Discrimination and categorization of photographs of natural objects by chimpanzees (Pan troglodytes)

Abstract Two experiments assessed the ability of fouradult female chimpanzees to categorize natural objects.Chimpanzees were initially trained to match different colorphotographs of familiar objects from four possible cate-gories. In training, all the comparison stimuli were fromthe same category in one condition, and from differentcategories in another condition. For all subjects, trainingperformance was consistently better for the “different cat-egory” than for the “same category” trials. Probe trialswere shown after training. In probe trials, the sample andpositive comparison stimuli were different items from thesame category, and the foils were selected from among thethree other test categories. Individual performance wasabove chance in probe trials, suggesting that categoriza-tion by chimpanzees may transcend perceptual resemblance.These results were later replicated with novel stimulusitems from the same four categories (experiment 2).Altogether, this research demonstrates that chimpanzeesgrouped perceptually different exemplars within the samecategory, and further suggests that these animals formedconceptual representations of the categories.

Keywords Object categorization · Photographs ·Primates · Cognition · Chimpanzees

Introduction

The ability to perceive relations among objects and to cat-egorize them is one of the fundamental elements of hu-

man cognition. Many studies have reported that nonhu-man animals – like humans – can form various types ofcategories. Some categories are based on natural objects:trees, water, people, cats, and flowers (e.g., Bhatt et al.1988; Herrnstein et al. 1976). Others are based on artifi-cial objects: car, chair, alphanumerical characters, evenpaintings by Monet or Picasso (e.g., Bhatt et al. 1988;Vauclair and Fagot 1996; Watanabe et al. 1995). To citeonly one of many possible examples, Yoshikubo (1985)showed that rhesus monkeys were able to distinguish in-dividuals of their own species from individuals of otherspecies of macaque monkeys. This suggests that the rhe-sus monkeys were able to discriminate and categorizespecies that resembled one another. In studies on animalcategorization, subjects have often been trained with asmall number of exemplars from each category, prior tobeing tested with novel exemplars. When subjects contin-ued to respond to the novel exemplars in the same waythat they had responded to the training exemplars, resultswere said to demonstrate open-ended categorization.These studies, however, lack an important control neededto definitively infer categorization: the demonstration thatexemplars from the same category are discriminably dif-ferent.

In humans, categorization is assumed to occur whenobservers respond in an equivalent manner to discrim-inably different stimuli (Behl-Chadha 1996). Only a fewanimal studies have addressed the issue of within-classdiscrimination (Thompson 1995; Vauclair and Fagot 1996;Wasserman et al. 1988). Wasserman et al. (1988) trainedpigeons to discriminate individual exemplars within eachof four categories. Results showed more errors for within-than between-category discrimination. Vauclair and Fagot(1996) showed that baboons were able to categorize thealphanumeric characters “B” and “3” in various fontstyles. After the first experiment, an identity matching-to-sample task was used to assess the issue of within-classdiscrimination. Results showed that exemplars from thesame category were discriminably different, suggestingthat baboons developed open-ended categorical proce-dures. Altogether, these studies suggest that nonhuman

Masayuki Tanaka

Discrimination and categorization of photographs of natural objects by chimpanzees (Pan troglodytes)

Anim Cogn (2001) 4 :201–211DOI 10.1007/s100710100106

Received: 29 July 2000 / Accepted after revision: 10 August 2001 / Published online: 5 October 2001

ORIGINAL ARTICLE

Electronic supplementary material to this paper can be obtainedby using the Springer Link server located athttp://dx.doi.org/10.1007/s100710100106

M. Tanaka (✉ )Department of Brain and Behavioral Sciences, Primate Research Institute, Kyoto University, Kanrin, Inuyama, Aichi 484-8506, Japane-mail: [email protected], Tel.: +81-568-630548, Fax: +81-568-630085

© Springer-Verlag 2001

primates and pigeons can sort perceptually different itemsinto the same class.

In humans, the class in which objects are classifiedmight depend upon relations between objects. That is, acollie might be classified as a dog in one context, but asan animal in another context. “Basic levels” of catego-rization (Rosch et al. 1976) emerge early in infancy in hu-mans (Behl-Chadha 1996). Roberts and Mazmanian(1988) reported that pigeons and monkeys had difficultydiscriminating between birds and other animals, or ani-mals and non-animals, although they learned to discrimi-nate between kingfishers and other birds. Results suggestthat the pigeons and the monkeys do not have basic levelsof categorization similar to those of humans.

The present study investigated categorization abilitiesin four adult female chimpanzees. Flowers, trees, weeds,and ground surface were used as experimental categoriesbecause the chimpanzees see these items every day andthey might promote category formation. Photographs offamiliar items were used in experiment 1. The subjectswere individually trained for within-class (e.g., flowers)and between-class discrimination (e.g., tree vs. weed).After training, probe trials were proposed. In probe trials,the sample and the positive comparison stimuli were dif-ferent items from the same category, and the foils were se-lected among the three other test categories. Experiment 2tested whether the subjects could discriminate and catego-rize the novel items from the same four categories.

Experiment 1

Methods

Subjects

The subjects were four adult female chimpanzees (Pantroglodytes), named Ai, Mari, Popo, and Pan (21, 21, 16,and 14 years old, respectively). They had previously beentested in various experiments on cognitive abilities (e.g.,Hashiya and Kojima 1997; Kawai and Matsuzawa 2000;Matsuzawa 1985; Tanaka 1996, 1997). They lived withseven other chimpanzees in an outdoor compound and at-tached indoor residence. They were not deprived of foodat any time during the present study. Care and use of thechimpanzees adhered to The Guide for the Care and Useof Laboratory Primates (1986) of the Primate ResearchInstitute, Kyoto University. Before the present study, allsubjects had already been trained in a matching-to-sampletask on the basis of identity, using the same apparatus asin this study.

Apparatus

Each chimpanzee was trained and tested in an experimen-tal booth (1.8 m wide, 1.8 m deep, and 2.0 m high). A 21-inch (53 cm) CRT display (Totoku, CV213PJ) with a

touch screen (MicroTouch) was installed on one wall ofthe booth. A universal feeder (Biomedica, BUF-310) de-livered small pieces of food reward (apples, or raisins)into a food tray below the display. A Pentium 200 com-puter (A-one, VX-5169) controlled the equipment.

Stimuli

Four categories of photographs were used: flower, tree,weed, and ground. Items shown on the photographs werepresent near the outdoor compound, and could be seen ona regular basis. Each category consisted of different typesof exemplars. For instance, the category “flower” con-tained Japanese cherry, dandelion, camellia, and azalea.We prepared three photographs for each type of exem-plar, resulting in a total of 48 pictures (3 photographs×4 types×4 categories; see Fig. 1 and electronic supplemen-tary material, ESM, S1, for a colour version). An effortwas made to have a heterogeneous set of images. To thataim, different items were photographed for each type ofexemplar and pictures varied in the number of items andcamera angle. All photographs were taken with a camera,and optically scanned (Epson, GT-6500WIN2). Each pho-tograph was 7×7 cm.

Procedure

Experiment 1 consisted of a training phase followed by atest phase.

Training phase. A trial began with the appearance of awarning stimulus (gray filled square, 4×4 cm) on thelower portion of the computer screen. After the subjecthad touched the warning stimulus, the display was re-freshed and a sample photograph appeared in the center ofthe screen. The subjects were then requested to touch thesample photograph to initiate presentation of four com-parison stimuli in the four corners of the display. A ses-sion comprised two tests of conditions, hereafter referredto as the different categories (D) and same category (S)conditions. In both D- and S-trials, the sample and correctcomparison stimuli showed two different photographs ofthe same type of exemplar from the same category (e.g.,Japanese cherry). When the subject chose the correctcomparison, a chime sounded and a food reward was de-livered. Choices of the incorrect comparisons were fol-lowed by a blackout of the display and a 3-s timeout. Theintertrial interval was 1 s.

In the D-condition, the distractors were from differentcategories than that of the sample. In the S condition, thedistractors were from the same category as the sample,but from different types of exemplar. For instance, sampleand comparison showed a Japanese cherry, but distractorsshowed a dandelion, camellia, or azalea.

Subjects were given 48 D-trials and 48 S-trials in thesame category condition in each session. Each of the 48photographs was presented once as a sample in S- and D

202

trials. The trials were mixed randomly and the position ofthe correct comparison was counterbalanced. The subjectswere expected to be able to match more than 80% correctin both S- and D-trials, but not all of them improved theirperformance in S-trials. Therefore, the training phase wasdiscontinued after the 20th session.

Test phase. The general testing procedure was the same asthat of training. Test trials were intermixed randomlyamong baseline S- and D-trials (similar to those of train-ing). On test trials, the match was from the same categoryas the sample, but from a different type of exemplar. Forexample, when the sample was a photograph of a Japanesecherry, the comparisons involved a photograph of a dan-

delion (same category as the sample) and photographs ofitems from tree, weed, and ground.

The stimuli used in the test trials were chosen as fol-lows. Sample and comparison stimuli used in the test tri-als were chosen considering performance achieved duringthe last ten training sessions. First, we retained only thetwo pictures of each category (total=8 types of exemplar)associated with the best matching performance when pre-sented as sample in S-trials. These items were used assamples in the test trials. Second, for each picture retainedas test sample, we selected the two comparison pictures(total of 16 types of exemplars) that were the least fre-quently selected by animals when an error was made in S-training-trials. These items were used as the positive

203

Fig.1 Exemplars of stimuliused in experiment 1. Therewere three photographs ofeach item, as shown in the bot-tom row (for a color version ofthis figure, see electronic sup-plementary material, ESM, S1)

comparisons in the test trials. Note that this procedure forstimulus selection ensured that the test sample and posi-tive comparison stimuli were discriminably different.

The test sessions consisted of 16 probe trials randomlyintermixed with 96 baseline trials (48 S- and 48 D-trials)similar to those of training. Each test sample stimulus waspresented twice during a session, once with each of twopositive comparison stimuli with which it was paired. Ineach probe trial, the three distractors were selected fromthe three categories of items different from the samplecategory. Testing involved 18 such sessions.

Results

Training phase

Figure 2 shows the individual percentage of correct S- andD-trials. Subjects consistently showed better performancein the D- (mean correct=92.9) than in the S-trials (meancorrect=60.7) in the last ten training sessions. All chim-panzees exceeded 80% correct or more in D-trials after

the 11th training session. Performance after the 11th ses-sion became lower than 80% in S-trials, but still exceededchance level (i.e., 25%) for each subject (binominal test,all Ps<0.05).

A category by test condition analysis of variance(ANOVA) was computed on performance data obtained in the last ten training sessions. This ANOVA revealed asignificant main effect of test conditions, F1, 24=36.6,P<0.001, but no significant effect of category F3, 24=1.13,P=0.359, and no significant condition by category interac-tion F3, 24=0.16, P=0.922, therefore suggesting similar re-sponse behaviors for the four categories of items.

Test phase

Subjects achieved 83.8% correct on average in baselinetrials. Individual baseline performance was above chancefor both D- (96.5% correct) and S- (71.1% correct, bino-mial tests, all Ps<0.001). Table 1 shows the total percent-age, of correct D- and S-trials (baseline). Each subjectshowed the same results as in the training. Categories(flower, tree, weed, and ground) by conditions (S-, D-)ANOVA performed on the number of correct trials re-vealed that the main effect of conditions was significantF1,24=26.8, P<0.0001, showing reduced performance in S- compared to D-trials.

Table 2 indicates individual response choices in S-base-line trials and in probe test trials. Considering baseline tri-als, Table 2 shows generally high performance for eachindividual item. These high performance levels indicatethat introduction of the probe trials had no disruptive ef-fect on chimpanzees’ responses.

More importantly, Table 2 also indicates that the posi-tive comparison stimulus in probe trials was often se-lected above chance. Preference for the positive compari-son stimulus emerged as significant 11, 5, 11, and 14 (outof 16) times for Ai, Mari, Popo, and Pan, respectively.This result suggests that the animals matched the sampleand comparison stimuli on a categorical basis.

Although the procedure adopted for selecting the sam-ple and positive comparison stimuli made it likely that

204

Fig.2 Percentage of correct responses in D- (Different) and S-tri-als (Same) for each subject in experiment 1

Table 1 The percentage of correct trials for each category and forthe total in the different categories (D-trials) and the same category(S-trials) conditions in the testing phase

Subject Trial type Category Total

Flower Tree Weed Ground

Ai Different 95.8 84.7 94.0 96.8 92.8Same 75.9 65.3 66.7 67.6 68.9

Mari Different 97.7 65.3 88.9 88.0 85.0Same 69.0 55.1 48.6 60.6 58.3

Popo Different 97.7 90.7 88.4 94.4 92.8Same 73.1 66.2 68.5 64.8 68.2

Pan Different 98.6 87.5 88.9 95.8 92.7Same 62.5 56.0 55.1 57.4 57.8

these forms were perceptually different, we searched inthe data set for information suggesting that this was in-deed the case. Thus, we analyzed the proportion ofchoices of each comparison stimulus shown in Table 2during baseline S-trials (in which they served as distrac-tors) and during probe trials (in which they served as pos-itive comparison stimuli). In baseline trials, Ai, Mari,Popo, and Pan selected these forms 5.79%, 13.4%, 9.49%,and 11.1% of times, respectively. These same items wereselected 63.5%, 55.9%, 53.8%, and 80.6% of times on av-erage by the same animals in probe trials. Wilcoxon testson individual data revealed that each comparison stimuluswas selected more often in probe than in baseline S-trials(Ai: Z=3.51, Mari: Z=3.51, Popo: Z=3.40, Pan: Z=3.51;all Ps<0.001). In other words, results demonstrate thatitems from the same category as the sample were avoidedin baseline trials, in which the comparison stimulus was aview of the exact same item as the sample, but preferredover the other distractors in probe trials, when there wasno better match because the sample item was no longershown as comparison.

Discussion

The results in the training phase revealed that the subjectswere able to choose the photographs of the same items asthe sample, although the photographs differed from oneanother. Results therefore suggest that the chimpanzeesused categorical procedures. The results however did notprovide clear-cut information on the physical or concep-tual cue that was referred to for responding. We alsofound enhanced performance in each subject in D- com-pared to S-trials, regardless of the category. The resultssuggest that the chimpanzees perceive items from thesame category as more similar than items from differentcategories.

The test trials showed two important results. First, thechimpanzees tended to choose items from the same cate-gory as the sample when the photograph of the samplewas not among the comparisons. Second, the same image

205

Table 2 Percentage correct for each subject and test sample item,and percentage of correct selection of positive comparison duringprobe test trials. Asterisks indicate that performance is abovechance level

Subject Baseline S-trials Test trials

Sample %Correct Comparison %Correct

Ai Flower1 94.4*** Flower2 55.6***Flower3 88.9***

Flower2 92.6*** Flower3 77.8***Flower4 27.8

Tree2 88.9*** Tree1 33.3Tree3 33.3

Tree3 63.0*** Tree1 55.6***Tree2 22.2

Weed2 57.4*** Weed3 27.8Weed4 94.4***

Weed3 96.3*** Weed1 72.2***Weed2 50.0*

Ground2 100*** Ground1 100***Ground4 100***

Ground4 83.3*** Ground2 83.3***Ground3 94.4***

Total 84.5*** 63.5***

Mari Flower1 66.7*** Flower2 44.4 Flower4 27.8

Flower2 94.4*** Flower1 55.6***Flower3 55.6***

Tree1 55.6*** Tree2 38.9Tree3 22.2

Tree3 29.6 Tree2 16.7Tree4 44.4

Weed2 50.0*** Weed1 44.4Weed4 72.2***

Weed3 64.8*** Weed1 27.8Weed2 72.2***

Ground2 92.6*** Ground1 38.9Ground4 38.9

Ground4 63.0*** Ground2 33.3Ground3 66.7***

Total 64.6*** 55.9***

Popo Flower1 92.6*** Flower3 5.56Flower4 5.56

Flower2 87.0*** Flower3 0Flower4 5.57

Tree1 68.5*** Tree2 66.7***Tree4 94.4***

Tree2 85.2*** Tree3 50.0*Tree4 66.7***

Weed1 64.8*** Weed3 61.1***Weed4 77.8***

Weed3 81.5*** Weed1 55.6***Weed2 27.8

Ground2 92.6*** Ground1 88.9***Ground4 100***


Total 79.2*** 53.8***

Pan Flower2 70.4*** Flower1 100***Flower4 100***

Flower3 61.1*** Flower1 94.4***Flower2 100***

Tree1 55.6*** Tree2 55.6***Tree3 77.8***

Tree2 94.4*** Tree1 66.7***Tree4 44.4

Weed1 59.3*** Weed2 83.3***Weed3 38.9

Weed2 59.3*** Weed1 88.9***Ground4 61.1***

Ground2 100*** Ground3 100***Ground4 83.3***


Total 69.9*** 80.6***

***P<0.001, *P<0.05 (binomial test)

Table 2 (continued)



was avoided when presented as comparison as baselinetrials. These two aspects of the data suggest that the set ofsample and comparison stimuli were perceptually differ-ent from one another, and that the subjects considered thecategories to which the items as a basis for response,rather than their strict physical resemblance. Interestingly,subjects were food-reinforced only after they had chosenthe same individual item as the sample in the trainingphase and baseline trials of the testing phase. The findingthat the chimpanzees matched on a categorical basis inprobe trials suggests that representations of the categorieswere already formed prior to the test.

Experiment 1 also suggests that the chimpanzees couldchange the levels of categorization according to the rela-tions between the stimuli. That is, the chimpanzees wereable to categorize not only the different photographs of

the same item but also different items from the same cat-egory. Such categorization with an embedded structure isfundamental for humans. The chimpanzees might haverudimentary abilities to organize a human-like hierarchi-cal category structure.

Of course, items from the same category resembledeach other more than items from different categories.Therefore, chimpanzees could have chosen the items onthe basis of perceptual resemblance. However, within-cat-egory similarity and between-category dissimilarity arenatural characteristics of basic categories (e.g., Rosch etal. 1976). Humans also may use perceptual resemblanceto classify items into categories. Because we used coloredphotographs, it is possible that color cues controlled dis-crimination. However, consideration of color cues wouldhave been of little help in test trials in choosing items

206

Fig.3 Exemplars of stimuliused in experiment 2. Therewere three photographs ofeach item, as shown in the bot-tom row (for a color version ofthis figure, see electronic sup-plementary material, ESM, S2)

from the same category as the sample. In addition, eachsubject showed very clear discrimination among itemsfrom both tree and weed categories (see Table 1), whichwere of similar colors (i.e., green). Results in the test tri-als suggest that the chimpanzees used some informationabout the natural categories which they found in the train-ing phase or had found before this study.

The results showed differences in discrimination per-formance among subjects. Indeed, Ai, and Pan showedbetter discrimination than Mari and Popo, especially in S-trials (see Table 1). Such differences between subjectshave often been shown in other cognitive tasks, and arenot restricted to the current study (e.g., Tanaka 1995, 1996).Regardless of individual differences in baseline perfor-mance, Mari and Popo showed the same effects as theother chimpanzees: they selected the items from the samecategory as the sample in test trials.

Experiment 2

Experiment 1 suggests that the chimpanzees could catego-rize “familiar” items in a human-like manner. Experiment 2investigated whether the chimpanzees could similarly cat-egorize novel items.

Methods

Subjects and apparatus

They were the same as in Experiment 1.

Stimuli

The stimuli were 48 novel (7×7 cm) color photographsfrom four categories (flower, tree, weed, and ground; Fig.3). These were all different from the photographsused in experiment 1. All items were novel to the subjects.The “flowers” and “trees” were foliage plants for garden-ing. “Weeds” were kinds of herbs that did not grow nearthe compound where the chimpanzees lived. The itemsfrom the “ground” category consisted of surfaces of tiledfloors, wooden floors, tatami mats, and carpets. As in ex-periment 1, each category consisted of four types of ex-emplar. We prepared three photographs for each type,which differed in camera angle. All photographs weretaken with a digital camera, and then stored as computerfiles.

Procedure

Experiment 2 consisted of two phases, the training andtesting phases. The procedure of each phase was same asin experiment 1 except that the new set of stimuli wasused.

Results

Training phase

Figure 4 shows the percentage of correct trials in S- andD-trials for each subject. All subjects consistently showedbetter performance in D- than in S-trials after session 1,and chose the correct match on more than 80% of the D-trials after session 2. Category (flower, tree, weed, andground) by condition (S-, D-) ANOVA performed on thenumber of correct trials in the last ten sessions revealedmain effects for categories, F3,24=13.3, P<0.001, and con-ditions, F1,24=67.1, P<0.001, and a significant categoriesby conditions interaction, F3,24=11.6, P<0.001. Post hocanalysis (Tukey HSD) showed that Mari, Popo, and Panachieved better performance with the flower than the treeor weed categories. Ai showed a significant difference inperformance only between flower and weed.

207

Fig.4 Percentage of correct responses in D- (Different) and S-tri-als (Same) for each subject in experiment 2

Test phase

Table 3 reports individual percentages of correct S- andD-trials obtained during the test phase. Each subjectshowed the same results as in the training phase: they con-sistently showed better performance in the D- than in theS-trials throughout the test phase. Performance dependedupon the test category; subjects showed very good perfor-mance with the “flower” category (mean correct=95.3%)and poor performance with the “weed” category (meancorrect=69.9%).

Table 4 shows individual response choices in S-base-line trials and in probe test trials. The format is the sameas that of Table 2. Considering baseline trials, Table 4shows generally high performance for each individualitem except for a few. These high performance levels in-dicate that introduction of the probe trials had little dis-ruptive effect on chimpanzees’ responses.

More importantly, Table 4 also indicates that the posi-tive comparison stimulus in probe trials was often se-lected at above chance levels. Preference for the positivecomparison stimulus emerged as significant 15, 13, 12,and 14 (out of 16) times for Ai, Mari, Popo, and Pan, re-spectively. Because the sample and match were two dif-ferent types of exemplar belonging to the same category,this result demonstrates that the animals matched the sam-ple and comparison stimuli on a categorical basis.

Although the procedure adopted for selecting the sam-ple and positive comparison stimuli made it likely thatthese forms were perceptually different, we searched inthe data set information suggesting that it was indeed thecase. Thus, we analyzed the proportion of choices of eachcomparison stimulus listed in Table 4, when these stimuliwere presented in baseline S-trials (in which they servedas distractors) and when they were presented in probe tri-als (in which they served as positive comparison stimu-lus). In baseline trials, Ai, Mari, Popo, and Pan selectedthese forms 3.00%, 19.9%, 13.4%, and 12.0% of times re-spectively. These same pictures were selected 80.6%,72.9%, 67.7%, and 76.4% of times on average by thesame animals in probe trials. Wilcoxon tests on individualdata showed that each comparison stimulus was selected

more often in probe than in baseline S-trials (Ai: Z=3.51,Mari: Z=3.40, Popo: Z=3.51, Pan: Z=3.51; all Ps<0.001).In other words, the results demonstrate that exemplarsfrom the same category as the sample were avoided inbaseline trials, in which the comparison stimulus was aview of the same item as the sample, but preferred overthe other distractors in probe trials, when there was nobetter match because the sample item was no longer beingshown as comparison.

Discussion

Experiment 2 confirmed and expanded the findings of ex-periment 1. First, it showed that the chimpanzees wereable to match different photographs of novel items.Second, results confirmed the chimpanzees were able todiscriminate the items from different categories more eas-ily than ones from the same category. These two findingsemerged although all items were novel for the chim-panzees. Each subject was able to choose the correct com-parison almost perfectly in D-trials. However, the perfor-mance of each subject varied with the category in S-trials.Items from the “weed” category were the most difficult todiscriminate; Mari, Popo and Pan apparently did not dis-criminate them. In contrast, each subject showed almostperfect scores in S-trials involving samples from theflower category. It is possible that perceptual resemblanceamong items within each category differed from one cate-gory to another. For example, the items from the flowercategory differed in the color of the petals whereas itemsfrom the weed category shared similar colors (i.e., verdantgreen, yellow, or green) although other features (e.g., theshapes of the blades) differed from one another.

Table 4 shows that, in the probe test trials, the subjectschose more items from the same category as the samplethan expected by chance. Complementarily, this table in-dicates that the subjects could discriminate between mostof the items used as samples or comparisons in S-trials.The results suggest that the chimpanzees could match thenovel items, at least from a subset of categories, as theydid in experiment 1 with familiar items. Categorizationprocesses are therefore applicable to both novel and fa-miliar items.

A statistical comparison between experiments 1 and 2would be meaningless because experiment 1 was run priorto experiment 2, and any difference between the two testscould reflect an effect of training or test order. Moreover,we have not attempted to equate the discriminability ofthe stimuli in these two experiments. The experimentaldesign therefore made it impossible to verify whether fa-miliar items were more correctly selected than novel onesin these two experiments. However, Table 4 shows that Aiperformed well in all testing conditions. At least, her re-sults suggest that she was able to sort novel items into cat-egories.

208

Table 3 The percentage of correct trials for each category and forthe total in the different categories (D-trials) and the same category(S-trials) conditions in the testing phase in experiment 2

Subject Trial type Category Total

Flower Tree Weed Ground

Ai Different 98.3 95.8 98.3 99.2 97.9Same 98.3 72.5 56.7 76.7 76.0

Mari Different 100 99.2 99.2 98.3 99.2Same 91.7 51.7 35.8 75.0 63.5

Popo Different 94.2 94.2 90.0 99.2 94.4Same 95.8 50.8 39.2 95.0 70.2

Pan Different 100 100 100 100 100Same 98.3 78.3 49.2 85.8 77.9

General discussion

The results of experiments 1 and 2 suggest the following:

1. Chimpanzees were able to match photographs of nat-ural objects, according to the category to which theybelonged.

2. Within-class discrimination was more difficult for thechimpanzees than between-class discrimination.

3. The experimental exemplars that the animals couldcategorize were discriminably different.

4. Selection of the response stimulus could either be madeconsidering the kinds of objects, or their category, de-pending upon the type of problem that had to besolved. In particular, chimpanzees could to some ex-tent select a photograph of an item from the same cat-egory as the sample when there was no photograph ofthe same item as the sample.

Thompson (1995) claimed that the notion that differentobjects have common class attributes, which permit themto be distinguished from each other, is at the core of con-ceptual categorization. So far, only few studies have ad-dressed this issue experimentally (Vauclair and Fagot1996; Wasserman et al. 1988), in contrast to the manystudies that have assessed concept formation in nonhumananimals. The main purpose of the present study was toverify that the subjects could effectively discriminateitems from the same category, before testing whether thesubjects could categorize the items.

The present study suggests that the chimpanzees mightcategorize real objects in the same manner as humans do.Of course, there might not be any ecological need for thechimpanzees to discriminate between trees and weeds,flowers and trees, or flowers and weeds. However, thesubjects quickly learned to discriminate items in D-trials.The results may suggest that (1) the chimpanzees had al-ready formed such categories prior to the present studyand that (2) the categories might have some significancefor the chimpanzees. The results of the present study con-trast with those of Roberts and Mazmanian (1988). Theseauthors showed that pigeons and monkeys had difficulties

209

Table 4 Percentage correct for each subject and test sample item,and percentage of correct selection of positive comparison duringprobe test trials. Asterisks indicate that performance is abovechance, as inferred by a binomial test



Ai Flower5 96.3*** Flower6 100***Flower7 88.9***

Flower6 98.1*** Flower7 66.7***Flower8 38.9

Tree5 57.4*** Tree7 77.8***Tree8 88.9***

Tree6 75.9*** Tree5 83.3***Tree7 83.3***

Weed6 88.9*** Weed7 83.3***Weed8 83.3***

Weed7 77.8*** Weed5 100***Weed6 94.4***


Ground7 94.4*** Ground5 27.8Ground8 88.9***

Total 85.4*** 80.6***

Mari Flower6 98.1*** Flower5 0Flower7 44.4

Flower8 98.1*** Flower6 33.3Flower7 50.0*

Tree5 81.5*** Tree7 83.3***Tree8 83.3***

Tree7 50.0*** Tree5 88.9***Tree8 100 ***

Weed6 24.1 Weed5 94.4***Weed7 100***

Weed7 42.6** Weed6 94.4***Weed8 100***



Total 71.1*** 72.9***

Popo Flower5 96.3*** Flower6 44.4Flower7 55.6***

Flower7 98.1*** Flower6 38.9Flower8 5.56

Tree5 81.5*** Tree7 83.3***Tree8 88.9***

Tree7 74.1*** Tree5 77.8***Tree6 100***

Weed5 27.8 Weed7 100***Weed8 100***

Weed7 48.1** Weed5 100***Weed6 94.4***

Ground5 100*** Ground6 50.0*Ground7 55.6***

Ground7 98.1** Ground5 11.1Ground6 77.8***

Total 78.0*** 67.7***

Pan Flower6 98.1*** Flower7 94.4***Flower8 66.7***

Flower7 92.6*** Flower5 83.3***Flower6 88.9***

Tree5 59.3*** Tree7 94.4***Tree8 83.3***

Tree7 79.6*** Tree5 72.2***Tree6 83.3***

Weed5 38.9* Weed7 94.4***Weed8 100***

Weed6 50.0*** Weed7 83.3***Weed8 94.4***


Ground7 94.4*** Ground5 5.56Ground8 100***

Total 76.4*** 76.4***

***P<0.001, **P<0.01, *P<0.05

Table 4 (continued)



in sorting stimuli at a level that would be considered “ba-sic” for humans (i.e., bird vs. other animals). Moreover,the nonhuman subjects failed to transfer to novel exem-plars. Of course, humans were able to discriminate birdfrom non-bird slides, and animal from non-animal slides,and to transfer to novel exemplars.

Humans organize categories at different levels. That is,humans could classify an object into categories of basiclevel (e.g., dog), superordinate level (e.g., animal), or sub-ordinate level (e.g., collie). Especially, humans are likelyto classify at the basic level, where there is higher within-category similarity and between-category dissimilarity(Rosch et al. 1976). The present study revealed that chim-panzees could indeed match the exemplars in both subor-dinate (e.g., dandelion) and basic-level categories (e.g.,flowers), apparently in the same way that humans do. Thepresent study did not make it clear whether the chim-panzees could match at the superordinate level. From ahuman perspective, the flower, tree, and weed categoriesmight be considered more natural than the ground cate-gory because they correspond to real objects existing innature. Moreover, the flower, tree, and weed categoriesbelong to the superordinate “plant” category. Interestingly,no difference emerged in terms of discrimination perfor-mance between the ground category and the more naturalflower, tree, and weed categories. The results suggest thatthe chimpanzees were not sensitive to the naturalisticcharacter of these latter categories, at least in the presentstudy.

Premack and Premack (1983) suggested that nonhu-man animals have a strong bias towardsresponding to ap-pearance. Language training appears to remove the biasso that the subject can respond according to “abstractcodes” (Premack 1983). According to Markman (1989),superordinate categories are difficult to learn, even by hu-man children, because exemplars from superordinate cat-egories can hardly be grouped considering perceptual re-semblance, stimuli being functionally rather than percep-tually equivalent. Tanaka (1997) showed, however, that achimpanzee could overcome the difficulty in categorizingitems on a functional basis, when previously trained to as-sociate arbitrary symbols with the functions. In support ofthis idea, Savage-Rumbaugh et al. (1980) showed thatlanguage-trained chimpanzees were able to classify ob-jects into food versus nonfood categories. If “language-training” makes a chimpanzee less likely to respond onthe basis of stimulus appearance, another language-trained chimpanzee, Ai, might be able to form categoriesat the superordinate level. However, in so far as we usethe same procedure as in the present study, it may be dif-ficult for the other chimpanzees to form superordinate cat-egories.

An important difference was found between experi-ments 1 and 2: only in experiment 2 did the effect of cat-egory emerge as significant. In our study, we could notcontrol for perceptual similarity, because the number ofdifferent species of trees, flowers, and weeds in the ani-mal environment was too limited to do so. Therefore, dif-ferences between experiments in terms of category effect

might be attributed to the fact that some categories weremore heterogeneous than others. Because there was nostrict control of perceptual resemblance in our experi-ments, it is possible that the chimpanzees failed to dis-criminate some of the items belonging to the same cate-gory. We maintain, however, that this possibility did notapply to the majority of the test items. First, the test stim-uli were selected from the training items that gave rise tothe best discrimination performance. Second, all thechimpanzees performed at above-chance levels in S-trialsduring the test, showing that they could discriminate thepositive comparison from the distractors. Third, perfor-mance in probe trials confirmed that the chimpanzeescould discriminate the positive comparison stimulus fromthe distractors. The results, moreover, demonstrated thatchimpanzees could match physically dissimilar items thatbelonged to the same category. Considering these findingsand the fact that stimuli were perceptually different, weare inclined to conclude that the chimpanzees did not cat-egorize items on the basis of their physical appearance,but rather developed more conceptual representations ofthe categories.

Acknowledgements We wish to thank Drs. T. Matsuzawa, M.Tomonaga, and other members of Department of Behavioral andBrain Sciences, Primate Research Institute, Kyoto for their helpfuladvice. I also thank Drs. J. Fagot, and I.H. Iversen for their readingand comments on earlier drafts and Mr. S. Nagumo for his techni-cal help in programming and interfacing. This study was supportedby Grants-in-Aid for Scientific Research, Ministry of Education,Science, and Culture 12002009, 12710037 to the authors, and10CE2005.

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Discrimination and categorization of photographs of natural objects by chimpanzees (Pan troglodytes)