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. 1619 C 2006 Springer Science+Business Media, LLC 0164-0291/06/1200-1619/0
1620 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 1622 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 1625 1626 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 1628 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 1630 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. 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