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Chimpanzee origami? Pith-folding at Toro-Semliki Wildlife Reserve, Uganda
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William C. McGrew1 and Kevin D. Hunt2
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Leverhulme Centre for Human Evolutionary Studies, Department of Biological
Anthropology, University of Cambridge, Fitzwilliam Street, Cambridge CB2 1QH, UK
e-mail: [email protected] telephone: 44 1223 764706 fax: 44 1223 764710
Department of Anthropology, Indiana University, Bloomington, IN 47405, USA
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Abstract All well-studied populations of wild chimpanzees (Pan troglodytes) orally
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compress detached vegetation between tongue and palate, then discard it after juices have
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been extracted (‘wadges’). Wadging is a form of food-processing, typically focussed on
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fibrous bark, pith, or fruit, that yields a characteristic remnant or artefact. Most wadges
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are jumbled, amorphous masses, but here we report a newly-discovered technique, from
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the chimpanzees of Semliki, Uganda. Various lengths of pith stripped from wild date
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palm (Phoenix reclinata) fronds are repeatedly folded to produce a characteristically-
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shaped artefact. (Systematic folding of plant matter for any reason is rare in apes, e.g.
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chimpanzees at Bossou, Guinea, fold leaves to extract water from tree-holes). Number of
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folds per wadge was correlated with length of pith, i.e. longer piths had more folds,
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presumably because it was necessary to fit the material into the confines of the mouth.
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However, 83% of wadges unexpectedly had an odd number of folds, a highly-skewed
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departure from an expected 50:50 distribution. There is no obvious reason why a
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concertina-style, folded object should have an odd or even number of folds, but the
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explanation was supplied by observations of another Ugandan population of
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chimpanzees. The Kanyawara community at Kibale sometimes processes the pith of
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papyrus (Cyperus papyrus) in a particular way that produces similarly-folded remnants.
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Key words Pan troglodytes, diet, food process, material culture, object manipulation
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Introduction
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Chimpanzees make and use tools on a daily basis, in a variety of arenas, but most of their
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tool manufacturing is transforming vegetation by stripping, peeling, splitting, crushing,
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clipping, etc. In the well-known examples of termite fish, ant dip, ant fish, leaf sponge,
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etc., the resultant tool is used in extractive foraging (McGrew 1992). In habituated
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populations, behavioural data accompany the artefacts, so that observers see precisely
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how the tools are made and why, e.g. twigs for fishing may be peeled off bark, in order to
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facilitate their smooth insertion into tiny holes.
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However, chimpanzees also modify vegetation when not making tools, and these
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activities also may leave behind puzzling artefacts. Such are the ‘wadges’ of the pith of
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the wild date palm, Phoenix reclinata, which we describe in this report from unhabituated
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subjects. (‘Wadge’ is a somewhat imprecise English word for such a bolus; ‘quid’ is
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probably better, as in chewing tobacco.) The artefact is obvious when encountered: A
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straight stem is bent repeatedly to alternating sides, concertina-style, so that it has a series
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of folds at acute angles (see Figure 1). These objects are the spat-out products of buccal
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compression, from which juices have been extracted by squeezing them between tongue
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and palate. Their ‘function’ is straight-forwardly nutritional, but the puzzles are why they
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take this distinctive shape and why there are so many wadges with an odd number of
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folds and so few with an even number of folds. We seek to study these puzzles here.
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The only previous report of similar origami is the leaf-folding done by the
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chimpanzees of Bossou, Guinea, who manufacture water-extracting tools (Tonooka et al.
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1994, Tonooka 2001; Biro et al. 2006; Sousa 2009). To make these tools, the apes fold
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sets of one to -four leaves (mostly of Hybophrynium braunianum) at about 3-cm
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intervals, while stuffing them into the mouth. This device is inserted by hand into a tree-
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hole containing water, then extracted and sucked, as a source of sustenance; the sequence
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may be repeated many times before the artefact is abandoned. (Such leaf-folding differs
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from leaf-sponging and leaf-spooning, although all three techniques yield drinking water.
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Tonooka, 2001).
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Wadging, in which a nutritious object is manipulated by mouth but not swallowed,
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seems to be a universal of chimpanzee food-processing technique. Goodall (1986, p. 238)
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described wadging at Gombe in detail, noting a variety of food-items, mostly fibrous
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plant-parts, e.g. figs, bark, etc. In some cases, chimpanzees add leaves to fleshy, ‘rich’-
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tasting foods, such as meat, eggs, honey, overripe fruit, to form a compound wadge. In
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this case, the compressed, homogenised bolus may be swallowed, but the usual result is
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an accumulation of amorphous, jumbled fibre-spheres left on the ground. (Presumably,
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wadges are ejected rather than ingested, because the fibrous mass of foliage is of low
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quality and would take up valuable gut space.) Humans similarly wadge the pith of
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Cyperus papyrus, which is described as “chewy and pleasant tasting” (van der Merwe et
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al. 2008).
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We have find found no previous record of accordion-folded wadges from previous
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studies of chimpanzees or any other species of non-human primates, either in nature or
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captivity. The aim of this report is to document these special wadges and to seek to infer
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how the artefacts end up with a non-random design, that is, a prevalence of odd-
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numbered folds. We predicted that the number of folds would be positively correlated
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with the length of the pithy stem involved; given a standard unit of folding distance, the
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longer the stem, the more folds required. However, we had no clue about the biased
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number of folds.
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Methods
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We studied the wild chimpanzees (Pan troglodytes schweinfurthii) of the Toro-Semliki
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Wildlife Reserve, Uganda, from May-November, 2008 (Hunt 2000). The apes were only
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partly habituated, but much progress was made, so that nest-to-nest follows were
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eventually achieved. If the chimpanzees could not un-nested at the beginning of the day,
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we searched for them, listening for calls and looking for their signs. Once contacted, we
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stayed with them for as long as possible. These apes occupy a largely open habitat, which
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is a mosaic of grassland, scrub, open woodland, galley forest, and swamp (Hunt and
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McGrew, 2002). Their subsistence patterns are mostly the same as those of other
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populations, but unusually among wild chimpanzees, they dig holes by hand in sandy
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riverbeds for drinking water (Hunt and McGrew 2002, McGrew et al. 2007).
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In tracking or following chimpanzees, we took note of any feeding traces left by
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them. For wild date palm wadges, we counted the numbers in an assemblage and noted
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the sources from whence the piths had been detached. Intact and fresh (less than 24 hr
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old) wadges were collected in plastic ziplock bags and returned to camp for processing.
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(In a parallel study, very fresh wadges were sealed in sterile containers for possible DNA
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extraction, on the assumption that buccal cells from the wadgers would be found. Payne,
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unpublished data) In camp, WCM measured wadges on a table-top, to the nearest 0.5 cm,
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from fold to fold, and counted the number of folds. Thus, a folded-pith wadge with three
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folds yielded four measureable segments.
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Results
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Remnants of the chimpanzees’ wadging of wild date palm were found on a daily basis
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over the six-month period of the study. A typical assemblage consisted of several mature
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fronds lying on the ground less than 5m from the presumed source plant. The presumed
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source plant was identified by what appeared to be freshly damaged ends of the intact
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fronds. Source plants were readily apparent because white pith revealed by the damage
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made a stark contrast to the green outer colour of the frond. A typical assemblage
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consisted of several mature fronds lying on the ground, havinge been torn from the
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targeted plant, which was often less than 5m away. It was easy to recognise the damaged
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ends of the intact fronds, as the revealed white pith was obvious in comparison with the
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green outer colour. Detached fronds were split length-wise and clipped cross-wise,
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producing segments of 8-50 cm length. All of the outer epithelium of the segments was
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peeled away, leaving only the grainy, moist, fibrous pith in strips of less than 1 cm
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diameter. There were few of these unused or incomplete strips of pith, by comparison
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with more than 30 wadges left strewn on the ground. (See Figure 1.) These artefacts were
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not scattered randomly, but were often concentrated in areas of less than 50 cm diameter,
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sometimes in a loose pile, as if the wadger had sat still and worked through a set of
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wadges before moving on. Fresh wadges had a distinctive pale gold colour and were
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moist; older wadges turned white and shrank in size as they dried. Desiccated wadges
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also expanded, changing the angles of the folds from acute to obtuse, unless the fibres
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were tangled up with one another.
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How did we know that these artefacts were made by chimpanzees, if we had no direct
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observations of their making? On several occasions, observers were within 5m of
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chimpanzees eating in thick undergrowth; we heard the distinctive sounds of fronds being
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detached and then we recovered fresh artefacts only minutes later. We often saw the
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chimpanzees wadging other vegetation, e.g. at least four species of tree-bark
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(unpublished data), showing their manifest capacities (but all of these wadges were the
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more typical jumbled balls described above). Many cases of wild date palm wadge
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recovery took place when we were tracking chimpanzees on the trail system, only
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minutes ahead of us. We found imprints of knuckle- and hand-prints in association with
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the wadges. No other animals at the study-site did such wadging, nor were there resident
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humans present to do so.
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We measured 110 wadges, which had 1-14 folds; 56 of these had 3 folds, making this
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the typical number. When fully extended, the length of the wadges ranged from 8-50 cm,
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with the mean wadge being 15.9 (SEM) cm long. The mean distance between folds (i.e.
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length of segment) was 3.8 cm. (SEM) (N = 563, range 1.5-6.5, median=3.5, mode=3.0)
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Figure 2 shows a positive correlation (Spearman’s rho, n=11, rs=0.98, p<.001, two-tailed)
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between mean length of wadge and the number of its folds, suggesting that segment-
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length is relatively standardised.
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Figure 3 shows that wadges with an odd number of folds (1,3,5,7, etc.) greatly
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exceeded in frequency the wadges with an even number of folds (2,4,6,8, etc). For every
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consecutive pairing, that is, 1 vs. 2 folds, 3 vs. 4 folds, etc., the prevalence of odd-number
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folds was greater. Overall, 91 (83%) wadges had 1-13 odd-numbered folds, while only 19
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(17%) had 2-14 even-numbered folds, which is a highly statistically significant difference
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(Binomial test, n=110, z=6.77, p<0.001, two-tailed) (This could not have been a
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collection bias, as the number of folds was obscured in the field and could not be
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ascertained until the artefact was extended back in camp.) This unexpected finding
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requires explanation.
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Discussion
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The implicit interpretation of leaf-folding at Bossou that the dimension of the fold is a
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function of the space available in the chimpanzee’s mouth makes sense for pith-folding
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too. The most economical way to pack a linear object into a much smaller space is to
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compact it to the maximum permissible length allowed, and the most efficient form of
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compaction is folding. Thus it is notable that the average length of fold of leaves at
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Bossou was 3 cm versus 3.8 cm for pith-folding at Semliki. But why is there any
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variation in length of wadge? One hypothesis is the bigger mouths will accommodate
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bigger (longer) wadges, and that mouth size is a function of maturation. Thus, we predict
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that physically mature apes make higher-volume wadges, but to test this depends on
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gaining behavioural data. So, the correlation between number of folds and length of
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wadge is likely just a matter of mechanical constraints.
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But what about the odd-even difference? It could be that chimpanzees consistently
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clipped lengths of pith that when folded to the optimal segment-length of about 3-4 cm
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were somehow biased toward an odd number of folds. This seems nonsensical. If an ape
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were sufficiently persnickety particular? fastidious? fussy? about producing exactly the
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right number of folds to fill up the mouth, then on average, all other things being equal,
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one would expect a 50:50 chance of odd or even, given individual variation in buccal
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volume.
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A solution to the oddness riddle emerged serendipitously from chance observations of
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habituated chimpanzees at another Ugandan field site, Kanyawara, in Kibale National
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Park. There, chimpanzees wadge the pith of papyrus stems; most of the time they produce
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the standard jumbled-mass wadges, but occasionally they fashion concertina-shaped,
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folded wadges that are identical in form to the Semliki ones (Bertolani, pers. comm.).
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(See Figure 4.) The technique is as follows: After clipping the stem to the final length, the
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ape folds it in half, with one end in the lips and the other in one hand. She then ‘feeds’
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the doubled-over stem into the mouth by the same hand, initial-fold first. Each new fold
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of the doubled-stem thus produces a pair of folds, which when added to the single initial
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fold, gives an odd-numbered total. Occasionally, the wadger does not bother with the
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initial fold, and just ‘feeds’ the stem into the mouth; presumably these artefacts have a
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50:50 chance of ending up with an odd or even number of folds.
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To what extent does the pith-folding at Semliki resemble the leaf-folding at Bossou?
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There are several differences (see Tonooka 2001): The folded leaf is a tool, the pleats of
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which increase the water-holding ability of the leaves that are folded. The folded pith is
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not a tool, and the folds seem to have no containing function. So, Semliki wadges are
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discarded after one use, whereas Bossou’s wadges may be re-used up to 122 times. At
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Semliki, only one species of plant is used for folding; at Bossou at least seven species are
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used. In making the wadge, at Semliki the raw material is folded over at its centre-point
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before being ‘fed’ into the mouth; at Bossou the raw material is stuffed directly into the
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mouth, starting at one end of the leaf. Most of the resulting differences can be explained
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by the differing functions of the activities.
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On the other hand, there are some similarities: Both procedures fold plant materials in
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order to fill up the buccal cavity, using folding as a technique to do so. The resulting
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pleats are evenly and similarly spaced, yielding a concertina-like artefact with alternating
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folds. The making of both types of artefact involves hand-mouth coordination. Most of
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these similarities can be explained by the similar biomechanics of the task.
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Why bother with such esoterica? Who cares if (whether?) the number of folds in a
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wadge is odd or even? One answer is that this is the stuff of culture, not just of gross
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differences across populations, but also of nuanced variation, when cultural traits are
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basically similar but subtly different. Such trivial variants occur all the time in human
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cultures, and may be the stuff of trivial cross-cultural misunderstandings (e.g. the
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orientation of the upraised, two-fingered salute or ‘V’-sign); the same may be true of non-
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human artefacts. This report is only a first ethnographic note, but it reminds us that
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culture is a layered phenomenon, and that if we operate on one level only, we may miss
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important features. Another answer is that scientists who work with artefacts, e.g.
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archaeologists, face great challenges in inferring how those artefacts were produced. This
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example of etho-archaeology reminds us how difficult it sometimes is to imagine the
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absent processes that result in material culture. Whether or not pith-folding is origami
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remains to be seen.
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Acknowledgements We thank: Uganda Wildlife Authority and Uganda National
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Council for Science and Technology for permission to work at Semliki, UWA rangers
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Alimosi Baluku, Patrick Biryomumsho, Charles Kasaija, Feliilex Pascal, Justus
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Orobokiritto, and Elly Rutaro for assistance in the field; Moses Comeboy, Eriikc
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Kasutama, Edson Katswamba, Jeremiah Ndutu for assistance in camp; Linda Marchant,
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Charlotte Payne, Timothy Webster for research collaboration and comments on the
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manuscript; National Science Foundation, Revealing Hominid Origins Initiative, XXX
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for funding; Lucie Salwiczek for graphics aid; Paco Bertolani for taking WCM into the
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swamp at Kanyawara.
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Figure legends:
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Fig. 1 Folded pith wadges of Phoenix reclinata made by chimpanzees at Toro-Semliki
Wildlife Reserve.
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Fig. 2 Average (overall) length of folded pith wadges of Phoenix reclinata as a function
of number of folds.
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Fig. 3 Frequency of folded pith wadges with odd versus even number of folds.
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Fig 4 Jumbled (standard) and folded pith wadges of Cyperus papyrus at Kanyawara,
Kibale National Park.
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