1 Chimpanzee origami? Pith-folding at Toro

Transcription

1 Chimpanzee origami? Pith-folding at Toro
1
1
Chimpanzee origami? Pith-folding at Toro-Semliki Wildlife Reserve, Uganda
2
3
William C. McGrew1 and Kevin D. Hunt2
4
5
6
7
8
1
9
2
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
10
11
Abstract All well-studied populations of wild chimpanzees (Pan troglodytes) orally
12
compress detached vegetation between tongue and palate, then discard it after juices have
13
been extracted (‘wadges’). Wadging is a form of food-processing, typically focussed on
14
fibrous bark, pith, or fruit, that yields a characteristic remnant or artefact. Most wadges
15
are jumbled, amorphous masses, but here we report a newly-discovered technique, from
16
the chimpanzees of Semliki, Uganda. Various lengths of pith stripped from wild date
17
palm (Phoenix reclinata) fronds are repeatedly folded to produce a characteristically-
18
shaped artefact. (Systematic folding of plant matter for any reason is rare in apes, e.g.
19
chimpanzees at Bossou, Guinea, fold leaves to extract water from tree-holes). Number of
20
folds per wadge was correlated with length of pith, i.e. longer piths had more folds,
21
presumably because it was necessary to fit the material into the confines of the mouth.
22
However, 83% of wadges unexpectedly had an odd number of folds, a highly-skewed
23
departure from an expected 50:50 distribution. There is no obvious reason why a
24
concertina-style, folded object should have an odd or even number of folds, but the
25
explanation was supplied by observations of another Ugandan population of
26
chimpanzees. The Kanyawara community at Kibale sometimes processes the pith of
27
papyrus (Cyperus papyrus) in a particular way that produces similarly-folded remnants.
28
29
30
31
32
Key words Pan troglodytes, diet, food process, material culture, object manipulation
2
33
Introduction
34
35
Chimpanzees make and use tools on a daily basis, in a variety of arenas, but most of their
36
tool manufacturing is transforming vegetation by stripping, peeling, splitting, crushing,
37
clipping, etc. In the well-known examples of termite fish, ant dip, ant fish, leaf sponge,
38
etc., the resultant tool is used in extractive foraging (McGrew 1992). In habituated
39
populations, behavioural data accompany the artefacts, so that observers see precisely
40
how the tools are made and why, e.g. twigs for fishing may be peeled off bark, in order to
41
facilitate their smooth insertion into tiny holes.
42
43
However, chimpanzees also modify vegetation when not making tools, and these
44
activities also may leave behind puzzling artefacts. Such are the ‘wadges’ of the pith of
45
the wild date palm, Phoenix reclinata, which we describe in this report from unhabituated
46
subjects. (‘Wadge’ is a somewhat imprecise English word for such a bolus; ‘quid’ is
47
probably better, as in chewing tobacco.) The artefact is obvious when encountered: A
48
straight stem is bent repeatedly to alternating sides, concertina-style, so that it has a series
49
of folds at acute angles (see Figure 1). These objects are the spat-out products of buccal
50
compression, from which juices have been extracted by squeezing them between tongue
51
and palate. Their ‘function’ is straight-forwardly nutritional, but the puzzles are why they
52
take this distinctive shape and why there are so many wadges with an odd number of
53
folds and so few with an even number of folds. We seek to study these puzzles here.
54
55
The only previous report of similar origami is the leaf-folding done by the
56
chimpanzees of Bossou, Guinea, who manufacture water-extracting tools (Tonooka et al.
57
1994, Tonooka 2001; Biro et al. 2006; Sousa 2009). To make these tools, the apes fold
58
sets of one to -four leaves (mostly of Hybophrynium braunianum) at about 3-cm
59
intervals, while stuffing them into the mouth. This device is inserted by hand into a tree-
60
hole containing water, then extracted and sucked, as a source of sustenance; the sequence
61
may be repeated many times before the artefact is abandoned. (Such leaf-folding differs
62
from leaf-sponging and leaf-spooning, although all three techniques yield drinking water.
63
Tonooka, 2001).
3
64
65
Wadging, in which a nutritious object is manipulated by mouth but not swallowed,
66
seems to be a universal of chimpanzee food-processing technique. Goodall (1986, p. 238)
67
described wadging at Gombe in detail, noting a variety of food-items, mostly fibrous
68
plant-parts, e.g. figs, bark, etc. In some cases, chimpanzees add leaves to fleshy, ‘rich’-
69
tasting foods, such as meat, eggs, honey, overripe fruit, to form a compound wadge. In
70
this case, the compressed, homogenised bolus may be swallowed, but the usual result is
71
an accumulation of amorphous, jumbled fibre-spheres left on the ground. (Presumably,
72
wadges are ejected rather than ingested, because the fibrous mass of foliage is of low
73
quality and would take up valuable gut space.) Humans similarly wadge the pith of
74
Cyperus papyrus, which is described as “chewy and pleasant tasting” (van der Merwe et
75
al. 2008).
76
77
We have find found no previous record of accordion-folded wadges from previous
78
studies of chimpanzees or any other species of non-human primates, either in nature or
79
captivity. The aim of this report is to document these special wadges and to seek to infer
80
how the artefacts end up with a non-random design, that is, a prevalence of odd-
81
numbered folds. We predicted that the number of folds would be positively correlated
82
with the length of the pithy stem involved; given a standard unit of folding distance, the
83
longer the stem, the more folds required. However, we had no clue about the biased
84
number of folds.
85
86
Methods
87
88
We studied the wild chimpanzees (Pan troglodytes schweinfurthii) of the Toro-Semliki
89
Wildlife Reserve, Uganda, from May-November, 2008 (Hunt 2000). The apes were only
90
partly habituated, but much progress was made, so that nest-to-nest follows were
91
eventually achieved. If the chimpanzees could not un-nested at the beginning of the day,
92
we searched for them, listening for calls and looking for their signs. Once contacted, we
93
stayed with them for as long as possible. These apes occupy a largely open habitat, which
94
is a mosaic of grassland, scrub, open woodland, galley forest, and swamp (Hunt and
4
95
McGrew, 2002). Their subsistence patterns are mostly the same as those of other
96
populations, but unusually among wild chimpanzees, they dig holes by hand in sandy
97
riverbeds for drinking water (Hunt and McGrew 2002, McGrew et al. 2007).
98
99
In tracking or following chimpanzees, we took note of any feeding traces left by
100
them. For wild date palm wadges, we counted the numbers in an assemblage and noted
101
the sources from whence the piths had been detached. Intact and fresh (less than 24 hr
102
old) wadges were collected in plastic ziplock bags and returned to camp for processing.
103
(In a parallel study, very fresh wadges were sealed in sterile containers for possible DNA
104
extraction, on the assumption that buccal cells from the wadgers would be found. Payne,
105
unpublished data) In camp, WCM measured wadges on a table-top, to the nearest 0.5 cm,
106
from fold to fold, and counted the number of folds. Thus, a folded-pith wadge with three
107
folds yielded four measureable segments.
108
109
Results
110
111
Remnants of the chimpanzees’ wadging of wild date palm were found on a daily basis
112
over the six-month period of the study. A typical assemblage consisted of several mature
113
fronds lying on the ground less than 5m from the presumed source plant. The presumed
114
source plant was identified by what appeared to be freshly damaged ends of the intact
115
fronds. Source plants were readily apparent because white pith revealed by the damage
116
made a stark contrast to the green outer colour of the frond. A typical assemblage
117
consisted of several mature fronds lying on the ground, havinge been torn from the
118
targeted plant, which was often less than 5m away. It was easy to recognise the damaged
119
ends of the intact fronds, as the revealed white pith was obvious in comparison with the
120
green outer colour. Detached fronds were split length-wise and clipped cross-wise,
121
producing segments of 8-50 cm length. All of the outer epithelium of the segments was
122
peeled away, leaving only the grainy, moist, fibrous pith in strips of less than 1 cm
123
diameter. There were few of these unused or incomplete strips of pith, by comparison
124
with more than 30 wadges left strewn on the ground. (See Figure 1.) These artefacts were
125
not scattered randomly, but were often concentrated in areas of less than 50 cm diameter,
5
126
sometimes in a loose pile, as if the wadger had sat still and worked through a set of
127
wadges before moving on. Fresh wadges had a distinctive pale gold colour and were
128
moist; older wadges turned white and shrank in size as they dried. Desiccated wadges
129
also expanded, changing the angles of the folds from acute to obtuse, unless the fibres
130
were tangled up with one another.
131
How did we know that these artefacts were made by chimpanzees, if we had no direct
132
observations of their making? On several occasions, observers were within 5m of
133
chimpanzees eating in thick undergrowth; we heard the distinctive sounds of fronds being
134
detached and then we recovered fresh artefacts only minutes later. We often saw the
135
chimpanzees wadging other vegetation, e.g. at least four species of tree-bark
136
(unpublished data), showing their manifest capacities (but all of these wadges were the
137
more typical jumbled balls described above). Many cases of wild date palm wadge
138
recovery took place when we were tracking chimpanzees on the trail system, only
139
minutes ahead of us. We found imprints of knuckle- and hand-prints in association with
140
the wadges. No other animals at the study-site did such wadging, nor were there resident
141
humans present to do so.
142
143
We measured 110 wadges, which had 1-14 folds; 56 of these had 3 folds, making this
144
the typical number. When fully extended, the length of the wadges ranged from 8-50 cm,
145
with the mean wadge being 15.9 (SEM) cm long. The mean distance between folds (i.e.
146
length of segment) was 3.8 cm. (SEM) (N = 563, range 1.5-6.5, median=3.5, mode=3.0)
147
Figure 2 shows a positive correlation (Spearman’s rho, n=11, rs=0.98, p<.001, two-tailed)
148
between mean length of wadge and the number of its folds, suggesting that segment-
149
length is relatively standardised.
150
151
Figure 3 shows that wadges with an odd number of folds (1,3,5,7, etc.) greatly
152
exceeded in frequency the wadges with an even number of folds (2,4,6,8, etc). For every
153
consecutive pairing, that is, 1 vs. 2 folds, 3 vs. 4 folds, etc., the prevalence of odd-number
154
folds was greater. Overall, 91 (83%) wadges had 1-13 odd-numbered folds, while only 19
155
(17%) had 2-14 even-numbered folds, which is a highly statistically significant difference
156
(Binomial test, n=110, z=6.77, p<0.001, two-tailed) (This could not have been a
6
157
collection bias, as the number of folds was obscured in the field and could not be
158
ascertained until the artefact was extended back in camp.) This unexpected finding
159
requires explanation.
160
161
Discussion
162
163
The implicit interpretation of leaf-folding at Bossou that the dimension of the fold is a
164
function of the space available in the chimpanzee’s mouth makes sense for pith-folding
165
too. The most economical way to pack a linear object into a much smaller space is to
166
compact it to the maximum permissible length allowed, and the most efficient form of
167
compaction is folding. Thus it is notable that the average length of fold of leaves at
168
Bossou was 3 cm versus 3.8 cm for pith-folding at Semliki. But why is there any
169
variation in length of wadge? One hypothesis is the bigger mouths will accommodate
170
bigger (longer) wadges, and that mouth size is a function of maturation. Thus, we predict
171
that physically mature apes make higher-volume wadges, but to test this depends on
172
gaining behavioural data. So, the correlation between number of folds and length of
173
wadge is likely just a matter of mechanical constraints.
174
175
But what about the odd-even difference? It could be that chimpanzees consistently
176
clipped lengths of pith that when folded to the optimal segment-length of about 3-4 cm
177
were somehow biased toward an odd number of folds. This seems nonsensical. If an ape
178
were sufficiently persnickety particular? fastidious? fussy? about producing exactly the
179
right number of folds to fill up the mouth, then on average, all other things being equal,
180
one would expect a 50:50 chance of odd or even, given individual variation in buccal
181
volume.
182
183
A solution to the oddness riddle emerged serendipitously from chance observations of
184
habituated chimpanzees at another Ugandan field site, Kanyawara, in Kibale National
185
Park. There, chimpanzees wadge the pith of papyrus stems; most of the time they produce
186
the standard jumbled-mass wadges, but occasionally they fashion concertina-shaped,
187
folded wadges that are identical in form to the Semliki ones (Bertolani, pers. comm.).
7
188
(See Figure 4.) The technique is as follows: After clipping the stem to the final length, the
189
ape folds it in half, with one end in the lips and the other in one hand. She then ‘feeds’
190
the doubled-over stem into the mouth by the same hand, initial-fold first. Each new fold
191
of the doubled-stem thus produces a pair of folds, which when added to the single initial
192
fold, gives an odd-numbered total. Occasionally, the wadger does not bother with the
193
initial fold, and just ‘feeds’ the stem into the mouth; presumably these artefacts have a
194
50:50 chance of ending up with an odd or even number of folds.
195
196
To what extent does the pith-folding at Semliki resemble the leaf-folding at Bossou?
197
There are several differences (see Tonooka 2001): The folded leaf is a tool, the pleats of
198
which increase the water-holding ability of the leaves that are folded. The folded pith is
199
not a tool, and the folds seem to have no containing function. So, Semliki wadges are
200
discarded after one use, whereas Bossou’s wadges may be re-used up to 122 times. At
201
Semliki, only one species of plant is used for folding; at Bossou at least seven species are
202
used. In making the wadge, at Semliki the raw material is folded over at its centre-point
203
before being ‘fed’ into the mouth; at Bossou the raw material is stuffed directly into the
204
mouth, starting at one end of the leaf. Most of the resulting differences can be explained
205
by the differing functions of the activities.
206
207
On the other hand, there are some similarities: Both procedures fold plant materials in
208
order to fill up the buccal cavity, using folding as a technique to do so. The resulting
209
pleats are evenly and similarly spaced, yielding a concertina-like artefact with alternating
210
folds. The making of both types of artefact involves hand-mouth coordination. Most of
211
these similarities can be explained by the similar biomechanics of the task.
212
213
Why bother with such esoterica? Who cares if (whether?) the number of folds in a
214
wadge is odd or even? One answer is that this is the stuff of culture, not just of gross
215
differences across populations, but also of nuanced variation, when cultural traits are
216
basically similar but subtly different. Such trivial variants occur all the time in human
217
cultures, and may be the stuff of trivial cross-cultural misunderstandings (e.g. the
218
orientation of the upraised, two-fingered salute or ‘V’-sign); the same may be true of non-
8
219
human artefacts. This report is only a first ethnographic note, but it reminds us that
220
culture is a layered phenomenon, and that if we operate on one level only, we may miss
221
important features. Another answer is that scientists who work with artefacts, e.g.
222
archaeologists, face great challenges in inferring how those artefacts were produced. This
223
example of etho-archaeology reminds us how difficult it sometimes is to imagine the
224
absent processes that result in material culture. Whether or not pith-folding is origami
225
remains to be seen.
226
227
Acknowledgements We thank: Uganda Wildlife Authority and Uganda National
228
Council for Science and Technology for permission to work at Semliki, UWA rangers
229
Alimosi Baluku, Patrick Biryomumsho, Charles Kasaija, Feliilex Pascal, Justus
230
Orobokiritto, and Elly Rutaro for assistance in the field; Moses Comeboy, Eriikc
231
Kasutama, Edson Katswamba, Jeremiah Ndutu for assistance in camp; Linda Marchant,
232
Charlotte Payne, Timothy Webster for research collaboration and comments on the
233
manuscript; National Science Foundation, Revealing Hominid Origins Initiative, XXX
234
for funding; Lucie Salwiczek for graphics aid; Paco Bertolani for taking WCM into the
235
swamp at Kanyawara.
236
237
References
238
239
Biro D, Sousa C, Matsuzawa T (2006) Ontogeny and cultural propagation of tool use by
240
wild chimpanzees at Bossou, Guinea: Case studies in nut cracking and leaf
241
folding. In: Matsuzawa T, Tomonaga M, Tanaka M (eds) Cognitive development
242
in chimpanzees. Springer, Tokyo, pp 476-508.
243
244
245
246
247
248
Goodall J (1986) The chimpanzees of Gombe: Patterns of behavior. Harvard University
Press, Cambridge, MA.
Hunt KD (2000) Initiation of a new chimpanzee study site at Semliki-Toro Wildlife
Reserve, Uganda. Pan Afr News 7:14-16.
Hunt KD, McGrew WC (2002) Chimpanzees in the dry habitats of Assirik, Senegal and
Semliki Wildlife Reserve, Uganda. In: Boesch C, Hohmann G, Marchant LF (eds)
9
249
Behavioural diversity in chimpanzees and bonobos. Cambridge University Press,
250
Cambridge, pp 35-51.
251
252
253
254
255
256
257
258
259
McGrew WC (1992) Chimpanzee material culture. Implications for human evolution.
Cambridge University Press, Cambridge.
McGrew WC, Marchant LF, Hunt KD (2007) Etho-archaeology of manual laterality:
Well digging by wild chimpanzees. Folia Primatol 78:240-244.
Sousa C (2009) Use of leaves for drinking water. In: Matsuzawa T, Sugiyama Y (eds.)
The chimpanzees of Bossou and Nimba. Springer, Tokyo, pp
Tonooka R (2001) Leaf-folding behavior for drinking water by wild chimpanzees (Pan
troglodytes verus) at Bossou, Guinea. Anim Cogn 4:325-334.
Tonooka R, Inoue N, Matsuzawa T (1994) [Leaf-folding behavior for drinking water by
260
wild chimpanzees at Bossou, Guinea: A field experiment and leaf selectivity.]
261
Primat Res 10:307-313. [Japanese, with English summary]
262
van der Merwe NJ, Masao FT, Bamford MK (2008) Isotopic evidence for contrasting
263
diets of early hominins Homo habilis and Australopithecus boisei of Tanzania. S
264
Afr J Sci 104:153-155.
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
10
280
281
282
283
284
285
286
287
288
Figure legends:
289
290
291
Fig. 1 Folded pith wadges of Phoenix reclinata made by chimpanzees at Toro-Semliki
Wildlife Reserve.
292
293
294
Fig. 2 Average (overall) length of folded pith wadges of Phoenix reclinata as a function
of number of folds.
295
296
Fig. 3 Frequency of folded pith wadges with odd versus even number of folds.
297
298
299
300
Fig 4 Jumbled (standard) and folded pith wadges of Cyperus papyrus at Kanyawara,
Kibale National Park.
11
Figure
12
Figure …
13
Figure:
14
Figure: