dip
Tools
Honey
probes
Total
Tool number
(n)
% of total
tool sites
% of
total
tools
13
46
52
12
56
25
102
Tool sites
(n)
Mean
length
(cm)
SEM
45
84.6
4.8
Dorylus
48
55
58.1
3.0
100
100
na
na
Terres-trial
stingless
bees
na
Insect
species
the tools discovered were slender branches ripped from nearby small trees. In
some cases their ends were blunted, and they were usually found without leaves,
which suggests that they had been stripped. Eleven of the ant tools and 11 of the
honey tools were found still projecting from insect holes.
Two additional kinds of toolsFthe ant club and honey hammer/club–were
also found. Because of the small sample size of these tools, they were excluded
from the quantitative analysis of tool length, although they are described in the
text. All tool sites, with one exception, were found in mixed forest, the most
common forest type. The exception was one ant-dip tool found in open forest.
Mixed forest was also more likely to contain chimpanzee nests than gorillas: in a
transect study conducted in the same forest region by Hicks et al. (unpublished
results), 74% of chimpanzee nests but only 12% of gorilla nests were found in
mixed forest. Please see Fig. 2 for a map of the distribution of the different tool
types across the southern section of the study area. As can be seen, the majority of
the tool sites were found in the southern forests adjacent to the Mbaéré River,
where no logging or logging surveys had yet occurred. Photographs of the four
different tool types can be seen in Fig. 3.
Tool Type 1: Ant-Dip Tool
Ant-dip tools were typically long, thin wands that apparently had been
stripped of their leaves and were found either protruding from or lying next to the
subterranean bivouacs of driver ants. The bivouacs were often located in holes or
mounds at the base of a tree. Two of the tools were found with leaves still
attached. Nine had been ripped from small trees found within 10 m of the ant
hole. The mean length for the ant-dip tools was 85 cm (SEM=4.8). The average
number of tools per site was 3.54 (SD=2.15), with a minimum of one and a
maximum of eight tools at a site. At some sites, the tools clearly dated from
different time periods, indicating that the chimpanzees had revisited ant-dipping
sites. At one site, following a contact with chimpanzees, the prints of an adult and
an immature chimpanzee were found in the dirt around a large driver ant
bivouac, and driver ants were swarming aggressively up the several sticks found
projecting from and lying around the ant holes. Older tools were found at this site
as well. The ant holes were often quite deep. The three holes that were measured
were 61, 91, and 91 cm deep, respectively. On two occasions the tools were found
in a foraging ant swarm away from the bivouac. On one of these occasions, it was
apparent from the footprints that the chimpanzee had bent down a small sapling
and sat on it while fishing for ants in the leaves (a ‘‘sapling seat’’?). At this site,
along with several stick tools, a vine tool was found projecting from the churned
228 / Hicks et al.
Fig. 2. Map of distribution of tool sites in the study area, north of the Mbaéré River.
dirt. This was one of only two cases of a vine being used by a chimpanzee to dip for
ants.
The Bofi names for the types of stick used to make 38 of the ant-dip tools
were recorded. Unfortunately, the Latin names were not available.
Tool Type 2: Honey Probe
The chimpanzees used honey tools to probe the ground and log holes of
stingless bees to withdraw wax and honey. Tools were found projecting from or
lying to the side of the holes, which often had bees still inside. Two honey tools at
two sites were found projecting from holes in rotting logs, and the rest were
associated with ground holes. None of the tools had leaves attached, and three
were found with stripped leaves lying beside the tool. Two tools at two sites were
found with their bark peeled off, strips of which were found lying beside the tools.
Seventeen of the tools had been ripped from small trees within 10 m of the honey
Chimpanzee Tool Use in the Ngotto Forest / 229
Fig. 3. a: Ant-dip site, with two ant-dip tools projecting from a bivouac at the base of a tree. b: Two
ant clubs lying next to a fallen solebumu nest. c, d: Two honey-probe tool sites. Ape hairs were
found inside the beehole (c), and bits of chewed wax can be seen around the tool (d). e, f: Two
images of the first honey hammer/club described in the text.
230 / Hicks et al.
Fig. 3. Continued.
site. The mean length of the honey probes was 58 cm (SEM=3.0). The average
number of tools per site was 4.67 (SD=3.9), with a minimum of one and a
maximum of 15 tools at a site. As with the ant tools, at some sites the tools dated
Chimpanzee Tool Use in the Ngotto Forest / 231
Fig. 3. Continued.
232 / Hicks et al.
from different time periods, indicating that the chimpanzees revisited honey sites
as well. At three fresh honey sites, wadges of beeswax were found lying beside the
open holes, and in two cases showed ape tooth marks. One of these ground holes
had been dug into, and the extracted dirt lay next to the hole.
Local humans, as well as the chimpanzees, harvested honey from the
subterranean stingless-bee nests (personal observation). The humans often used
their hands to dig a meter or more into the nests to obtain honey. Chimpanzee
excavations were shallow in comparison to the human excavations, and were
apparently limited to the upper chamber of the hive. The five ground honey holes
that were measured were approximately 28, 8, 8, 31, and 25 cm deep, respectively.
These holes were much shallower than the ant holes.
The chimpanzees used various types of small trees to make honey-probe
tools. The Bofi names for the types of stick used to make 51 of the honey tools
were recorded. Latin names were not available.
Ant-Dip Tool and Honey-Probe Comparison
The ant-dip tools were significantly longer than the honey probes: t=4.739,
Po.01 (n1=46, n2=56). With the exception of the mobaba tree (which was used to
make 10.5% and 13.7% of the ant and honey tools, respectively), there was no
overlap in the type of tree used in constructing the ant-dip tools and honey
probes.
Tool Type 3: Honey Hammer/Club
Two examples of the honey hammer/club were found, each at a different site.
Although these tools differed from one another in length and shape, they were
included in the same category because both apparently were used to forcefully
pound into arboreal beehives.
On 19 April 2002, immediately following an observation of three adult
chimpanzees feeding on figs, a large number of chimpanzees vocalizing excitedly
along with a chimpanzee apparently using a honey-hammer/club were heard to
our north. For approximately 3 min, the loud hammering sound coming from the
upper canopy continued. According to a Griman assistant, a chimpanzee was
trying to open up an arboreal beehive, a behavior the assistant said he had seen
before. As we approached, we heard a loud crash from the direction of the
hammering, followed by the scream of a chimpanzee apparently falling out of the
canopy. Upon investigating, we found that an enormous dead tree had just fallen.
Wet sap from the vines that had come down with the tree was splattered across
the scene. Inside a knot halfway up the tree, bees were visible flying around the
entrance to their hive. Beneath the tree, one of the assistants found what
appeared to be a tool made from a short section of tree branch. The tool was
shaped like a dagger and fit neatly in the hand. It measured 35.6 cm in length and
2.4 cm in diameter. The end of the tool was freshly blunted, with the tip frayed
and pushed back, consistent with it having been used to pound against a tree
trunk.
The second hammer/club was found on another day at the base of a tree, lying
on top of a pile of freshly severed vines. The assistants said that a chimpanzee had
slashed the vines with his or her teeth to clear the space around the beehive 15 m
up in the tree. Apparently the chimpanzee had pounded on the nest with the thick
club to open it, and then dropped the club on top of the pile of vines below.
Chimpanzee Tool Use in the Ngotto Forest / 233
We could see the stinging bees (possibly Apis mellifera) still swarming around the
mouth of their hive. The club measured 88.9 cm in length.
Tool Type 4: Ant Club
We discovered beneath two recently-made chimpanzee nests a large
solebumu ant nest that had been knocked down that day from the limb of an
overhead moseteke tree. The solebumu nest was a brown, brittle, football-shaped
nest constructed from what we assumed to be leaves. It resembled the arboreal
Crematogaster nests described by Goodall [1964, 1986]. A large moseteke stick had
been ripped from a fallen branch 5 m to the east, and apparently had been used as
a club to punch six deep holes into the ant nest. This club, lying just beside the
fallen ant nest, was still swarming with ants, which are quick and painful biters.
A second, thinner moseteke stick tool was found lying next to the nest as well, also
covered in ants. The larger of the two tools was approximately three times as
thick as the other, and measured 91.4 cm in length. The smaller tool, which
resembled an ant-dip tool, was 83.8 cm in length.
DISCUSSION
As has been found in other populations of Pan troglodytes [Boesch & BoeschAcherman, 2000; Humle & Matsuzawa, 2002; Stanford et al., 2000], chimpanzees
of the Ngotto Forest selected or constructed tools of different lengths depending
on the function of the tool. The tools used to dip for ants were longer than those
used to probe for honey.
Ecological Factors
No difference was found in the habitat types in which the tool types were
encountered (they were mixed forest only, with one exception). However, other
subtle differences in habitat cannot be ruled out. Other than the mobaba tree
(which was used to make 10.5% and 13.7% of the ant and honey tools,
respectively), there was no overlap in Bofi tree types used to construct the antdip and honey probe tools. Selection of different plant species as raw materials for
the two tool types cannot be ruled out, as was documented at Mount Assirik,
Senegal, by McBeath and McGrew [1982], and this may explain the difference in
the tool lengths.
Between-Site Comparisons: Ant-Dip Tools and Honey Probes
As can be seen in Table III, the Ngotto chimpanzees used longer ant-dip
tools than the chimpanzees at Tai, Gombe, and Bossou. The chimpanzees of
Gombe and Tai use different methods to dip for ants [Boesch, 1996]. The Tai
chimpanzees sweep the ants directly from short rods with their lips (the mouthsweep technique). The Gombe chimpanzees pull long wands through their free
hand, catching the ants in that hand and then putting them in their mouths (the
pull-through technique) [McGrew, 1974]. At Bossou, the chimpanzees use both
techniques: wands are used for the pull-through technique, and rods are used for
the mouth-sweep technique [Humle & Matsuzawa, 2002]. In that study, the pullthrough technique was used in situations in which the chimpanzees were more
likely to get bitten, such as at the ant bivouac. They used this technique most
often with the aggressive black driver ants, and the mouth-sweep technique was
used with the less-aggressive driver ant species of the area. The length of the
234 / Hicks et al.
TABLE III. Driver Ant Dip Tool Between-Site Comparison
n
Mean length
(cm)
SEM
Range (cm)
Reference
Ngotto
Tai
46
35
84.6
23.9
4.8
–
24–152
11–50
Gombe
Bossou
13
189
66.0
53.7
–
1.5
15–113
23–154
This study
Boesch and Boesch
[1990]
McGrew [1974]
Humle and Matsuzawa
[2002]
TABLE IV. Honey Tool Between-Site Comparison
Ngotto
Tai
Boesch
Bwindi stinging bees
(Apis mellifera)
Bwindi stingless bees
(Meliponula bocandei)
n
Mean length
(cm)
12
45
SEM
Range (cm)
Reference
58.1
28.1
3
–
6.5–99
14–60
9
60.0
–
25–85
12
27.0
–
14–70.5
This study
Boesch and Boesch
[1990]
Stanford et al.
[2000]
Stanford et al.
[2000]
sticks measured in this study suggests that the Ngotto Forest chimpanzees use
the pull-through method.
As shown in Table IV, the mean length of the Ngotto honey probes is similar
to that of the sticks used by the Bwindi chimpanzees to probe for the honey of
stinging bees [Stanford et al., 2000]. Within the Bwindi population, a significant
difference exists between the lengths of the longer sticks used to acquire stingingbee honey and the shorter sticks used to acquire stingless-bee honey. The Bwindi
short sticks are almost as long as the Tai honey tools.
Between-Site Comparisons: Honey Hammer/Clubs, Ant Clubs, and
Ant-Fishing Perches
Tools resembling the ant clubs and honey hammer/clubs of Ngotto have been
observed at other sites. In the extreme southwest corner of Central African
Republic, Central chimpanzees were observed by Fay and Carroll [1994] at the
Bai Hokou and the Ndakan research sites using wooden hammers of up to 10 cm
in diameter to smash open both arboreal and terrestrial nests of meliponine bees.
Fay and Carroll also found circumstantial evidence of chimpanzee honeyhammering across the border in the People’s Republic of Congo, indicating that
P. t. troglodytes engages in this behavior across a wide geographical area. If the
Ngotto Forest chimpanzees, who live 210 km to the north-northeast of Ndakan,
are included as honey-hammerers, the area across which this type of tool use
occurs becomes enormous, and honey-hammering could be identified as a
characteristic large-scale tool use pattern of the Central chimpanzee. There have
been few observations of this behavior in the other subspecies of chimpanzee. J.
Hart, in a personal communication to McGrew [1992], described finding stick
Chimpanzee Tool Use in the Ngotto Forest / 235
clubs in the Ituri Forest that had been used by Eastern chimpanzees to open up
beehives in hollow trees. Brewer and McGrew [1990] reported the use of a branch
as a chisel by a forest-born, rehabilitated female Western chimpanzee on an island
in the Gambia. The chisel, which was part of a tool set of four different-sized
branches and vines, was used to pound into the batumen of an arboreal stinglessbee nest to acquire honey.
There are very few reports of the use of clubs to punch holes into arboreal ant
nests in chimpanzees. Goodall [1986] observed chimpanzees at Gombe using
sticks to force openings into the arboreal nests of ants, possibly in the same
manner in which the two ant clubs found at Ngotto were used.
Only once was a possible example of a ‘‘sapling seat’’ seen in the Ngotto
Forest, but the Griman assistants claimed that the chimpanzees of the area
commonly bent saplings as perches while fishing for driver ants. This behavior
was described in the Gombe chimpanzees by McGrew [1974].
Insect Holes
With regard to the ant and stingless-bee ground holes, humans sometimes
dug down to a meter beneath the ground to excavate the deeper chambers of
stingless-bee holes, while the chimpanzees apparently were limited to probing
into the upper chamber (the part easily accessible from the surface). The ant
holes were much deeper than the upper chambers of the stingless-bee holes. This
may be why the chimpanzees were using longer sticks for ants. A future study
could compare the depths of insect colonies at different chimpanzee study sites to
correlate them with tool length.
Termite Fishing
Termite fishing is a common practice in many chimpanzee populations across
Africa [McGrew, 1992]. No termite-fishing tools were observed during the 7month study at Ngotto. No obvious Macrotermes mounds were observed by T.
Hicks in the forest. However, since the study was conducted primarily in the dry/
early wet season, we cannot rule out the possibility that the Ngotto chimpanzees
engaged in some sort of termite fishing during the rainy season.
Future Research
Future studies of the Ngotto Forest chimpanzees should focus on direct
observations of the apes constructing and using their tools. This would help
determine whether the chimpanzees reuse old tools that have been left at
frequently-visited sites (as documented at Gombe by McGrew [1974]), and
whether they modify their tools prior to use, as was observed by Goodall [1964]
and McGrew [1974]. Direct observations of chimpanzees using honey hammer/
clubs and ant clubs would be valuable as well. Unfortunately, with the whole of
their forest currently in the process of being logged, the future survival of this
population and its tool traditions are not assured.
CONCLUSIONS
The evidence presented here is a further demonstration of the impressive
behavioral diversity of chimpanzee populations [Boesch, 1996; Whiten et al.,
2001]. Chimpanzee culture is a powerful explanatory factor for this diversity. By
continuing to document the full range of chimpanzee material and behavioral
236 / Hicks et al.
culture in both little-known and habituated populations [McGrew et al., 2003], we
move toward increasing our understanding of the depth and complexity of
tradition beyond Homo sapiens.
ACKNOWLEDGMENTS
Data from this paper were originally presented to Central Washington
University in partial completion of a Master of Science degree. We thank Bernard
Voyemakoa, the Ministre et Directeur Général des Recherches Scientifiques et
Technologiques of Central African Republic, for allowing us to conduct our
research in the Ngotto Forest. We also thank Hillary Fouts, Alain Penelon of
ECOFAC, Chief Dakamoli, Dibesco Pilandon, Ngasa Bernard, Pilabo Martin,
Wanley Bemina, Samedi Claude, Fami Eve, Justin Kula, Tim Tikouzou, J.-B.
Bobi, and Brice for their help with the project.
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