Ethology 87, 237-248 (1991) 0 1991 Paul Pare? Scicntific Publishers, Berlin and Hamburg ISSN 0179.1613

Primate Research Institute, Kyoto University, Inuyama, Aichi

Flexibility of Wild C himpanzee Nut-cracking Behavior Using Stone Hammers and Anvils: an Experimental Analysis

OSAMU SAKURA & TETSURO MATSUZAWA

S A K U R A , 0. & ~IATSUZAWA, T. 1991: Flexibility of wild chimpanzee nut-cracking behavior using \ tone hammers and anvils: an experimental analysis. Ethology 87, 237-248.

Abstract

Nur-cracking bch.ivior in wild chimpanzees (Pun troglodyte$ w e ~ u s ) using natural stone ham- mers and anvils was studied through transect survey and experimental analysis at Bossu, Guinea. ’Transect surveys revealcd that hammers were significantly smaller than anvils, although the role division was not strict. Analysis of experiments such as separating nuts and stones found under palm trees revealed the chimbanzces’ flexibility in tool use. T h e behavioral patterns of transportation, discrimination, and the substitution of tools were not stereotyped. This flexibility appears to stem trom the chimpanzee<’ understanding of the relationship between tools and referents, i. e. the tunction of tool$.

Corresponding author: Osamu Sakura, Laboratory of Social Life Science, Mitsubishi Kasei Institute of Life Sciences, 11 Minamiooya, Machida, Tokyo, 194 Japan.

Introduction

Animal tool-using behavior has been reported in several species (see WILSON 1975; BECK 1980; BONNER 1980 for reviews), chiefly in the chimpanzee, for many of which detailed long-term field surveys have revealed surprising complexity (GOODALL 1973, 1986; NISHIDA & HIRAIWA 1982; NISHIDA 1987). There are sexual differences (e. g. MCGREW 1979; BOESCH & BOESCH 1981, 1984 a), optimality in tool choice (BOESCH & BOESCH 1982), variability related to habitat conditions (e.g. MCGREW et al. 1979; KORTLANDT 1986), seasonal changes in technique (e.g. UEHARA 1982), as well as evidence of mental mapping (BOESCH & BOESCH 1984 b), highly sophisticated tools (SUGIYAMA 1985), and “meta-tool” use, that is, a tool-use for another tool (MATSUZAWA 1989 a).

Since KOEHLEK’S (1 925) classical study several researchers have carried out esperimental work on chimpanzee tool-using behavior in captivity, which have revealed some determinants of tool-use and the transmission processes (e.g.

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23 8 OSAMU SAKURA & TETSURO MATSUZAWA

SUMITA et al. 1985; HANNAH & MCGREW 1987). In the wild, however, tool-use by chimpanzees is still open to experimental analysis, and chimpanzees’ mental awareness of the relation between tools and referents has not been studied.

This paper reports the first attempt to analyze tool-using behavior in wild chimpanzees experimentally. We examined the use of natural stones as hammers and anvils in nut-cracking. Nut-cracking behavior has been reported in chimpan- zees from the Western African region to the west of Dahome Gap, although there are slight differences among populations (Ivory Coast: BEATTY 1951 ; STRUHSAKER & HUNKELER 1971; RAHM 1971; BOESCH 1978; BOESCH & BOESCH 1981, 1982, 1984 a, b; Guinea: SUGIYAMA & KOMAN, 1979 a; SUGIYAMA 1981; KORTLANDT 1986; KORTLANDT & HOLZHAUS 1987; Liberia: ANDERSON et al. 1983; Sierra Leone: WHITESIDES 1985).

Based on observational surveys, we describe the kind of stones used for nut- cracking, and report the flexibility of tool-using behavior using data from experimental manipulations. H o w did chimps modify their behavior when the ecological environment, consisting of stones and nuts, was experimentally man- ipulated? Such analysis of tool-using in wild chimpanzees should be useful in discussing its origin and evolution, as well as in speculating on the primary factor promoting hominization.

Materials and Methods

Subject

The subjects were members of a group of wild chimpanzees in Bossu, Guinea, West Africa. SUGIYAMA and his colleagues have studied these primates since 1976 (see SUGIYAMA 1989 for a review). SUCIYAMA & KOMAN (1979 a, b) and KORTLANDT (1986) described their habitat detail; the former found that they cracked the hard shells of oil palm (Efueis grctneenszs) seeds with natural stone hammers and anvils to eat the embryos.

O u r study lasted 6 months, from Sep. 1987 to Mar. 1988. Fresh traces of palm-nut cracking were observed in only 4 months, from Dec. 1987 to Mar. 1988. The original group consisted of 21 individuals, including infants. A 6.5-year-old male died in Jan. (MATSUZAWA et al. 1990), leaving 20. Many oil palm trees grow naturally in the secondary forests of the chimpanzees’ ranging area. Although we succeeded in directly observing some nut-cracking behavior, clear direct observation was rather difficult (only three times in total in 6 months). Dense bush at cracking sites and the degree of habituation did not allow us to approach the chimps within 10 m on the ground, so that most of the study depended on the traces of nut cracking.

Transect Survey

To collect background data for the experiments a transect survey was made. In a preliminary survey we chose 34 “target” palm trees frequently used by the chimps for nut-cracking. We made routes (ca. 2 km) connecting these, and marked all stones (n = 88) which had been used as tools. Clear signs were the many fragments of cracked seed shells on and around the stones, and bits of palm fruit sticking to the stones (see also SUGIYAMA & KOMAN 1979 b). Of the marked 88 stones, 67 were easily identified as either hammers or anvils: 35 were used as hammer only, 32 as anvil only. The other 21 stones were of ambiguous role, o r had been used as both hammer and anvil in one bout of nut- cracking.

Tools were defined from the position of the stone surface with fresh traces of palm fruit and shell. If the striking surface with such pieces faced upwards, the stone was considered an anvil; if it faced down, a hammer. All other stones around an anvil were categorized as ‘stones of ambiguous

Flexibility of Chimpanzee Nut-cracking Behavior 239

role’. We believe that our criterion discriminating between hammers and anvils was quite adequate, as hammers were not left with the striking surface upwards, and even if they had been, pieces of shell could not stick to .I hammer surface. Although it was easy to discriminate accurately between hammers and anvils, it qhould be noted that our criterion recorded only the last role of a stone. If a stone had served first as hammer, then as anvil, only ‘anvil’ (or ‘ambiguous’) would be recorded.

We performed monthly transect surveys four times. The following measurements were taken of each marked stone: I ) length: the longest diameter of striking surface (surface with depressions), 2) width: diameter ot the striking surface at rightangles to the length, 3) thickness: the longest axis at rightangles to the striking surfaces, 6 ) number of depressions, and 7) distance of site from the nearest palm tree and other hammer/anvils, 8) usage (hammer or anvil), and 9) any change in position.

Experimental Analysis

Next we constructed an ‘outdoor laboratory’ in the chimp ranging area, to analyze nut-cracking behavior experimentally. The following three manipulations were made in 1988:

Expertrncwt I: Amassing stones: O n Jan. 12 we gathered all stones (n = 10) used for nut- cracking under a palm tree (# G17), and one suitable but unused stone. Nuts were scattered widely under the tree and wc‘ added 10 kg to motivate cracking.

Experzrnwzr 2; Separating stones and nurs: O n Feb. 13 we gathered stones (n = 25) and palm nuts (20 kg) into sepal-are piles about 3 ni apart near palm tree # B19 where the chimps often cracked nuts (Fig. 1). Stones ranged from 0.15- >10 kg, and palm nut age ranged from a few days after falling (fresh red ones with outer fruit) to several weeks after falling (shelled and naked, without fruit).

Exper iment 3: Setting nut5 and stones in the open: On Feb. 12, in a place often visited by the chimps, but with no palm trees near, we deposited a pair of hammers and anvils of moderate size and some 200 palm nuts about t m off. The site was a big fallen tree of Anzngr*eria robusta often used as a resting place by travelling chimps.

Each site was checked at least once daily and the original condition was restored whenever traces of nut-cracking were observed.

Results

Transect Survey

Fig. 2 gives the frequency distribution of the sizes of stones used as tools. Table 1 shows the result of size comparison of hammers and anvils, excluding stones which had served as both o r whose role was ambiguous. Anvils were significantly larger than hammers in all aspects (Mann-Whitney U test, two-

I q . 1: (

Set-up for Exp. 2 details see text)

240 OSAMU SAKURA & TETSURO MATSUZAWA

10

5

5 10 15 2 0 2 5 4 0

L E N G T H ( c a )

( C )

15

10

5

5 10 15

T H I C K N E S S ( c a ) 0 5 10

U E I G H T ( k g )

Fig. 2: Frequency distribution of sizes of stones used for cracking palm seeds, 1987-1988. Arrows: X, solid lines: hammers, dotted lines: anvils. See text for definitions of measured aspects

tailed), and anvil sizes also varied significantly more than hammers (variance ratio F). The frequency distribution of depressions and striking surfaces per stone are shown in Fig. 3. Depressions per hammer were significantly fewer than in anvils, while the number of striking surfaces did not differ significantly (Table 1).

20

10

1 2 3 4 5 6 7 8

NO. D E P R E S S I O N S

1 2 3

NO. U S E D S U R F A C E S

Fig. 3: Frequency distribution of number of depressions per stone (A) and number of used surfaces per stone (B). Hatched bars: anvils, open bars: hammers

Flexibility of Chimpanzee Nut-cracking Behavior 24 1

Table I: Comparisons between hammers and anvils (2 t S D)

Items Hammer Anvil Difference (n = 35) (n = 32) ?) Variance")

Length (cm) 12.0 rt 2.65 16.1 5 6.47 p < 0.01 (Z = 2.99) p < 0.01 (F = 5.95) Width (cm) 7 .7r t1 .86 1 1 . 1 f 4 . 3 8 p < O . O l ( Z = 4 . 1 3 ) p < O . O l ( F = 5.53) Thickness (cm) 5 .2 k 1.08 7.3 f 2.97 p < 0.01 (Z = 3.66) p < 0.01 (F = 7.57) Weight (kg) 0.7 2 0.33 2.2 rt 2.21') p < 0.01 (2 = 4.29) p < 0.01 (I; = 46.26) No. of depressions 1.6 i 0.63 2.6 ? 1.80 p < 0.05 (2 = 2.24) p < 0.01 (F = 8.16) No. of used surtaces 1.5 rt 0.55 1.4 rt 0.61 ns (Z = 1.00) ns (F = 1.12)

") Mann-Whitney U-rest, two-tailed, b, variance ratio, ') n = 31

Table 2 shows the reuse frequency of stones. Four new unmarked stones appeared and three marked stones disappeared, but such observations are not included in Table 2. Four instances of marked stone transportation were recorded: two were 2 m shifts (to a neighbouring palm tree) and two were of about 50 m. The sizes of reused stones did not differ significantly from those not reused (Table 2) .

T R E E

4 H A M M E R

A N V I L ,._.. -- ....

I I m

I i g 4 Set-up and result of Exp. I . The '.. arrowed stone appeared to have been u\ed a t first with the single stone 11,

..... .___ _... -*.

subsequently with stone IT1 (see text) S T 0 N E S U P P L Y

Erhnlogv, Vnl 87 (3- 4 ) 16

242 OSAMU SAKURA & TETSURO MATSUZAWA

1;lexibiIit~- of Chimpanzee Nut-cracking Behavior 243

Of 34 palm trees 14 (41 Y o ) afforded traces of nut-cracking. The mean number of stones per tree was 4.3 (range 1-13). Distance between stones and trees was 0.5-3 m. Although we have n o quantitative data for stone availability, it was not hard to find suitable stones in the Bossou forests. Sometimes local people piled stones in the forests after cultivation. Some of the palm trees with nut-cracking traces had such piles nearby.

Experimental Analysis

The following observations followed our experimental manipulation of stones and palm nuts.

Exp . I : We found new traces of nut-cracking 16 days after the set-up, with some 200 cracked palm seeds. The stones we had placed together were dispersed (Fig. 4). Transportation distances were 1-5 m and the distances between traces were 0.5-4 m. The new hammer-anvil pairs differed completely from the previous ones, moreover two stones formerly used as anvils were reused as hammers. The formerly unused stone was used in the new event. I t should be noted that in each pair anvil was larger than hammer. A stone located as an anvil had new traces on both sides (arrowed in Fig. 4). Another stone was discovered alone, without a counterpart, suggesting that a hammer-anvil pair was exchanged at least once during a nut-cracking bout.

Exp. 2: New traces of nut-cracking were found two days after the set-up. A pair of stones was moved to our test nut pile, where there were now about 20 cracked palm seeds (Fig. 5). Another two pairs of stones were moved about 20-30 cm from their original locations and used at the place of stone supply, to which about 10 nuts were shifted (Fig. 6). The chimps’ palm seed preferences were also clarified. Those cracked were neither very fresh nor very old, but about one to two weeks after falling, although we had prepared seeds variously aged from new to old.

Exp. 3: (1) Two new traces of nut-cracking were found on the fallen tree on the morning following set-up. The Anlnguevia surface had been used as an anvil

l r g . 5 . Exp. 2: stones cracking palm seeds I

ported to the place of supply

for :rans-

seed

, 6 ::-

Fig. 6: Exp. 2: palm s have been transported to location of stone supply

cracked there

Fig. ing face on t

7: Trace of nut-crack- with wooden anvil, sur- of fallen Anznguena tree

he day following set-up of Exp. 3

eeds the and

(Fig. 7). Stones used had served as hammers only in both cases. T w o cracking spots on the trunk each had cracked palm seeds; the first spot was 4 m from the stone supply, the second 2.5 m from the first. The tree-trunk anvil spots were as hard as stone. Scrutiny and attempts to crack seeds on the trunk revealed that such hard spots had a special white appearance unlike the rest of the trunk, and that only such spots could function as anvils, because the softness of the rest absorbed the seeds. We found no evidence that the chimps sought these suitable ‘anvils’ through trial and error.

( 2 ) 9 days after the first response a 6.5-year-old male (Pr) came to the ‘outdoor laboratory’ and cracked palm seeds with a pair of stones on the surface of the fallen tree, 1 m from the stone supply. Pr used only stones (a hammer in the right hand) and not the tree surface at that time. When he spotted the observer (TM) he stopped cracking and moved away. A couple of traces of palm seeds were recognized, the choice of nuts was similar to that of Exp. 2 .

245

Discussion

O u r data concerning stone size was similar to previous results by SUCIYAMA (1981) and KoK’ri.Awi, & HOLZHAUS (1987) in Bossou. However, no comparable data was available for hammer and anvil sizes, as previous studies had not clearly distinguished between the two. The ratio of palm trees with nut-cracking traces (41 %)) is also congruent with SUCIYAMA (31 ‘YO: 39/127). In general, the Bossou chimpanzees have not changed their nut-cracking behavior within a decade.

O u r transect survey revealed a significant difference in size between ham- mers and anvils. This suggests that chimpanzees choose stones for their optimal size, as BOKSCH & I~OI:SCH (1 982) found in chimp nut-cracking behavior in the Tai forest, Ivory Coast. The greater variance in anvil size suggests that the criterion for hammers might be more strict. Anvils need stability as a platform for cracking, and shape might be a more important factor than size. Although it is unclear why anvils had more depressions than hammers, it may simply be because anvils are bigger.

In comparison to the results from Tai, Ivory Coast (BOESCH & BOESCH 1982, 1984 a, b), the characteristics of nut-cracking in Bossou are summarized in the following three points: 1) the stones used are smaller; 2 ) only stones are used, no wood; and 3) transportation of tools is less frequent and the distances are shorter. For the first point, the size difference is quite clear: in BOSSOU, no hammers were heavier than 3 kg, while in Tai 57 hammers (58 %) for Panda nuts and 20 hammers (32 Yo) for C o d a nuts were heavier than 3 kg (ratio in Tai calculated from BOESCH & BOESCH 1982). This can be explained by the difference in nuts being cracked. Palm seeds, the main nuts in BOSSOU, are softer than Pandu and Coulu nuts, the main objects in Tai (BOESCH & BOESCH 1982). O n the second point, the exclusive use of stones may be due to their availability. As mentioned, it is easy to find suitable stones for palm-cracking, while suitable root anvils are quite rare in the secondary forest of Bossou. Chimpanzees might recognize the .ivailability and location of stones in their “mental map” (BOESCH & BOESCH I984 b). This may also explain the third point. The high availability of stones in I3ossou largely obviates the need for tool transportation. These explanations agree with the interpretation by B O ~ S C H & BOESCH (1982) that optimization in time and energy costs ma) produce population differences in nut-cracking behaviour between Bossou and Tai.

O u r experiments revealed other kinds of optimization, in inter-individual distances during cracking bouts, and palm nut ripeness. The chimps stayed 1-3 m apart in Lxp. 1 , reminiscent of the inter-individual spacing by which chimps avoid mutual interference (cf. BOESCH & BOESCH 1984 a). Concerning nut preference, they chose the moderately aged palm nuts in Exp. 2. Newer ones \yere harder, and more difficult to crack, and embryos were difficult to get at, often sticking to the insides of shells. Older nuts were usually worm-eaten and wmetimes empty, without embryos (see BOKH & BOMCH 1984 b on chimp preference for Coulu nuts).

However, i t should be emphasized that such optimization is proved by the flexibility revealed through our experiments. This flexibility is due to the recogni-

246 O S A h l U SAKUKA & TrTSURO MATSUZAWA

tion of the relation between tools (i.e. stones) and referents (i,e. palm seeds). In other words, the chimps recognize the “function” of tools. This is evidenced by the following three nut-cracking phenomena. The first is transportation. To bring the two together, chimps sometimes moved tools, sometimes nuts. They did not have a stereotyped pattern of transportation (Exp. 2). The second is tool discrimination. Sometimes a certain stone was used first as anvil, then as hammer (Exp. 1). This means they discriminated whether the stone should function as hammer or anvil. There were no single concepts such as “the stone is a hammer.”

The third phenomenon is the substitution of tools. The chimps could use the tree as anvil when no stone was available (Exp. 3). This result is especially important because the use of wooden anvils had never been observed in Bossou during the 12-year study (SUGIYAMA 1981; SUGIYAMA & KOMAN 1979 a, 1987; FUSHIMI et al. 1990), although reported from Liberia (ANDERSON et al. 1983) and the Ivory Coast (STRUHSAKER & HUNKELER 1971; BOESCH 1978; BOESCH & BOESCH 1981, 1982, 1984 a, b). The new behavior, using a wooden anvil, appeared the first morning after the set-up of Exp. 3 , confirming that the absence of this behavior in Bossou is not due to the chimps’ limited ability. Although KORTLAND (1 986) states that differences in the origin of tool-using behavior among chimpanzees are fixed as inter-group variabilities, his argument is not valid in this case.

We also emphasize that chimpanzees who have never cracked nuts with wood-tools used the hardest spots as anvils without any trial-and-error process. This also shows that they recognize the “function” of stones as tools, rather than seeing the “stones” themselves as tools. The variety and flexibility proved in the present study illuminates the uniqueness of the chimpanzees’ tool-use behavior. Tool-using behavior has been reported in many other animal species (WILSON 1975; BONNER 1980; MUNDINGER 1980), but it seemed to be sterotyped and resulted from the transfer or slight modification of an existing behavior (ALCOCK 1984; NISHIDA 1987). There is little evidence available that other animals can recognize the function of tools as chimpanzees can.

O u r experimental manipulations distorted some parameters of nut-cracking behavior from the optimal values. The animals adopted various ways of returning the shifted conditions to optimality. We believe that developmental rather than genetic variability realized the diverse (extended) phenotypes (DAWKINS 1982) in tool-using behavior. The various “bypasses” are based on the chimpanzees’ ability to recognize the relation between “tools” and “referents.” This ability, similar to that of chimp recognition of the relation between “symbols” and “referents” (MATSUZAWA 1985, 1989 b), might be important in producing “cross- cultural variations” (MCGREW et al. 1978, 1979; NISHIDA & HIRAIWA 1982; SUGIYAMA 1985) in chimpanzee tool-using behavior and therefore in promoting cultural evolution in early hominids.

Acknowledgements We are grateful to Direction de la Recherche Scientifique et Technique, Republique de Guinee

(Directeur: Dr . Fod6 SOUMAH) for their official support of the study; to Y. SUGIYAMA for his direction of the field study; Y. SUGIYAMA and two anonymous reviewers for their helpful comments

Flexibility of Chimpanzee Nut-cracking Behavior 247

on earlier drafts; to 1 . KOMAN, G. GUMI, and T. KAMARA for their assistance during the field study; and to K. BE~VI:NUTO for her help in English revision. This study was financed by Grant-in-Aid for Overseas Field Research, No. 63043045 (Director: Y. SUGIYAMA) from the Ministry of Education, Science and Culture, Japan.

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Received: July 10, 1989

Accepted: December 21, 1990 (H.-U. Reyer)