C 2006)
International Journal of Primatology, Vol. 27, No. 6, December 2006 (
DOI: 10.1007/s10764-006-9091-7
Archaeological Analysis Does Not Support
Intentionality in the Production of Brushed
Ends on Chimpanzee Termiting Tools
Jason L. Heaton1,2,5 and Travis Rayne Pickering2,3,4
Received April 26, 2005; revision September 7, 2005; accepted October 18, 2005; Published Online December 5, 2006
Some chimpanzees use 2 types of tools to extract underground termites for
consumption. Chimpanzees insert thin, flexible probes into tunnels or holes
in termite mounds (fishing), and sometimes use stouter, rigid sticks to first
puncture the holes and also possibly to fish. Many puncturing sticks have
distinctive “brushed” ends. Researchers have hypothesized that chimpanzees
create the brushed ends intentionally to increase their affixibility to biting termites (Sugiyama, 1985). The results of our archaeological analysis of a large
collection of puncturing sticks used by Central African chimpanzees falsifies
this hypothesis, and instead agrees with the recent behavioral observations of
Sanz et al. (2004; cf. Bermejo and Illera, 1999) that brushing is a coincidental result of procuring sticks from vegetation sources. The results highlight
the positive contribution of an archaeological approach to problems in chimpanzee material culture and emphasize to primatologists the value of curating
artifacts.
KEY WORDS: archaeological approach; artifacts; Central Africa; chimpanzees; extractive
foraging.
1 Department
of Biology, Samford University, 800 Lakeshore Drive, Birmingham, Alabama
35229.
2 Stone Age Institute, 1392 West Dittemore Road, Gosport (Bloomington), Indiana 47433.
3 Department of Anthropology, University of Wisconsin-Madison, 1180 Observatory Drive,
5240 Social Science Building, Madison, Wisconsin 53706.
4 Institute of Human Evolution, University of the Witwatersrand, WITS 2050, Johannesburg,
South Africa.
5 To whom correspondence should be addressed; e-mail: jlheaton@samford.edu.
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C 2006 Springer Science+Business Media, LLC
0164-0291/06/1200-1619/0
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Heaton and Pickering
INTRODUCTION
Primatologists have seemingly been more willing than archaeologists to draw similarities between chimpanzee and early hominid behavior (Boesch and Boesch, 1989; Kortlandt, 1980, 1986; McGrew, 1979,
1981, 1991, 1992, 2004; Stanford, 1995; Suzuki, 1975; Teleki, 1974, 1975;
Wrangham, 1987; Wynn and McGrew, 1989). However, a few archaeologists have successfully applied behavioral studies of chimpanzees to their
investigations of the Stone Age (Isaac, 1987; Schick et al., 1999; Steele, 1989;
Toth and Schick, 1986; Toth et al., 1993). We approach the interface of primatology and archaeology from yet another direction. We aim to demonstrate that a traditional archaeological-inferential approach to 1 component
of chimpanzee material culture, brush sticks, is a way in which one can productively test hypotheses of chimpanzee cognition that are based on scant
behavioral observations.
Chimpanzees use a wide assortment of tools that vary regionally
among populations, with a general sense that the regional differences are
attributable to distinct cultural traditions (McGrew, 1992; Whiten et al.,
1999; contra Byrne, 1995). Chimpanzees show ingenuity and flexibility in
using the tools to fulfill basic daily needs and to solve problems. Well known
among numerous examples of tool use are Goodall’s (1964) classic observations of Gombe chimpanzees fashioning vines, twigs, and bark into probes,
used subsequently to fish for insects dwelling underground. Insect fishing
occurs in several chimpanzee populations and is one of the most technologically variable patterns of extractive foraging (McGrew, 1992; Whiten et al.,
1999). In particular, fishing for underground termites (usually Macrotermes
spp.) is linked to at least 2 distinct tool types: 1) the thin, flexible probes that
are inserted into tunnels or holes in termite mounds and 2) stouter, more
rigid sticks, which may be primary implements by which the holes are punctured into which the probes could be inserted (Bermejo and Illera, 1999;
Byrne, 1995; Fay and Carroll, 1994; Jones and Sabater Pı́, 1969; McGrew
et al., 1979; Muroyama, 1991; Sabater Pı́, 1974; Sanz et al., 2004; Sugiyama,
1985; Suzuki et al., 1995).
Both tool types sometimes exhibit frayed or brushed ends. Bermejo
and Illera (1999) inferred that brushing on the thin probes recovered from
the Lossi Forest (Congo) had been “made by [chimpanzee] chewing because teeth marks or saliva were often found on the brush area” (p. 623);
behavioral observations of chimpanzees from the Goualougo Triangle (Republic of Congo) later corroborated the observation (Sanz et al., 2004).
However, Bermejo and Illera (1999, p. 265) also stressed that the origin of
the brushing on the puncturing sticks was enigmatic, which McGrew (1992,
p. 157) had elaborated earlier:
Intentionality and Brushed Ends on Chimpanzee Tools
1621
A variation of the probing [puncturing] stick is the brush-stick. . . . The end of the
tool inserted into the [termite] mound was frayed to resemble a paint-brush, apparently to increase its ‘affixibility’ to the biting insect defenders. . . What is not clear
is whether such fraying was done deliberately by pounding with a hammer-stone
(Sugiyama, 1985) or chewing with the [chimpanzee] molars. . ., or was an inadvertent
by-product of wear through repeated use (McGrew and Collins, 1985). Behavioral
data are needed.
Sanz et al. (2004) provided the behavioral data 12 yr later; they
employed video monitoring technology to observe chimpanzees in the
Goualougo Triangle making and using probe and puncturing/perforating
sticks. The observations confirmed the hypothesis that puncturing sticks
are, in fact, used for that function. They also seemed to falsify the hypothesis
that brushing on the puncturing sticks was a result of intentional behavior
by chimpanzees aimed at increasing their affixibility.
We have recovered what Sugiyama (1985) described as ‘brush sticks,’ but these did
not seem to have a particular function in termite predation. Rather, it seems that
the sticks’ appearance (puncturing sticks with frayed ends) was a by-product of the
sticks’ removal from the tree source. Bermejo and Illera (1999) also suggest that the
fraying at the end of the sticks was caused by breaking the tree stem with a backand-forth movement. Rather than serving as an important component in the termite
extraction process, these ends quickly become blunted with repeated insertion into
the mound. Our assertion that the brushes on puncturing sticks are not important in
puncturing or fishing is further supported by Sugiyama (1985) and our observations
that all these tools that were found inserted into the nest matrix had the “brush”
end in the air. (Sanz et al., 2004, p. 579).
We present a material and technological analysis of a large assemblage
of brush puncturing sticks collected in Central Africa during the 1960s. Our
results corroborate the important inferences of brush formation, which are
based on visual observations in the field. Such concordance from variable
lines of evidence provides the strongest tests of competing hypotheses. In
addition, we hope our results provide a convincing example of the successful application of archaeological methodology to problems in primatology,
and demonstrate again the productivity of a multidirectional flow of intellectual engagement from workers specializing in disparate research on the
Hominidae.
MATERIALS AND METHODS
Our sample of brush puncturing sticks is the first described for any population of chimpanzees, and consists of 160 specimens collected from various termite (Macrotermes muelleri) mounds in the Okorobikó Mountains
(146 specimens), and the Ayamiken region (12 specimens) of Equatorial
Guinea, and from Dipikar Island (2 specimens), in neighboring Cameroon,
between 1967 and 1968 (Jones and Sabater Pı́, 1969; Sabater Pı́, 1974). The
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Heaton and Pickering
collection is currently housed in the Natural History Museum of Tulane
University (USA), and henceforth we refer to it as the TU sample. The
TU sample showed no evidence of curatorial cleaning, and many specimens were wrapped individually or in bundles bound with string in Central
African newspapers dating to the time of collection.
We measured the length (cm), diameter (mm), and weight (g) of each
specimen. Next, using standard archaeological methods, we scrutinized the
entire surface of each specimen using a × 10 hand lens under a strong,
oblique light source (Blumenschine et al., 1996). We recorded the presence
of adhering matrix (soil) on one or both ends of each specimen, as probable
evidence of puncturing, perforating or digging behavior. The adhering matrix is caked tightly to the sticks, and it would take considerable focused effort to remove it; there is no evidence that this occurred at any stage during
the recovery, transport, and curation of the TU sample. Thus, we are confident that our data on current matrix distribution accurately reflect its original distribution at the time of tool abandonment by chimpanzees. In addition, we closely examined tool ends for striations overlaid on the wood’s
natural morphology and other surface modifications, e.g., tooth marks, that
might have resulted from digging or dental manipulation. We also noted
the incidences of stripped bark and bending and partial breaks that occur mid-shaft on sticks. Finally, we used the observations generated for the
procedures in the preceding text to classify each end of every specimen as
rounded, mashed, rounded and mashed, brushed, or nonmodified (Fig. 1).
A rounded end is a smoothed or tapered point, blunted but lacking any appreciable fraying or splaying of terminal wood fibers. A mashed end shows
little or no rounding but displays significant fringing and lateral and backward bending of terminal wood fibers. A rounded and mashed end combines a rounded termination with some incipient fringing and splaying or
flexion of wood fibers. A brushed end possesses a significantly frayed termination with no or usually fairly unidirectional flexion of long and discretely
separated wood fibers. A nonmodified end displays none of the features.
RESULTS AND DISCUSSION
Average specimen length for the TU sample is mid-range of observed values for puncturing sticks from other chimpanzee sites, such as
Goualougo (Sanz et al., 2004), Guga (Suzuki et al., 1995), Lossi (Bermejo
and Illera, 1999), Bai Hokou, (Fay and Carroll, 1994), Campo (Muroyama,
1991; Sugiyama, 1985), and Belinga (McGrew and Rogers, 1983). For
samples with those sites, mean stick diameters are slightly smaller in all
cases than in the TU sample, regardless of which subsample, brushed and
Intentionality and Brushed Ends on Chimpanzee Tools
1623
Fig. 1. Categories of damage to the ends of sticks in the TU collection: (a) rounded; (b)
rounded and mashed; (c) mashed; (d) brushed.
nonbrushed, is compared. The dispersion of length and diameter values of
the group is similar to that at other sites with similar tool sets (Table I).
In the TU sample, a slight majority of sticks (64.3%) show modification or evidence of use, i.e., adhering matrix, on both ends of the specimen.
Brushed specimens comprise the greatest portion (62.5%) of the tool sample, and the brushed subsample has the greatest amount of variability with
respect to its occurrence with other damage types. Brushes occur with almost all other types of end damage, but the most frequently observed pair
is brush ends with mashing at the opposite end (Table II; Fig. 2). Only a
small portion (4.4 %) of the sample exhibits brushing only.
After our extensive examination of the assemblage, we concluded that
distribution of nonmorphological striations was not always a useful indicator for inferring a stick’s working end(s), i.e., an end that made deliberate
contact with a termite mound. Striations with an ambiguous appearance
and orientation occur on ca. 29% of the total sample. Thus, we determined
working ends of sticks by the presence of matrix coating. Though specimens
exhibiting 2 modified ends based on gross morphology, i.e., rounding, mashing, or brushing, are predominant in the TU sample, we observed adhering
1624
Table I. Lengths (cm) and diameters (mm) of puncturing sticks in the TU sample and those of comparative sitesa
Bai Hakou, Central Africa Republic
Belinga, Gabon
Campo, Cameroon
Campo, Cameroon
Goualougo, Republic of Congo
Guga, Republic of Congo
Lossi, Republic of Congo
Tulane University (TU) sampleb
N
Length (cm)
74
2
110
81
332
66
169
160
58.7 ± 18.3
68, 76
46.8 ± 8.3
44.5 ± 9.2
40.6 ± 14.2
52.7 ± 15.1
56.6 ± 12.9
47.9 ± 12.6
a Adapted from Sanz et al. (2004, p. 577, Table I).
b Sample contains specimens from Okorobikó, Cameroon,
N
72
2
111
81
315
35
159
160
Diameter (mm)
12.3 ± 3.5
14, 18
9.9 ± 1.6
9.0 ± 1.6
9.6 ± 2.3
10.9 ± 2.8
9.8 ± 2.3
13 ± 3
References
Fay and Carroll, 1994
McGrew and Rogers, 1983
Sugiyama, 1985
Muroyama, 1991
Sanz et al., 2004
Suzuki et al., 1995
Bermejo and Illera, 1999
This study
previously described by Jones and Sabater Pı́ (1969); McGrew et al.
(1979); Sabater Pı́ (1974).
Heaton and Pickering
Second modified end
Second
end same
First modified end Both ends brushed
Both ends mashed
Both ends rounded
Unmodified
One end brushed
One end mashed
One end mashed
One end mashed and rounded
One end rounded
Total
Unmodified
10
4
2
16
One end
mashed
One end
mashed and
rounded
One end
rounded
2
3
3
7
3
13
16
12
54
51
18
14
1
55
18
15
Total
10
6
6
3
90
3
13
16
13
160
Intentionality and Brushed Ends on Chimpanzee Tools
Table II. Observed frequencies of end modifications in the TU sample of puncturing sticks
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Heaton and Pickering
100
Mashed
Mashed and Rounded
Rounded
Brush
No Damage
Number of specimens
80
60
40
20
0
One
Both
Brush Modifications
Fig. 2. Types of stick end modifications on the opposing ends of brush-tips in the TU
collection.
matrix on only 1 end for ca. 74% of the sample. Table III is a summary of
matrix distribution by type of modified end. Of the 171 modified ends that
are matrix-coated, only 19.3% (n = 33) are brushed. Thus, relative to other
types of modified ends, it is inferred that brushed ends rarely made direct,
intentional contact with termite mounds. This makes sense, given that unlike a blunt end, a brush will bend fairly uselessly under the force of pounding or digging. An additional observation is that matrix occurs on brushes
on every specimen with 2 brushed ends; with such sticks, chimpanzees had
no choice but to use 1 of these otherwise undesirable ends. In contrast, matrix occurs on brushes in only 24 specimens (26.7 %) that have brushes at
only 1 end.
We observed adhering matrix at a similar frequency among sticks with
an unmodified end (28%) and sticks with 1 brushed end (26.7%). Based
Intentionality and Brushed Ends on Chimpanzee Tools
1627
Table III. Matrix distribution of modified ends in the TU sample of puncturing sticks
Adhering matrix
Stick type
Total
Both ends brushed
Both ends mashed
Both ends rounded
Unmodified
One end brushed
One end mashed
One end mashed and rounded
One end rounded
Not present
Present
Total
10
2
0
41
66
5
5
4
133
10
9
9
16
24
66
29
24
187
20
11
9
57
90
71
34
28
320
on the observations, a case could be made that unmodified sticks are inserted into mounds at roughly the same frequency as brushed specimens,
which could potentially counter the argument that chimpanzees intentionally manufactured brushed ends to attain greater affixibility to biting termites. However, what is really needed to assert the hypothesis with confidence is experimental work testing whether brushing does, in fact, enhance
affixibility when compared to other stick end morphologies; we are currently conducting work on the issue.
In addition, because of their consistent internal structure, woody twigs
and branches fracture in predictable ways. Two types of forces, tensile and
compressive, are transferred through a limb by the stress of lateral-bending
(Fig. 3)—a very likely way in which chimpanzees detach branches from vegetation sources that are then used as tools (Sugiyama, 1985, p. 367, Fig. 8)
The side of the limb that experiences the tensile force fails first, causing an
uneven, and often incomplete break. Depending on the (taxon-specific?)
mechanical properties of the limb, brushing—separation and sometimes the
associated deformation of wood fibers—may occur during a fracture event.
A clear example of brush damage induced by lateral bending of a specimen
in the TU sample is Fig. 4. In contrast, the fibers on the compressive side of a
stick produced by lateral-bending do not fracture, or fracture incompletely,
when a limb is stressed. In addition to lateral bending, a limb subjected to
torsion will splinter around its circumference, resulting in a brush for the
entire end of the specimen (Record, 1914). Five of the TU specimens show
brush features that are consistent with typical breakage from twisting, suggesting strongly that chimpanzees did not produce the brushes intentionally
but formed them during torsional removal from source vegetation (Fig. 5).
Further, some varieties of wood in the TU sample appear mechanically
predisposed to brushing. For example, Figure 5a shows a non-working end
on a stick specimen with defined separation of its wood fibers, but that lacks
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Heaton and Pickering
Fig. 3. Damage to a stick midshaft in the TU collection that illustrates the mechanics of wood
fracture resulting from the strain of lateral bending.
splaying, bending or other distortion of those fibers, as seen in some other
brushes. This suggests that torsional forces had little or nothing to do with
the creation of this morphological form, but rather the exposed wood fibers
are unbundling naturally in the absence of bark to keep them binded together. We refer to this phenomenon as incipient brushing and it accounts
for 54 (49.1%) of the total 110 brushed ends in the TU sample.
Also supporting the contention that brushing results primarily from unintentional actions in puncture stick procurement and use by chimpanzees
is the presence of mid-shaft brushing on 4 specimens (Fig. 5d). The sample
is small but suggestive. The mid-shaft brushing might be related to unsuccessful or simply abandoned attempts to remove the branch from its source
stalk at that position. Alternatively, the midshaft damage may represent incomplete stress fractures caused by energy transfer through the sticks as
chimpanzees dug and pounded with them.
Altogether, our study supports the observationally based contentions
of Bermejo and Illera (1999) and Sanz et al. (2004) that brushing on puncturing sticks is a by-product of their detachment from vegetation sources—and
possibly some partial fracturing as a consequence of non-brush use—and
not a result of intentional design by chimpanzees.
Intentionality and Brushed Ends on Chimpanzee Tools
1629
Fig. 4. Stick from the TU collection that preserves evidence of a lateral bending fracture and
its associated brushing.
CONCLUSIONS
Observations of chimpanzees have demonstrated that tools are used
habitually in a wide variety of settings (Beck, 1980; McGrew, 1992, 2004).
In addition, chimpanzees are capable of choosing between alternative techniques that can most effectively accomplish a task, and researchers have
suggested this type of behavioral flexibility is evidence of real intelligence
(Byrne, 1995). Thus, the idea of chimpanzees intentionally creating brushes
on puncturing sticks to increase termite foraging yields is probably not beyond their estimated cognitive capabilities. In fact, there is direct observational evidence that the smaller fishing probes used in termite predation are
intentionally modified with brushing (Sanz et al., 2004). However, no such
observational data support the hypothesis of chimpanzee intent resulting
in brushing on puncturing sticks. Similarly, the results of our material and
technological analysis of a large sample of puncturing sticks from Equatorial Guinea and Cameroon also show that the form and distribution of
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Heaton and Pickering
Fig. 5. Examples of unintentional brushing on sticks in the TU collection: (a) incipient brushing; (b) and (c) torsional removal damage; (d) midshaft splintering and brushing (see text for
details).
brushing on the tools are inadvertent by-products. The lines of evidence
that we provide converge: 1) brushing is associated with adhering matrix
on only ca. 20% of the puncture stick ends; 2) brushing is parsimoniously
explained by the response of wood to the mechanical stresses of limb detachment from source vegetation; 3) brushing is commonly expressed as
incipient brushing—straight, but separated wood fibers on nonworking but
barkless ends; and 4) brushing sometimes occurs on the definitively nonworking mid-shaft surfaces of sticks. Considering the behavioral and technological observations in combination, the most parsimonious explanation
of brushing on puncturing sticks is that it is simply the unintended result of
limb removal from a vegetation source (cf. Bermejo and Illera, 1999; Sanz
et al., 2004).
In conclusion, we believe that other debates on the potential and
limitations of the chimpanzee mind, previously predicated solely or largely
on behavioral observations, might be usefully supplemented with a more
rigorous investigation of the actual technology as reflected in artifacts.
Intentionality and Brushed Ends on Chimpanzee Tools
1631
Progress in such debates would benefit from more systematic and detailed
description of tool types. The use of consistent intraspecimen features to
form typological variables, much like that for the archaeological study of
lithic remains, will advance our specific understanding of damage and/or
modifications resulting from chimpanzee tool-using behavior. Direct observation of chimpanzee behavior is the best source of data, but technological
studies of their residual material culture can be a powerful complementary
source of hypothesis testing. Thus, we urge primatologists to, whenever
possible, collect and curate chimpanzee artifacts to study themselves or in
collaboration with archaeologists.
ACKNOWLEDGMENTS
We thank Hank Bart and the Natural History Museum of Tulane
University for permission to analyze the TU collection and for facilitating
its convenient study. We thank Clyde Jones and Jorge Sabater Pı́ for
encouragement to undertake this work; it was through their early field
efforts that this material is available to primatologists and paleoanthropologists alike. We thank Kevin Hunt and Bill McGrew for much advice
and encouragement and Bill McGrew, Linda Marchant, Jim Moore, and
Craig Stanford for helpful comments on an early draft of the article and 2
anonymous reviewers for comments on a later draft. J. L. Heaton thanks
Dan Osborne for his statistical advice and Anne Heaton for help with
specimen photography, Nick Toth and Kathy Schick for teaching him
the value of experimental archaeology and its application to problems
of paleoanthropology, and his family for their unconditional support of
his research. T. R. Pickering initiated the study as a faculty member in
the Department of Anthropology at Tulane University and is grateful to
his former colleagues there, Will Andrews, Harvey Bricker, Dan Healan,
Trenton Holliday and John Verano, for their camaraderie and support
during that time. T. R. Pickering thanks his family and Nick Toth and
Kathy Schick for their continued and unwavering support of his work.
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