Pan troglodytes) and young human children ( Homo …...young human children (Homo sapiens sapiens) Patrizia Potì,1 Misato Hayashi2 and Tetsuro Matsuzawa2 1. Institute of Cognitive

Developmental Science 12:4 (2009), pp 536–548 DOI: 10.1111/j.1467-7687.2008.00797.x

© 2009 The Authors. Journal compilation © 2009 Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA.

Blackwell Publishing LtdPAPERChimpanzees’ and children’s construction skillsSpatial construction skills of chimpanzees (Pan troglodytes) and young human children (Homo sapiens sapiens)

Patrizia Potì,1 Misato Hayashi2 and Tetsuro Matsuzawa2

1. Institute of Cognitive Sciences and Technologies – CNR, Rome, Italy

2. Primate Research Institute of Kyoto University (KUPRI), Japan

Abstract

Spatial construction tasks are basic tests of visual-spatial processing. Two studies have assessed spatial construction skills in

chimpanzees (Pan troglodytes) and young children (Homo sapiens sapiens) with a block modelling task. Study 1a subjects

were three young chimpanzees and five adult chimpanzees. Study 1b subjects were 30 human children belonging to five age groups

(24, 30, 36, 42, 48 months). Subjects were given three model constructions to reproduce: Line, Cross-Stack and Arch, which

differed in type and number of spatial relations and dimensions, but required comparable configurational understanding. Subjects’

constructions were rated for accuracy. Our results show that: (1) chimpanzees are relatively advanced in constructing in the

vertical dimension; (2) Among chimpanzees only adults make accurate copies of constructions; (3) Chimpanzees do not develop

in the direction of constructing in two dimensions as human children do starting from age 30 months. The pattern of development

of construction skills in chimpanzees partially diverges from that of human children and indicates that spatial analysis and spatial

representation are partially different in the two species.

Key words: chimpanzees, human children, construction skills, spatial configurations, spatial representation.Introduction

Combining objects with each other is a basic way forhumans and nonhuman primates to acquire and expressknowledge about spatial relations. Spatial constructiontasks are basic tests of visual-spatial processing. Spatialanalysis has been defined as ‘the ability to specify boththe parts and the overall configuration of a visuallypresented pattern and to understand how the parts arerelated to form a whole. It thus involves the ability to bothsegment a pattern into constituent parts and to integratethose parts into a coherent whole’ (Stiles & Stern, 2001,p. 158). In constructing spatial configurations, spatialanalysis goes beyond spatial perception and form recog-nition and implies the representation of simultaneousspatial relations among distinct elements (Vereeken, 1961).

Studies of early spontaneous block play by younghuman children have focused on the construction of spa-tial relations and on developmental change in thenumber and type of spatial relations human childrenconstruct and in the number of spatial dimensions alongwhich they construct (e.g. Forman, 1982; Guanella, 1934;Stiles-Davis, 1988). These studies showed that humanchildren begin to stack blocks one on top of the other atabout 12 months, whereas they start to make lines atabout 18 months. To explain this developmental pattern,some authors have suggested that constructing spatialrelations in the vertical (V) dimension requires less coor-dination and planning than constructing relations in the

horizontal (H) dimension (Sugarman, 1983; Stiles-Davis,1988). Constructing relations in the V dimension (e.g.making stacks), actually only requires one place, onedimension and one direction of object grouping: thedirection in which to construct is one and mandatory,and, as each new object is added, it is only necessary toconsider its relation to the object immediately support-ing it. Conversely, spatial relations in the H plane can beconstructed in more than one direction (e.g. by addingobjects to a line on both sides) and dimension (e.g.constructing enclosures). However, although stacks aredeveloped before lines, some relations in the verticaldimension are developed later than lines. For example,crossing two blocks has been observed in children’sspontaneous play between ages 30 and 36 months(Vereeken, 1961).

These studies also showed that between ages 30 and36 months human children systematically coordinatemultiple relations between separate constructions orcombine two constructions into one (Sugarman, 1983).Finally, human children construct one-dimensional con-structions before two-dimensional constructions. Betweenages 3 and 4 years human children start to systematicallybuild both in the V and the H dimension within thesame construction (Guanella, 1934).

Previous studies on young children’s constructionalabilities using block-modelling tasks (e.g. Stiles & Stern,2001) reported similar findings, though they focused onsomewhat different aspects of spatial constructions by

Address for correspondence: Patrizia Potì, via Ulisse Aldrovandi, 16b, 00157, Roma, Italy; e-mail: [email protected]

Chimpanzees’ and children’s construction skills 537

© 2009 The Authors. Journal compilation © 2009 Blackwell Publishing Ltd.

children, like number and length of components. Alsothese studies showed that the information in the spatialarray varies according to the number of blocks and theirrelative positions, and that with increasing age humanchildren produce increasingly complex constructions.

Some studies have investigated chimpanzees’ con-structional abilities through spontaneous block play(e.g. Hayashi & Matsuzawa, 2003; Potì, 1996; Potì &Langer, 2001; Potì, 2005; Takeshita, 2001). These studiesshowed that the chimpanzees’ pattern of developmentof constructional abilities partially diverges from that ofyoung human children in the relative age of emergenceof different spatial relations and/or the processes used bythe subjects to construct those relations. In particular,chimpanzees are relatively more advanced in construct-ing insertion relations (starting from age 8–11 months)than support relations, which they construct spontane-ously only after the age of 2 years (Hayashi, 2007;Hayashi & Matsuzawa, 2003). Other studies showedthat, after that age, chimpanzees are relatively moreadvanced in constructing in the V dimension than in theH dimension. Whereas chimpanzees construct supportrelations at higher level than human children aged 24months, they remain at a level roughly comparable tothat of human children age 2 years when constructingnext-to relations, considering either the spatial configu-rations produced (Potì & Langer, 2001) or the processesby which these constructions are achieved (Potì, 2005).

Finally, since chimpanzees spontaneously constructand reproduce different types of spatial relations, such asnext-to or support relations, they give evidence ofpossessing the requisites to represent these spatial rela-tions. However, chimpanzees do not spontaneously repro-duce combinations of these relations; for example, theydo not reproduce bi-dimensional constructions. It wassuggested that chimpanzees show specific limits in under-standing multiple spatial relations and coordinatingmultiple positions in the manipulative space (Potì, 2005).

The subjects of some of the previous studies on spon-taneous block constructions were chimpanzees raisedin highly enriched environments from a very early age(Matsuzawa, 2003; Matsuzawa, Tomonaga & Tanaka,2006; Savage-Rumbaugh 1986; Rumbaugh, Savage-Rumbaugh & Sevcik, 1994; Savage-Rumbaugh &Lewin 1994). The method of observing spontaneousconstructions has its merits, but it may not be the mosteffective one to elicit especially advanced constructionsin chimpanzees, and it should be supplemented by othermethods. Another method, widely used with children,is to provide the subjects with a model construction toreproduce.

The method of presenting chimpanzees with modelsto be copied seems to be viable. Indeed, chimpanzeeshave been shown to emulate a demonstrator and toreproduce the environmental results of object relationsin the actions of others (e.g. Call, Carpenter & Toma-sello, 2005). In the present study the focus is not on thesocial learning abilities of chimpanzees, but we build

upon the evidence that chimpanzees can emulate ahuman or chimpanzee demonstrator. In particular, Myowa-Yamakoshi and Matsuzawa (1999) showed that chimpan-zees could reproduce an action demonstrated in the firsttrial in the condition in which an object was to bemanipulated vis-à-vis another object, though not veryoften. We adopted a similar ‘do this’ procedure in face-to-face situations requiring chimpanzees to reproducespatial configurations with three objects.

In the present study we aimed at getting fuller insightinto chimpanzees’ abilities to analyse and reconstructspatial configurations in the context of object manipula-tion and into the differences between chimpanzees andhumans in developing the representation of spatial rela-tions. In particular we wanted to assess further whetherchimpanzees are relatively more advanced in representingsupport relations than next-to relations compared withhuman children, and whether chimpanzees have difficultiescoordinating relations along two spatial dimensions,such as coordinating object–object relations in the V andthe H dimension simultaneously. Adopting the sameblock modelling task we were able to make a directcomparison between the two species. Also the rearingconditions of the chimpanzee subjects involved in thestudy contributed to the reliability and fairness of thecomparison (Matsuzawa, 2003).

As modelled constructions we chose three construc-tions, Line, Cross-Stack, and Arch, which comprise thesame number of elements and similarly require anunderstanding of spatial configuration as a whole whilevarying in number (1 vs. 2) and type of spatial relations(next-to in Line, cross-and-support in Cross-Stack, andnext-to and support in Arch). In particular, in the Vdimension, we chose the cross-and-support relationinstead of a simple support relation because simplemulti-object stacks may only require comparing twoobjects at a time (e.g. Stiles-Davis, 1988), whereas across-stack requires considering the whole configuration,much like the line and the arch.

Study 1a

Methods

Participants

The subjects were eight chimpanzees (Pan troglodytes),five adults and three young chimpanzees living in agroup of 14 individuals at the Primate Research Instituteof Kyoto University (KUPRI), Japan. At the time oftesting the age range of the adults was 22 to 29 years, theage range of young chimpanzees was 4 years and 8months to 5 years. The young chimpanzees were twofemales, Pal and Cleo, and one male, Ayumu, and theywere all reared by their biological mothers in the group.The adults were four females, Ai, Pan, Pendesa andPopo, and one male, Akira. The chimpanzees lived in an

538 Patrizia Potì et al.


outdoor compound (approx. 700 m2) enriched by about500 planted trees belonging to about 60 species and by15 m high climbing frames. Housing and feeding condi-tions were in accordance with the Guide for the Care and

Use of Laboratory Primates produced by the PrimateResearch Institute, Kyoto University (2nd edn., 2002).A small outdoor booth provided access to the mainbuilding through an underground tunnel. The subjectchimpanzee living in the outdoor compound was allowedto decide whether or not to come to the test booth toparticipate in the cognitive tasks.

All subjects had already participated in a number ofstudies and cognitive tasks including object manipula-tion tasks in face-to-face situations (see summaries inMatsuzawa, 2003; and Matsuzawa, Tomonaga &Tanaka, 2006). In particular, the four adult females hadparticipated in a study involving the manipulation ofvarious kinds of objects (Myowa-Yamakoshi & Matsuzawa,2000). Moreover, two adult females (Ai and Pan) andthe three young chimpanzees had participated in astudy on the development of object manipulation in thefirst two years of age (Hayashi & Matsuzawa, 2003),and, with the addition of the male adult, they also par-ticipated in a successive study more thoroughly investi-gating the cognitive processes underlying block-stackingbehaviour (Hayashi, 2007). Two adult females, the adultmale and the three young chimpanzees of the presentstudy had therefore had previous experience in manipu-lating cubic and cylindrical blocks. The other two adultfemales (Popo and Pendesa), however, had no comparableexperience. The last two cited studies showed that theabilities to stack up blocks were present from an earlyage in these chimpanzees (in fact between the ages of 3and 4 years) and these abilities interacted with experiencein object manipulation. The adult subjects were able tostack up blocks from the start, readily adapting to newshapes to be stacked.

Material

The blocks consisted of rectangular parallelepipedsmeasuring 3 × 5 × 12 cm. They presented three surfacesof different sizes labeled ‘A’ (the 12 × 5 cm surface), ‘B’(the 12 × 3 cm surface), and ‘C’ (the 3 × 5 cm surface).Three blocks were used by the human model and thesame number of blocks were given to the subject.

Design

Three constructions were used as models, each comprisingthree blocks: Line, Cross-Stack and Arch (see Figure 1for description). The three models differed in numberand type of spatial relations and dimension of extension.Line and Cross-Stack implied only one spatial relationamong the objects and one dimension, but the Line anext-to relation in the horizontal (H) dimension, and theCross-Stack a cross-and-support relation in the vertical(V) dimension. On the other hand, Arch involved both

next-to and support relations and two dimensions (HV).However, all three models demanded an understandingof the overall spatial configuration in order to be success-fully reproduced. Whereas Line and Arch-like construc-tions were previously used as models in other studieswith human children (e.g. Stiles & Stern, 2001), theCross-Stack was not. We chose the Cross-Stack inplace of a simple stack for several reasons. First, theCross-Stack requires taking the whole configuration intoconsideration, much like the Line and the Arch. In fact,doing a three-block cross-stack requires comparing threeblocks at a time, whereas doing a three-block stackrequires comparing only two blocks at a time; so, a cross-stack is more complex than a stack. Second, the youngchimpanzees and some of the adult chimpanzees par-ticipating in the present study already had considerableexperience in stacking cubes. So, it was necessary to pro-pose a different configuration in the vertical dimension.Third, other chimpanzees had been observed to sponta-neously stack and cross two blocks (Potì, personalobservation), so this specific spatial relation is within thechimpanzees’ reach.

Each subject received three separate sessions, and ineach session he/she received 12 trials with only onemodel. All subjects received the three models in differ-ent, but balanced orders. Moreover, there were severalpossible instantiations of each model, depending on howthe blocks were oriented with respect to the floor andbetween themselves. We chose two types of orientationfor each model, as illustrated in Figure 1. One adult(Akira) and two young chimpanzees (Cleo and Pal)received both Types, while all remaining subjects receivedonly Type I for each model.

Procedure

Testing took place in one of two adjacent and connectedbooths, each measuring 2 m × 3 m. The wall of the testingroom was covered with transparent panels with metalframes in order to allow filming from the outside. Thetwin booths allowed an infant to be separated from themother according to their own free will. In one of thebooths a human tester was able to give cognitive tests toan infant chimpanzee in a face-to-face situation withoutinterference from the mother (Matsuzawa et al., 2006;Matsuzawa, 2003; Matsuzawa, 2001). Each subject wastested individually. There were two experimenters. Onewas inside the room together with the subject while theother recorded the trials on videotape from outside theroom. During the sessions, the experimenter insidethe playroom and a subject sat face to face, at a distanceproviding enough space for the model construction andthe copy construction to face each other without con-tact. Throughout their rearing history all the subjectshad learned to sit on a wooden seat when they partici-pated in a cognitive test. The experimenter built a modelwith three blocks in front of the subject and then gavethree other blocks to the subject, placing the blocks with



the A surface on the floor and not in contact with oneanother. The experimenter said ‘do this!’, touching themodel to focus the subject’s attention on the model andthen pointing to blocks in front of the subject as arequest to the subject to do something with his blocks.The model remained in the same place throughout thetrial. Each trial lasted 30 seconds, starting as soon as thesubject started manipulating the blocks. The end wassignalled by a kitchen timer. At the end of each trial thesubject was praised and received a food reward whateverthe level of matching produced, and the experimentertook the blocks back, before starting a new trial. Duringa trial, if the chimpanzee tried to give the blocks back to

the experimenter before the 30 seconds had elapsed, theexperimenter put the blocks back on the floor. Theexperimenter encouraged the subject to copy the modelby repeating ‘do this’ and touching the model and thenpointing to the blocks in front of the subject. The experi-menter also provided verbal guidance to the subject bysaying ‘yes’ or ‘no’ as the subject was making a con-struction. The experimenter prevented the subject frombiting the blocks by saying ‘no’ and ‘do not bite’. Theexperimenter prevented the subject from taking theblocks from the model by saying ‘no’ and from puttinghis/her blocks on those of the model by saying ‘no’ andgiving the blocks back to the subject.

Line (Three horizontally aligned blocks)

Accurate construction = Same Orientation, Three Blocks Aligned: Three blocks placed horizontally in a straight line and oriented with respect to the floor as in the model, with 80% minimal contact surface and an angular deviation of 0° ± 15°. Partially accurate construction = Different Orientation: Three blocks aligned, but oriented differently with respect to the floor than in the model. Two Blocks Aligned: Three blocks placed next to one another, only two aligned. Orientation may also differ.Inaccurate construction = Any other construction.

Cross-Stack (Three stacked blocks crossing each other)

Accurate construction = Same Orientation, Three Blocks Crossing: Three blocks stacked crossing each other at an angle of 90° ± 15° and oriented to the table as in the model.Partially accurate construction = Different Orientation: Three cross-stacked blocks oriented differently with respect to the floor than in the model. Two Blocks Crossing: Three blocks stacked, but only two crossing each other. Orientation may also differ.Inaccurate construction = Any other construction.

Arch (Two blocks spanned by a third supported one)

Accurate construction = Same Orientation, Relations of Support and Proximity of Bottom Blocks to the Upper One: Two blocks are placed next to each other not in contact but spanned by a third which they both support, and all blocks are oriented with respect to the floor as in the model.Partially accurate construction = Different Orientation, Relations of Support and Proximity of Bottom Blocks to the Upper One: Two blocks placed next to each other not in contact and spanned by a third which they both support, but one or more blocks are oriented differently with respect to the floor than in the model. Support: Two blocks are put next to each other in contact and both supporting a third block. Proximity: The bottom blocks are placed next to each other not in contact and a third block is placed on both, but only one of the bottom blocks is sufficient support for the top one. In the Support and the Proximity types the blocks can also be oriented differently with respect to the floor than in the model.Inaccurate construction = Any other construction.

Figure 1 Model constructions.



Data analysis

Coding. A construction was coded whenever all threeblocks were put together in contact, and the construc-tion was stable, that is, the construction was observablefor at least 1 second after the subject released the blocks(even in the case of the subject again contacting theconstruction without making any change). For eachconstruction, the orientation of the blocks to the floorand the spatial relations among the blocks was coded. Inall other instances no construction was coded, for examplewhether the subject manipulated the blocks individu-ally, or put the three blocks into contact, but did notrelease his/her hands, and/or kept moving the blocksrestlessly. A new construction was coded whenever anychange occurred in the contact surfaces, the spatial rela-tions among the blocks or the order of placement of the

blocks. At each trial either no construction or one ormore constructions were made.

Construction accuracy. Each construction in each trialwas coded as accurate, partially accurate, or inaccurate,depending on the model, as illustrated in Figure 1. Then,to rate the accuracy of subjects’ constructions, the bestconstruction in each trial was scored using a 0–1–2 scoresystem: an accurate construction was assigned a score of2, a partially accurate construction was assigned a scoreof 1, an inaccurate construction or no construction wereassigned a score of 0. Thus, the mean accuracy rangedbetween 0 and 2.

Although the partially accurate constructions were allassigned a score of 1, there were different types. Examplesof partially accurate constructions by chimpanzeesor human children are drawn in Figure 2. In the type

Figure 2 Examples of partially accurate constructions by model.



called Different Orientation the difference from themodel was in the orientation of the blocks to the surfaceand to each other. In the type called Two Blocks, forLine and Cross-Stack, a similar spatial relation as in themodel was only realized between two of the three blockscombined: with Line two blocks were aligned out ofthree blocks placed next to one another, with Cross-stackone block was put on and crossing another out of threestacked blocks. In the types called Proximity or Support(only with Arch) only one relation instead of two wasrealized between the three blocks. In the Proximity typethe blocks underneath were next-to each other in proximity,but only one was sufficient support for the top block. Inthe Support type both the blocks below were a necessarysupport for the top block, but they were in contact with

each other. With all models, the Orientation type wasmore advanced than others because a spatial configura-tion was realized among all three blocks that was similarto the model configuration. Finally, examples of inaccurateconstructions are given in Figure 3.

Spatial dimension. We also considered all subjects’ con-structions with respect to the spatial dimensions alongwhich they constructed, regardless of whether they wereaccurate, partially accurate or inaccurate. We coded eachconstruction as constructed in the horizontal dimension(H), the vertical dimension (V), or along both the hori-zontal and the vertical dimensions (HV). It was expectedthat subjects would realize constructions in the H dimen-sion when presented with Line, in the V dimension when

Figure 3 Examples of inaccurate constructions by model.



presented with Cross-Stack, and in both dimensions whenpresented with Arch.

Reliability. Two independent observers separately coded20% of all trials. Cohen’s kappa was used to assessinter-observer agreement for each dependent measureseparately. For construction accuracy, each observer ratedthe highest score reached in each trial using the three-category scale (i.e. 0–1–2). Cohen’s kappa was .82.For spatial dimension, each observer coded all construc-tions per trials, using the three-category coding (i.e.H–V–HV). Agreement was calculated on the construc-tions coded by both observers. Cohen’s kappa was .97.

Results

Construction accuracy

A two-way Analysis of Variance was carried out usingAge (young vs. adult) as the between-subjects variableand Construction (Line, Cross-Stack, Arch) as the within-subject variable. The dependent variable was the meanscore per trial obtained by each subject. The main effectfor Construction, F(2, 12) = 4.57, p = .03, was significant.Tukey Honestly Significant Difference (HSD) Tests,p < .05, showed a significantly higher score for Cross-Stack (.269) than Arch (.023). Also a significantAge × Construction interaction was found: F(2, 12) = 4.20,p = .041. Figure 4 shows the Age × Constructioninteraction. Tukey Unequal N HSD tests showed thatadult chimpanzees had a higher score with Line (.45)than with Arch (.033). Moreoveor, as illustrated inTable 1, only adult chimpanzees made some accurate orpartially accurate lines. Figures 5 and 6 show examplesof an accurate line and an accurate cross-stack, respec-tively, by an adult chimpanzee (see Figures 5 and 6).

With respect to partially accurate constructions madeby chimpanzees (young or adult), all were of the simplesttypes. Partially accurate lines comprised only two alignedblocks out of three blocks put next to one another incontact, and oriented as in the model. Partially accurate

cross-stacks included only two stacked and crossedblocks of three stacked blocks. The partially accuratearch-like constructions were of the Proximity type: twoblocks were put next to each other not in contact andsimilarly oriented to the floor, and a third block was puton both, but one of the bottom blocks provided suffi-cient support for the top block. These results show thatwhen they made partially accurate constructions, chim-panzees did not reproduce the overall configuration witha different orientation from the model.

Regarding the inaccurate constructions with theCross-Stack model, these were not prominently alignedstacks of three blocks. In fact, three-block aligned stacksaccounted for 5.7% of the young chimpanzees’ inaccu-rate constructions and 14% of the adult chimpanzees’inaccurate constructions.

Spatial dimension

We analysed the dimensions along which chimpanzeesconstructed when presented with each model, includingaccurate, partially accurate or inaccurate constructions.Figure 7 shows the percentage of constructions in thedifferent dimensions with each model graphed by age.Young chimpanzees mostly made one-dimensional

Table 1 Frequency of accurate, partially accurate and inaccurate constructions by model by chimpanzees

Subject

Line Cross-Stack Arch

AccuratePartially accurate Inaccurate Accurate

Partially accurate Inaccurate Accurate

Partially accurate Inaccurate

AdultAi 1 6 20 0 4 28 0 1 39Akira 0 0 9 0 3 8 0 0 8Pan 0 4 4 0 2 17 0 0 18Pendesa 2 6 8 0 2 32 0 1 13Popo 3 6 7 1 8 6 0 0 26

Overall 6 22 48 1 19 91 0 2 104Young

Ayumu 0 0 3 0 5 4 0 0 3Cleo 0 0 10 0 2 5 0 0 16Pal 0 0 51 0 6 26 0 1 40

Overall 0 0 64 0 13 35 0 1 59

Figure 4 The Age × Construction interaction for the mean accuracy score of chimpanzees. Model constructions are Line, Cross-Stack (C.-S.) and Arch.



constructions extending in the V dimension, no matter whatmodel was present. On the other hand, adult chimpanzeesconstructed in either the H or the V dimension. Whenpresented with Line, adult chimpanzees made a majorityof constructions in the H dimension; when presentedwith Cross-Stack or Arch adult chimpanzees made amajority of constructions in the V dimension. So, withregard to the spatial dimension in which they constructed,adult chimpanzees were clearly more influenced by themodel than young chimpanzees.

Study 1b

This study was designed to generate comparabledata on human children. We wanted to assess develop-mental trends in young human children presentedwith the same modelling task as chimpanzees, and tocompare children’s performance with that of chim-panzees. We basically used the same method as inStudy 1a, with minor differences that are illustratedbelow.

Figure 5 Popo making an accurate Line.

Figure 6 Popo making an accurate Cross-Stack.



Methods

Participants

A total of 30 human children participated in the study.Six human children belonged to each of five age groups:24, 30, 36, 42, and 48 months. Overall there were 16females and 14 males. Each child was tested within 1month of the stated age (e.g. human children in the30-month-old group were between the ages of 29 and31 months of age). Human children were recruited fromItalian state preschools.

Material

This was the same as in Study 1a.

Design

The same three models were used as in Study 1a: Line,Cross-Stack and Arch. Subjects received a block of six

trials for each model. Each subject received one or twoor all three models in the same day, depending on his/her collaboration. Subjects of each age class received thethree models in different but balanced orders, that is, eachblock of trials with a model was given first, or second orthird an equal number of times. Moreover, half of thesubjects received Type I and the other half received TypeII of each model. Subjects were randomly assigned to thetype and the order of the models within each age class.

Procedure

The procedure was very similar to that of Experiment 1a.Each subject was tested individually and seated across atable from the experimenter. The experimenter built amodel with three blocks in front of the child and thengave him/her three other blocks by placing them with thesame surface (A) on the table and not in contact. Whengiving the child the blocks the experimenter asked him/her ‘Can you do the same?’ The model remained in thesame place during the trial. Each trial lasted 30 secondsstarting from the child’s first manipulation of the blocks.During a trial, if the child tried to give the blocks backto the experimenter, before the 30 seconds had elapsed,the experimenter put the blocks back on the table. Theexperimenter did not allow the child to take the blocksfrom the model and also asked the child not to put his/her blocks in contact with those of the model. Theexperimenter also encouraged the child to copy themodel by saying ‘Is it the same? Can you do the same?’The end of a trial was signalled by a kitchen timer. Atthe end of each trial the subject was praised regardlessof the level of the matching produced, and the experi-menter took all the blocks back, before starting a new trial.

Data analysis

This was the same as in Study 1a.

Reliability. Two independent observers separatelycoded 20% of all trials. Cohen’s kappa was used to assessinter-observer agreement separately for each dependentmeasure. For construction accuracy, each observer ratedthe highest score reached in each trial using the three-category scale (i.e. 0–1–2). Cohen’s kappa was .94. Forspatial dimension, each observer coded all constructionsper trial, using the three-category coding (i.e. H–V– HV).Agreement was calculated on the constructions coded byboth observers. Cohen’s kappa was .98.

Results

Construction accuracy

A two-way Analysis of Variance, using Age (24, 30, 36, 42,48 months) as the between-subjects variable and Con-struction (Line, Cross-Stack, Arch) as the within-subjectvariable was conducted. The main effects for Construction,

Figure 7 Proportion of all constructions by young and adult chimpanzees in the vertical dimension (V), the horizontal-vertical (HV) and the horizontal dimension (H) in the presence of each model construction: Line, Cross-Stack or Arch. The correct constructions would be made in the H dimension in the presence of Line, in the V dimension in the presence of Cross-Stack, and in the HV dimensions in the presence of Arch.



F(2, 50) = 8.07, p < .001, and Age, F(4, 25) = 16.18,p < .001, were significant. Tukey HSD tests, p < .05,showed significant differences between Line (Mean score= 1.39) and Cross-Stack (.98), and between Line andArch (1.09). Thus, overall, human children copied Linemore easily than either Cross-Stack or Arch. TukeyHSD tests, p < .05, also showed significant differencesbetween age 24 months (.25) and all other ages; betweenage 30 months (.94) and ages 42 and 48 months (1.62and 1.81, respectively); between age 36 months (1.16)and age 48 months. Thus, within the age range con-sidered, there seemed to be three developmental steps atages 30, 42, and 48 months.

Table 2 shows the overall frequency of accurate,partially accurate and inaccurate constructions made byhuman children. Human children aged 24 months madesome accurate lines, but only partially accurate cross-stacks and no arch-like constructions. Human childrenstarted to make accurate cross-stacks or arches from age30 months.

The types of partially accurate constructions that weremade by human children at the different ages with eachmodel are shown in Table 3. The more advanced partiallyaccurate constructions (i.e. with a different orientationthan in the model) were present at different ages depend-ing on the model. Differently oriented lines were alreadybeing made at age 24 months, differently oriented archeswere being made by age 30 months, and differentlyoriented cross-stacks by age 42 months. These resultsconfirm those for accurate constructions by children.

We also directly compared chimpanzees’ and children’sconstruction accuracy. A mixed two-way Analysis ofvariance was carried out using Experimental group

(Ch, H24, H30, H36, H42, H48) as the between-subjectsvariable and Construction as the within-subject variable.The main effects for Experimental Group, F(5, 32) = 25.74,p < .0001, and Construction, F(2, 64) = 9.27, p < .001,and the interaction of Experimental group × Construc-tion, F(10, 64) = 2.06, p < .05, were all significant. TukeyUnequal N HSD tests, p < .05, showed that chimpan-zees’ and 24-month-old children’s scores were not signi-ficantly different for any particular construction. Moreover,human children at age 30 months had a significantlyhigher score than chimpanzees for Line and Arch, butnot for Cross-Stack. It was only from age 42 monthsthat children’s mean score was significantly higher thanthat of chimpanzees for Cross-Stack.

Spatial dimension

We analysed the dimensions along which human childrenconstructed when presented with each model. In thisanalysis we considered all subjects’ constructions, includ-ing accurate, partially accurate or inaccurate construc-tions. This analysis further showed age-related trends inhow human children were influenced by the models.Figure 8 shows the percentage of constructions in thedifferent dimensions with each model graphed by age.As expected, with the Line model human children atall ages mostly constructed in the H dimension, whilewith the Cross-Stack model they mainly constructed inthe V dimension. Regarding the Arch model, at age 24months, human children mostly made one-dimensionalconstructions in the V dimension. However, from age 30months, human children also started constructing in twodimensions when presented with the model Arch.

Table 2 Frequency of accurate, partially accurate and inaccurate constructions by model by human children

Age


AccuratePartially accurate Inaccurate Accurate

Partially accurate Inaccurate Accurate

Partially accurate Inaccurate

24 Mos. 5 14 4 0 5 25 0 0 2630 Mos. 23 5 8 3 17 20 12 6 1936 Mos. 20 12 14 13 3 20 25 0 2142 Mos. 32 2 9 23 11 3 23 7 1148 Mos. 31 3 4 32 4 5 30 6 11All ages 111 36 39 71 40 73 90 19 88

Table 3 Overall frequency of different types of partially accurate constructions by human children

Age


≠ OrientationTwo Blocks

Aligned ≠ OrientationTwo Blocks

Crossing ≠ OrientationSupport or Proximity

24 Mos. 6 8 0 5 0 030 Mos. 5 0 0 17 4 236 Mos. 3 9 0 3 0 042 Mos. 2 0 4 7 6 148 Mos. 2 1 3 1 2 4All ages 18 18 7 33 12 7



Discussion

The chimpanzee pattern

We show that spatial modelling tasks can be usedsuccessfully to study spatial analytic abilities in apes.Our results extend those of previous studies on chim-panzees’ constructions in several important ways. First,we found developmental differences between young andadult chimpanzees in their construction skills. Onlyadult chimpanzees made some accurate constructions,thus showing the ability to copy a spatial configuration.With respect to the type of spatial relations copied, wefound that chimpanzees constructed cross-stacks beforethey constructed lines, although young chimpanzeesmade the cross-and-support relation only between twoblocks. The young chimpanzees constructed mainly inthe vertical dimension. The salience of the V dimensionfor the young chimpanzees was probably partly due totheir previous experience in stacking blocks. However,

developmental factors must also have been at work. Theyoung chimpanzees had actually developed the ability tostack up blocks from a very early age, either spontaneously(Pal, at age 2 years and 7 months) or immediately aftera human tester began rewarding stacking behaviour(Cleo and Ayumu, age 3 years and 1 month) (Hayashi,2007). Moreover, both the shape of the blocks and thestack-and-cross relation were new to the chimpanzees.

Second, we document a higher level of chimpanzees’construction skills than previously reported. We showthat the cross-and-support relation is within the chim-panzees’ reach at an early age (at 5 years or less) at alevel comparable to that of human children aged 24months. Moreover, we observed rare bi-dimensionalconstructions by chimpanzees. This finding indicatesthat constructing in two dimensions constitutes a cogni-tive challenge for chimpanzees and probably taps theupper boundary of their construction skills. This, inturn, strongly limits chimpanzees’ potential to build com-plex spatial structures, such as enclosed spaces or surfaces.

Our finding about chimpanzees being relatively advancedconstructing in the V dimension compared with the Hdimension may seem at odds with chimpanzees’ nestbuilding skills. All chimpanzees build nests in the wildwhich are complex structures extending in both the H andthe V dimension. In fact, nest building does not necessarilyrequire the constructive skills we have tested in our tasks.Chimpanzees build nests in a very conservative way(Baldwin, Sabater, McGrew & Tutin, 1981). Their techniquewas described by Lawick-Goodall (1962) in great detail.On a foundation, not made but chosen by chimpanzees(e.g. a horizontal fork from which several leafy branchesfan out) several smaller branches are successively bentwith hands or feet towards and under oneself. Given thateach branch remains attached to a tree, chimpanzees musthold them in place by standing on them to prevent thebranch unfolding. The result is a rough interweaving ofbranches. Moreover, as the chimpanzee bends the brancheshe/she moves either in a circle or from one side to anotherso that the end product has a roughly circular shape allaround the animal. Finally, small twigs and leaves are added,often put under some parts of the body to make the nestmore comfortable. The whole process is very rapid (i.e.5 minutes at most). It is clear from this description thateach step of nest building requires coordinating differentactions with various parts of the body on the branches.Branch interweaving is made possible by the shape andflexibility of the branches themselves. The end result isincidental to coordinated actions-on-branches withoutprecise placement of detached objects. In conclusion,although learning and practice are necessary for nestbuilding (Bernstein, 1962; Lawick-Goodall, 1968), theunderlying cognitive processes are simpler than thoseinvolved in arranging detached objects in precise spatialrelation to one another. Wild-born chimpanzee infantsactually start to build rudimentary nests during playbetween ages 8 months (Lawick-Goodall, 1968) and 12months (Plooij, 1984). This is about 2 years earlier than

Figure 8 Proportion of all constructions by human children at different ages in the vertical dimension (V), the horizontal-vertical (HV) and the horizontal dimension (H) in the presence of each model construction: Line, Cross-Stack or Arch. The correct constructions would be made in the H dimension in the presence of Line, in the V dimension in the presence of Cross-Stack, and in the HV dimensions in the presence of Arch.



the age at which first spontaneous block stackingbehaviour is observed (Hayashi & Matsuzawa, 2003).

The human pattern

We found age-related trends between ages 2 and 4 yearsin the frequency and level of human children’s construc-tions to be a function of the type and number of spatialrelations of the constructions modelled. The youngestage group made accurate lines, but only partially accu-rate cross-stacks and no arches. They constructed alongone dimension at a time. At age 30 months, humanchildren took an important developmental step: theystarted to make accurate cross-stacks and arches and toconstruct either in one or two dimensions, depending onthe model. Subsequent development at older ages wasfeatured by a progressive increase in the accuracy ofconstructions. In practice, human children start to makeaccurate lines before they make accurate cross-stacks,and human children make lines before they make anyarch-like construction.

Our findings are not inconsistent with the well-documented finding that human children start to makestacks before they make lines. As already discussed, stacksare simpler constructions than either cross-stacks orlines. Stacks (however high) only require considering asupport relation between two blocks at a time. Our resultsfurther show that, for humans, cross-stacks are morecomplex constructions than simple lines. Accurate cross-stacks and lines both require considering a spatial relationamong three blocks at a time. However, to achieve this,cross-stacks also require that the orientation of the longestaxis of the blocks be alternated so that the middle blocklies at an angle of 90° to the other two.

Other lines of evidence in the previous literature onchildren’s spatial constructions are clearly consistentwith our findings. First, it was reported that humanchildren spontaneously construct crossing relations in theV dimension between ages 30 and 36 months (Vereeken,1961). This is consistent with our finding that humanchildren aged 30 months make accurate cross-stacks in amodelling task. We further show that human childrenaged 24 months can sometimes make partially accuratecross-stacks. Second, it has been reported that humanchildren construct one kind of spatial relation in theirgroupings along one dimension before they producegroupings with more than one relation type in twospatial dimensions (Guanella, 1934; Stiles-Davis, 1988).Other studies reported that human children make accu-rate lines at age 24 months, before they make accuratearch-like constructions at age 30 months, although thosestudies adopted constructions with more blocks thanours (Stiles-Davis, 1988; Stiles & Stern, 2001).

Comparison between chimpanzees and children

Chimpanzees and human children both can cope with ablock modelling task. We can make a direct comparison

between chimpanzees’ and children’s development ofconstruction skills. The chimpanzees’ developmental patterndiffers partially from that of children. First, chimpanzeesare relatively more advanced in copying in the V dimen-sion than in the H dimension. Whereas human childrenstart reproducing (accurate) lines before they reproduce(accurate) cross-stacks or (accurate) arches, chimpanzeesstart reproducing (partially accurate) cross-stacks beforethey reproduce lines (either accurate or partially accurateones). Moreover, young chimpanzees make cross-stacksat a level comparable to that of human children aged24 months, whereas it is only from age 42 months thathuman children are more accurate than chimpanzees inmaking cross-stacks. Only adult chimpanzees makeaccurate next-to constructions and at a level comparableto that of human children aged 24 months, but lowerthan that of human children aged 30 months.

Second, although chimpanzees sometimes make bi-dimensional constructions, it is clear that they do notdevelop in the direction of constructing in two dimensionsas human children do starting from age 30 months. Ourfindings about chimpanzees’ relative advance in construct-ing in the V dimension and difficulty in constructing intwo dimensions are consistent with literature reportsabout spontaneous spatial constructions by chimpanzees(Potì, Langer, Savage-Rumbaugh & Brakke, 1999; Potì& Langer, 2001; Potì, 2005), and indicate that spatialanalysis and spatial representation are at least partiallydifferent in human children and in chimpanzees.

In view of our findings, we suggest that future studiesshould further investigate the processes by whichchimpanzees construct and integrate different parts ofa construction, analogously to what has been done withhuman children (e.g. Stiles & Stern, 2001). To do sowould require modelling new constructions withoutallowing the subject to witness the constructive processby which the experimenter makes a model construction.

Acknowledgements

The present study was supported by grants from theMinistry of Education, Science, and Culture in Japan(#12002009 - #16002001), JSPS-gCOE A06 and 21COE(A14) from the biodiversity research and from JSPS-HOPE. It was also supported in part by Research Fellow-ship 16-1059 from the Japan Society for the Promotionof Science for Young Scientists given to the second author.

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Received: 6 December 2006 Accepted: 7 May 2008

Documents

Pan troglodytes) and young human children ( Homo …...young human children (Homo sapiens sapiens) Patrizia Potì,1 Misato Hayashi2 and Tetsuro Matsuzawa2 1. Institute of Cognitive