Object Permanence Tests on Gibbons (Hylobatidae) - CiteSeerX

Journal of Comparative Psychology 2008, Vol. 122, No. 4, 403– 417

Copyright 2008 by the American Psychological Association 0735-7036/08/$12.00 DOI: 10.1037/0735-7036.122.4.403

Object Permanence Tests on Gibbons (Hylobatidae) Anna Fedor

Gabriella Skolla´r

Eo¨tvo¨s Lorańd University of Sciences

Szeged University of Sciences

No´ra Szerencsy

Ma´ria Ujhelyi

Eo¨tvo¨s Lorańd University of Sciences

Semmelweis University

Ten gibbons of various species (Symphalangus syndactylus, Hylobates lar, Nomascus gabriellae, and Nomascus leucogenys) were tested on object permanence tasks. Three identical wooden boxes, presented in a linear line, were used to hide pieces of food. The authors conducted single visible, single invisible, double invisible, and control displacements, in both random and nonrandom order. During invisible displacements, the experimenter hid the object in her hand before putting it into a box. The performance of gibbons was better than expected by chance in all the tests, except for the randomly ordered double displacement. However, individual analysis of performance showed great variability across subjects, and only 1 gibbon is assumed to have solved single visible and single invisible displacements without recourse to a strategy that the control test eliminated. Keywords: object permanence, gibbons, Piagetian tests, mental representation

& Cole, 2002; Piaget, 1937/1954). Success on single visible displacements indicates Stage 4b of object permanence, and subjects at this stage commit the A-not-B or perseveration error, which means that if the object is hidden in various places during sequential trials, the subjects will search in the previously successful place. They overcome this kind of error later, at Stage 5a, when they are able to solve sequential visible displacements. Further into this stage (Stage 5b), subjects can solve single invisible displacements, but they fail to solve sequential invisible ones either because they commit the perseveration error or because they commit other types of errors. Solving sequential invisible displacements marks the beginning of Stage 6a. Subjects at Stage 6b can solve double invisible displacements, meaning that they can mentally reconstruct the movements of an unperceived object (Dore´ & Dumas, 1987). However, from the original object permanence tasks further developed by Uzgiris and Hunt (1975), it cannot be assumed that subjects are searching just for something of interest or for the particular object that had been hidden (Pepperberg & Funk, 1990). To test whether subjects mentally represent and remember the identity of the object, other types of tasks need to be completed in which the experimenter surreptitiously replaces the object during invisible displacements. If subjects are surprised at finding something other than what had been hidden, it demonstrates a complete understanding of object permanence (e.g., Funk, 1996; Pepperberg, Willner, & Gravitz, 1997). Infants younger than 9 months do not search for an occluded object. Piaget (1937/1954) attributed this to the lack of object permanence; children do not search for an object behind a screen because they do not understand that it still exists when out of sight. However, using visual habituation tasks, which do not require reaching, Baillargeon, Spelke and Wasserman (1985) found that even 5-month-old infants seem to understand that an object still exists when occluded. According to them, 5-month-olds fail the Piagetian search task not because of the lack of object permanence

Object permanence—the understanding that objects are separate entities that continue to exist when out of sight (e.g., Funk, 1996; Piaget, 1937/1954)—is most frequently studied with the help of Piaget’s object permanence tasks, in which subjects have to find objects that were previously hidden by the experimenter. The two main types of this task are visible and invisible displacements. During visible displacements, the experimenter hides an object in full view of the subject, whereas during invisible displacements, the experimenter first puts the object in a container, then moves the container behind a screen and drops the object inconspicuously behind the screen. Displacements can be single, when the object visits only one screen, and double or triple, when the object visits two or three screens, respectively. In the Piagetian classification, there are six stages within the sensorimotor period during which sensorimotor intelligence develops from the simplest reflexes to mental representation (e.g., Cole

Anna Fedor and No´ra Szerencsy, Eo¨tvo¨s Lorańd University of Sciences, Budapest, Hungary; Gabriella Skolla´r, Szeged University of Sciences, Szeged, Hungary; Ma´ria Ujhelyi, Institute of Behavioural Sciences, Semmelweis University, Budapest, Hungary. Gabriella Skolla´r is now at the Gibbon Conservation Center, Santa Clarita, California. This research was supported by the authors; the videocamera and the videotapes were provided by Semmelweis University. We thank the zoo directors for their permission to work with the gibbons. We are also grateful to the zoo staff for their support and for their assistance with the construction of the apparatus. We sincerely thank Bjo¨rn Merker who provided helpful comments on this article. We also thank some members of the “CS” Association for help in correcting grammatical errors. Correspondence concerning this article should be addressed to Anna ¨ kolo´giai Tansze´k, Pa´zmańy Fedor, ELTE TTK No¨veńyrendszertani e´s O Pe´ter stny. 1/C, 1117, Budapest, Hungary. E-mail: [email protected] 403

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´ R, SZERENCSY, AND UJHELYI FEDOR, SKOLLA

abilities but because either they lack the short-term memory required or they are unable to perform coordinated actions. Diamond (1995) found that 6-month-old infants indeed have good recognition memory even in tasks that require reaching for the reward and attributed the failure to other factors, such as the inability to execute a means– end action sequence, relational skills, or speed of processing. Moreover, according to Bogartz, Shinskey, and Schilling (2000); Cashon and Cohen (2000); and Schilling (2000), infants do not use the possibility or impossibility of events but instead novelty and familiarity preference to determine their looking times. Rivera, Wakeley and Langer (1999) also questioned Baillargeon et al.’s (1985) claim that in their study the behavior of infants was due to representational reasoning but suggested rather that it is because of simple perceptual preferences. These findings undermine the Piagetian supposition (Piaget, 1970) that the different domains of cognition undergo the same stages at the same time and attract attention to the fact that solving standard object permanence tasks requires other modalities too, not only object permanence. Because Piaget used object manipulation almost exclusively as an index of cognitive functioning, a lack of manipulative skills could cause the cognitive abilities being tested to be underestimated if experimental tasks pose inappropriate manipulation requirements. According to Tomasello and Call (1997), the Piagetian theory of cognitive stages could be useful for analyzing primate cognition within particular domains, but its use in constructing a single unidimensional scale of intelligence should be questioned. In sum, cognitive experiments have to be designed in such a way that deficiencies not in question do not influence performance. But this is not the only thing one has to be careful of. It is also important to exclude the possibility that tasks can be solved with some other abilities not being examined, or at least to be aware of this possibility when analyzing results. In the case of object permanence tests, there are several factors that have to be taken into account if we want to be sure that results reflect true object permanence abilities. First of all, we have to decrease as much as possible the possibility that some Clever Hans effect is responsible for the performance. Inadvertent cues from the experimenter when performing experiments face-to-face to the subject—such as eye gaze or subtle body movements—might help subjects to find the correct response (e.g., Wood, Moriaty, Gardner, & Gardner, 1980). To exclude this possibility, experimenters have to control their body movements very consciously when performing experiments. Moreover, they have to make sure that their eye movements do not influence subjects, so either they have to control their eye movements, too, or they should wear blindfolds, sunglasses, or baseball caps. Collier-Baker, Davis, and Suddendorf (2004; Collier-Baker, Davis, Nielsen, & Suddendorf, 2006) controlled for experimenter cues in single invisible displacement tests with dogs and chimpanzees; they suspended an opaque curtain between the subject and the experimenter or the experimenter wore a welding mask, respectively. Their results indicated that neither of the species used experimenter cues to solve the task. We suppose that gibbons would be even less sensitive to these kinds of cues because they are not accustomed to paying attention to humans. Second, perceptual cues could also lead the subject’s behavior. The experimenter should perform displacements without making any noise and should hide the object in such a way that it cannot

be seen from the subject’s perspective. Odor cues can be ruled out if the object (usually pieces of food) comes into contact with the apparatus and the hiding places before the tests, for example during familiarization periods; contamination and remaining crumbles can mask the odor of hidden food (e.g., Pepperberg, 2002). However, because odor cues seem to be unimportant even for dogs (Gagnon & Dore´, 1992), which are known for their olfactory sensitivity, we suppose that gibbons would not rely on olfaction either. Third, it is very important to be able to discriminate between representational versus nonrepresentational problem solving during Piagetian tasks. The tasks are assumed to be solved by using mental representation (inferring the place of the object through real understanding of its movements and its permanence); however, in certain circumstances it is possible to solve some of them by simple associations, or associative strategies. For example, if the object (usually pieces of food) is always hidden in the same box, subjects might learn to associate the reward with that particular box (simple place learning through trial-and-error learning). So even though they might not understand that the reward they get is the same piece of food that was in the experimenter’s hand before the displacement, their performance could be still above chance on the task, especially if there were many trials. For this reason, it is important to reduce the number of trials per task or to analyze first trials separately to see if performance improved over trials. Learning can be ruled out if performance did not improve over trials; however, there is a possibility that if the stimulus associated with the reward was efficiently strong, subjects would acquire these local rules guiding their behavior through one-trial learning, which cannot be detected by analyzing first trials separately (e.g., Gagnon & Dore´, 1992). In this case, control displacements are necessary to exclude the possibility that success is caused by simple association instead of mental representation of the object and its movements. Several studies have attempted to control for associative strategies and rule learning in different species. For example, systematic search behind all screens visited by the object is usually used as a criterion for the idea that subjects mentally reconstruct the path followed by the object and are capable of inferring the possible hiding locations (Piaget, 1937/1954). However, if the object had visited all screens available, it is possible to achieve the same searching pattern by using a simple “searchunder-all-screens” strategy (Call, 2001). Natale, Antinucci, Spinozzi, and Potı´ (1986) constructed their experiments so as to control for this strategy by making some hiding places not logically possible. They introduced “false” trials to control for the “choose-the-last-box-touched-by-theexperimenter” strategy. In these trials, the experimenter, after placing the object in the displacement device, lifted one of the hiding boxes without manipulating the displacement device at all. Natale et al. found that the Japanese macaque (Macaca fuscata) tested failed the task (selected the touched box in half of the trials), whereas a gorilla never made this kind of error. Then they used “nonlinear” trials to control for the “choose-the-box-adjacent-tothe-displacement-device” strategy. In these trials, the displacement device was placed next to the irrelevant box (previously it was placed next to the target box), which caused the macaque but not the gorilla to search in this box after examining the displacement device.

OBJECT PERMANENCE TESTS ON GIBBONS (HYLOBATIDAE)

Collier-Baker et al. (2004, 2006) used similar stringent controls in their experiments with dogs and chimpanzees, respectively. When they controlled for the last-box rule, the object was placed in the target box with a displacement device, then the displacement device was shown empty as in single invisible displacement trials; however, it then visited an irrelevant box before being moved to its final position. When controlling for the adjacency rule, they never put the displacement device adjacent to the target box, but always adjacent to one of the irrelevant boxes. They introduced an additional control for the “choose-the-first-box-visited-by-thedisplacement-device” strategy. In this task, the experimenter moved the displacement device to a box, but did not deposit the object there. Then, after showing that it still contained the object, the experimenter deposited the object in another box. Chimpanzees were not misled by any of the controls, whereas dogs seemed to rely on the adjacency rule instead of mentally representing the object’s past trajectory. Watson et al. (2001) used another paradigm to test for true understanding of invisible displacements in an exhaustive search task. They found that dogs rely on associative guidance, whereas children rely, to a certain degree, on logical guidance when searching for hidden objects. In sum, control displacements and the analysis of first trials are necessary to assess true object permanence abilities. If performance is not above chance, it means that the subject could solve the task neither by mental representation nor by associative strategies. If it is above chance and performance increases over trials, it means that the subject acquired local rules through learning. If there are no signs of learning, it can still be the case that the subject used a local rule from the first trial that does not require learning. In this case, the subject would fail in the first trial of a control displacement designed in a way that it still contains the stimulus on which the local rule is based but is insolvable with that particular rule. Subjects with true object permanence understanding would not fail in control trials and would not show the signs of learning. However, several distinctive local rules might be available for subjects, and although it is possible that they switch between these strategies as the task changes, it is not probable that this switch happens in the first trial. It is assumed that subjects try new strategies after failing with the previous one.

Object Permanence in Great Apes and Monkeys Object permanence tests have been widely used to assess the object permanence abilities of animals in cross-sectional studies (for a review, see, e.g., Dore´ & Dumas, 1987; Dore´ & Goulet, 1998). In so doing, it is essential to suitably modify and adapt the traditional Piagetian tests (designed for human children) to the species’ motor and attentional characteristics (e.g., Pepperberg, 2002), to motivate the animal to search for the hidden object (e.g., Dore´ & Dumas, 1987), and to include appropriate controls for excluding the possibility that subjects solve the task by using nonrepresentational associative strategies. On the basis of object permanence tests on primates, it was proposed that there is a dichotomy, namely that great apes reach Stage 6, whereas monkeys are confined to Stage 5 (e.g., de Blois & Novak, 1994; de Blois, Novak, & Bond, 1998; Natale et al., 1986). Longitudinal observations and object permanence tests have confirmed that gorillas (Natale et al., 1986; Redshaw, 1978), chimpanzees (Call, 2001; Mathieu, Bouchard, Granger, & Hersco-

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vitch, 1976; Wood et al., 1980), and orangutans (Call, 2001; de Blois et al. 1998) reach Stage 6 of object permanence. However, orangutans (Call, 2001; de Blois et al., 1998) and chimpanzees (Call, 2001) had problems with nonadjacent double invisible displacements, in which the object visits the left and the right boxes (of three boxes arranged in a line in the frontoparallel plane). This was attributed by de Blois et al. (1998) to memory problems, whereas according to Call (2001), a response bias and inhibition problem was responsible for this poor performance (see a more detailed discussion of this problem in the Summary section). Earlier experiments claiming that some monkey species also solved invisible displacements (Mathieu et al., 1976; Parker, 1977; Vaughter, Smotherman, & Ordy, 1972; Wise, Wise, & Zimmermann, 1974) usually used a large number of trials. This raises the possibility that monkeys solved the tasks with trial-and-error learning. In recent studies, both New World and Old World monkeys could solve visible displacements but performed poorly in invisible displacements: crab-eating macaques (Macaca fascicularis; Natale & Antinucci, 1989; Schino, Spinozzi, & Berlinguer, 1990), rhesus macaques (Macaca mulatta; de Blois & Novak, 1994), Japanese macaques (Macaca fuscata; Natale & Antinucci, 1989; Natale et al., 1986), and capuchin monkeys (Cebus apella; Dumas & Brunet, 1994; Natale & Antinucci, 1989; Schino et al., 1990). In invisible displacements, monkeys usually searched randomly (e.g., Natale & Antinucci, 1989) or developed different nonrepresentational strategies like always searching at the same place (e.g., de Blois & Novak, 1994). However, in a computerized task, rhesus macaques could predict the path of an occluded object, suggesting that they could represent unperceived object movements (Fillion, Washburn, & Gullege, 1996). In a recent study (Mendes & Huber, 2004), 2 out of 11 common marmosets (Callithrix jacchus) achieved very high scores in successive invisible displacement tasks, suggesting that this species is also able to represent the existence and the movements of invisible objects. Neiworth et al. (2003) claimed that cotton top tamarins (Saguinus oedipus) also reach Stage 6 of object permanence; however, their methodology was unusual in that they opened the displacement device between the two boxes of all double invisible displacements. It is questionable whether such displacements can be regarded as double invisible if the inside of the displacement device was shown to subjects during displacements (they can be viewed, rather, as two separate single invisible displacements). In sum, it does not seem to be justified to divide primates along the monkey–ape line concerning object permanence abilities. Moreover, some psittacine bird species have also shown considerable object permanence skills, which suggests that Stage 6 object permanence might not be confined to great apes. In Pepperberg et al.’s studies (Pepperberg & Funk, 1990; Pepperberg et al., 1997), grey parrots (Psittacus erithacus), an Illiger mini macaw (Ara maracana), and a cockatiel (Nymphicus hollandicus) all met the criteria for full Stage 6 object permanence according to the Uzgiris and Hunt (1975) scale; however, not all control conditions described above were administered.

Cognition of Small Apes The cognitive capacities of gibbons remain little studied, especially as regards their social cognition. Studies of the latter are confined to the domain of self-concept, although even these must

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be regarded as preliminary. Lethmate and Du¨cker (1973) found that gibbons could not recognize themselves in mirrors, whereas Ujhelyi, Merker, Buk, and Geissman (2000) made observations on the behavior of gibbons in the presence of mirrors that suggest that their capacities in this regard may require further studying before categorical conclusions are drawn. There are few studies about the physical cognition of small apes that can be considered relevant to object permanence tests. For example, in Beck’s (1967) experiment gibbons performed better than chimpanzees on patterned string-pulling tasks and behaved in a way that appeared insightful. Furthermore, Torigoe (1985) observed 74 different primate species in the presence of a set of objects and determined capuchin monkeys and apes (including small apes) to be the most manipulative of all. They picked up, mouthed, transported, rolled, rubbed, slid the object against a substrate (floor or cage bars), draped, dropped, swung, struck, and threw it. Our observations with gibbons show that they play in various ways with new objects, however with great individual differences. One subject (Gyurci) put a plastic bucket on his head, rolled it with his feet lying on his back, scooped water with it, tried to drink from it, brought it up to a tree and poured the water out from there. However, there were subjects that showed hardly any interest in the plastic buckets. There are very few observations about the tool using of gibbons. Gibbons drop branches on intruders and sometimes they roll up leaves to sponge up liquids in the wild (Tingpalong, Watson, Whitmire, Chapple, & Marshall, 1981). Rumbaugh (1970) observed a gibbon using a cloth as a sponge and a rope to make a swing in captivity. There is a recent study on object manipulation (Cunningham, Anderson, & Mootnick, 2006) of hoolock gibbons. In the first experiment, hoolocks learned to pull a rake to obtain an out-of-reach food item. One of the 4 subjects succeeded in the task on the second try, which suggests that he did not use trial-and-error learning but mentally represented the action and the outcome beforehand. In the second experiment, gibbons were given a choice between a rake that offered the chance of reinforcement and another that if pulled would result in the reward being lost in a trap. Two of the subjects performed consistently above chance (even in the first block of 10 trials), but the other 2 subjects also gradually learned to solve the task in later blocks. Their causal understanding of three factors—the rake, the reward, and a trap—was moderately better than that of chimpanzees (Limongelli, Boysen, & Visalberghi, 1995) and capuchins (Fujita, Kuroshima, & Asai, 2003). Regarding memory, Harlow and colleagues (Harlow, 1932; Harlow, Uehling, & Maslow, 1932; Maslow & Harlow, 1932) tested different species of primates on delayed spatial choice problems. If the reward was placed in one of several easily distinguishable boxes, gibbons could choose the right one even after 30 min delay. There have not been any conclusive studies of object permanence in small apes (Dore´ & Dumas, 1987). The only study (Snyder, Birchette, & Achenbach, 1978) had just 1 subject (a Hylobates lar), used an excessive number of trials, and did not analyze the first block of trials separately. For these reasons, it is impossible to conclude whether search behavior was based on trial-and-error learning or on true object permanence (Dore´ & Dumas, 1987). The main goal of the study reported here was to gather new information regarding the cognitive abilities of small apes. We are interested in comparing the performance of hylobatids on object

permanence tasks with that of other primates, so we designed our experiments to be comparable with previous studies (e.g., Call, 2001; de Blois et al., 1998). Accordingly, we performed single visible, single invisible, double invisible, and control displacements in both random and nonrandom order. We chose gibbons as subjects not only because they are an underrepresented group in cognitive studies but also because of their unique phylogenetic position. Since they are taxonomically apes, but intermediate between great apes and Old World monkeys (Cunningham et al., 2006), it would be highly interesting from a comparative point of view to assess their abilities in these frequently used tasks.

Method Subjects The subjects were 10 gibbons of various species (2 Symphalangus syndactylus, 4 Hylobates lar, 1 Nomascus gabriellae, 2 Nomascus leucogenys, and 1 Nomascus gabriellae x leucogenys hybrid; see Table 1), 6 male and 4 female. Six animals were captive born. Five of these were reared by their biological mother, and 1 (Gyurci) was reared by a human. The remaining 4 animals were wild born. The age of subjects was between 7 and 30 years at the time of the experiments, although in case of the wild-born gibbons their exact age is not known. There are several studies discussing the behavioral differences in apes with different life histories (for contrasting views, see, e.g., Tomasello, Savage-Rumbaugh, & Kruger, 1993, and Bering, 2004). Because in our study there was only 1 human-reared subject, we could not compare its performance statistically with that of mother-reared subjects, but we repeated all statistical tests excluding the human-reared subject to examine its possible effect on average performance. Because it is very difficult to find a sufficiently large sample from the same species, we chose to simply unite all species available into one single group of “gibbons” to allow for a sensible statistical treatment of the data obtained. Even though this approach is controversial because possible differences in the behavior of species could be viewed as an artifact of grouping techniques, it is a legitimate approach; in fact, the Hylobatidae represent a quite homogeneous, monophyletic taxonomic group (Chivers, 1985; Roos & Geissmann, 2001). Subjects were housed at six different Hungarian zoos in Budapest, Debrecen, Gyo˝r, Ja´szbereńy, Nyı´regyha´za, and Szeged. Testing was conducted through the mesh wall of the animals’ habitual enclosure. There were animals living alone (Buci and Todi), nonrelatives living together (Gyurci and Dodo, Kaszat and Kormos with a nontested Hylobates lar, and Goofy and Hilda), and families living together (Jerry and Lenocska with two of their offspring). First, we always tried to isolate the animals not involved in the experiment so that the subjects stayed alone in their habitual cage. However, in some cases this caused so much trauma to the subject that it refused to pay attention to the experiment. In such cases, we allowed the others to stay in the cage during experimentation and we fed them at the other side of the cage or simply waited with the trials until they engaged in other activities and only the subject stayed close to the apparatus. Because of these precautions, other individuals could not watch whole trials uninterruptedly but could still glimpse the procedure from time to time. Because they could not observe trials from the beginning until the solution, we do not


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Table 1 Data of the Subjects Included in the Study Subject

Species

Sex

Agea

Birthplace

Zoob

Goofy Hilda Todi Dodo Gyurci Kaszat Kormos Jerry Lenocska Buci

Symphalangus syndactylus Symphalangus syndactylus Nomascus gabriellae Nomascus leucogenys N. gabriellae 3 leucogenys hybrid Nomascus leucogenys Hylobates lar Hylobates lar Hylobates lar Hylobates lar

m f m m m m f m f f

30 30 12 19 7 7 17 17 20 25

Wild Wild Wild Wild Captivity Captivity Captivity Captivity Captivity Captivity

Budapest Budapest Nyı´regyha´za Gyo˝r Gyo˝r Szeged Szeged Debrecen Debrecen Ja´szbereńy

Note. m 5 male, f 5 female. a Age is given in years. The age of the wild-born animals is approximate; however, they were captured in their juvenile stage, when it is easier to assess their age from the color of their fur, the status of their teeth, and their sexual organs. b Subjects in the same zoo lived in the same cage.

think that learning could take place. Usually, it was not difficult to have only 1 gibbon close to the apparatus because the dominant animal monopolized it, but because of this we also could not test all of the animals in all of the zoos. However, fortunately, sometimes the dominant animal lost its interest in the task after a few hours of experimentation, and we could then test the others in the time left in the day (which is why we could test 2 animals in most of the zoos; see Table 1). Five subjects (Goofy, Hilda, Todi, Dodo, and Buci) had been involved in previous experiments with mirrors (mirrors as tools to find objects and self-recognition in mirrors; Ujhelyi et al., 2000). All of the subjects have access to freely movable objects on a daily basis, mainly food, but sometimes they get little balls, boxes, and so forth to play with. The contact they have with humans is usually restricted to zoo keepers during feeding (except for the hand-reared gibbon, which lived with a human until the age of 2).

often, subjects sat in front of an extreme box; in these cases they had to change position if they wanted to flip the other extreme box. The experimenter performed all tests with her hands. In the case of our subjects, a freely manipulable container would be impossible to work with. If the container had been small enough to get through the mesh wall of the enclosures, gibbons would have taken it in (as they took in everything they could grab), and we could not have taken it out again. If the container had been larger, they would have become frustrated at not being able to get it. A sliding container that could be fixed at the final positions would have been a solution to this problem, but would have been problematic for other reasons. If a container slides behind the boxes, it cannot be seen except for the small fraction of its trajectory when it is between two boxes if one observes the displacement from the level of the board. However, if it is observed from a little bit higher, the whole trajectory can be seen. These two conditions cannot be

Materials Care was taken to exclude the possibility that the lack of certain motor skills caused unsuccessful performance. Reaching for an object is part of the motor repertoire of gibbons, but their hand is highly adapted to hanging, which makes it hard for them to pick up small objects. That is why we designed our hiding boxes in a particular way. Three identical wooden boxes (30 cm 3 15 cm 3 15 cm) were used as hiding places for the objects. They were fixed in a row on a wooden board (130 cm long) with 20 cm between them (see Figure 1). Each box was fixed to the board with a hinge, and one side of the box was open to hide the objects. The open side of the boxes faced the experimenter when she performed the displacements so the subjects could not see inside the boxes. After the displacement was completed, the experimenter pushed the board against the wire-mesh wall of the enclosure, near enough for the gibbons to reach it. The gibbon could then turn the boxes so their open side faced upward to make it easy to retrieve the object. Usually, subjects sat in front of the middle of the board; hence, they could reach and flip all boxes without moving. They usually, however, moved closer to the boxes to retrieve the reward. Less

Figure 1. The experimental setup with two Hylobates lar in Debrecen Zoo (Lenocska and her untested offspring).

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regarded as the same because in double displacements, for example, when the container visits the two extreme boxes, the difficulty of the task is increased if the container “disappears” behind the middle box. Because we could not control the precise position of our subjects (they often climbed on the mesh wall), but we wanted all trials to be comparable, we decided to use not a sliding container, but the experimenter’s hand. Moving the hand above the boxes is similarly visible from any position and it is not regarded as a retrievable object by the gibbons. There are studies comparing the effect of using a container as a displacement device instead of the hand. De Blois et al. (1998) found that orangutans performed better on double invisible displacements with a cup than with the hand. They suggested that it was because the cup, as a novel object, captured the subjects’ attention more than the hand. They could not make the same comparison on squirrel monkeys because they were distracted by the experimenter’s hand. On the contrary, de Blois and Novak (1994) found that in general, rhesus monkeys performed better in the hand condition, probably because it was a better discriminative stimulus than the container because of its having been associated with food rewards during feeding. We cannot estimate the effect of the hand in our study, but because it could be a discriminative stimulus, it would be interesting to compare the hand and the container condition in a future study with a different testing set up. The objects that were hidden consisted of a variety of treats that were selected according to the keepers’ knowledge of subjects’ preferences (e.g., banana, orange, apple, grape, plum, peanuts, or cookies). It was very easy to hide the objects noiselessly so that auditory signals could not help the gibbons to find them. Olfactory cues are also unlikely to have influenced subjects because all the apparatus was contaminated with fruit during the adaptation period and crumbles from peanuts and cookies remained in and around the boxes that could mask the odor of hidden food. After every displacement, the experimenter stepped back and turned her head down, looking downward to rule out that looking at the correct box would help the animals. Almost all of the trials were videotaped (accidentally, a few were not) and one of us took notes during testing.

Procedure Adaptation. The apparatus was pushed near the mesh wall of the enclosure so that subjects could freely manipulate it. We fed the animals so they would become familiar with us and put pieces of food on the apparatus (on the boxes, on the board between the boxes, and in the overturned boxes) to raise their interest. We never simulated any of the trials because we did not want learning to take place. For this reason, we did not put food into the boxes when the open side faced the experimenter, but only after turning them over so that the open side faced upward and the gibbons could look into them. Some gibbons turned the boxes accidentally when reaching for the food on top of them, and others tried to turn the boxes after observing us turning them several times. Some gibbons were too timid to touch the boxes and others were frightened by the slam of the turning boxes in the beginning, but they were curious, too, so they got used to the apparatus after a while. The adaptation period usually lasted for a few hours until the subjects learned how to turn the boxes and became familiar with us and with the apparatus. Testing began on the next day.

Testing. We conducted four types of displacements: single visible displacements (SVDs), single invisible displacements (SIDs), control displacements (CDs), and double invisible displacements (DIDs). All displacements are described in detail in the Protocol section. During visible displacements, the experimenter held the object between her fingers and simply put it in a box in full view of the subject. During invisible displacements, however, she closed her hand first, so that the subject could not see the reward, and then she put it in a box. Each displacement started by showing the reward to the animal and ended with the experimenter showing her empty hand. Both showing the reward and showing the hand occurred high above the boxes (at the height of the experimenter’s face) approximately in the center of the array of boxes. In CDs and DIDs, the showing of the hand after touching the first box occurred at the same height halfway between the two boxes involved. There were two series of every type of displacement, and both consisted of 15 trials: a random (r) series and a nonrandom (nr) series. During the random series, the object was placed five times in each of the boxes, but it was not placed in the same box in more than 2 sequential trials. During nonrandom series, the object was placed five times in each of the boxes again, but always five times in the same box successively. The order of boxes was M–R–L, R–L–M, or L–R–M (with R 5 right box, M 5 middle box, and L 5 left box), randomized across subjects and tests. We baited the boxes in this order—so that left and right boxes (the extreme boxes) were baited after each other—to decrease the possibility of place learning, so that subjects did not become accustomed to one end of the apparatus. In DIDs, when in 1 trial there was a baited box and a touched nonbaited box, every box was involved in a series five times as the baited one and five times as the touched nonbaited one. For example, the control nonrandom series when the object is hidden in the first touched box could be RL, RL, RL, RM, RM, LR, LR, LM, LM, LM, ML, ML, MR, MR, MR. As one can see, there were 5 trials of each of the possible trajectories (concerning the boxes visited by the experimenter in which extreme boxes are the left and the right box), that is, 5 extreme– middle, 5 middle– extreme, and 5 extreme– extreme trials; of these, 10 were adjacent trials (the extreme–middle and the middle– extreme displacements) and 5 were nonadjacent trials (the extreme– extreme displacements). If a gibbon became inattentive during a trial (went away, started to sing, etc.) or did not give a response, the trial was scored as invalid and was repeated at the end of the series. Usually we waited 5 min for the response after the reward was placed in the box before aborting the trial; however, gibbons usually went away or engaged in other activities earlier if they were not interested in the task. The experimenter remained standing back, with her head turned downward while waiting for response. We aborted the trial immediately (even during the trial) if the subject had not looked toward the boxes or had engaged in other activities during the trial. The percentage of invalid trials gives an indication of the subjects’ attentional abilities and motivation.

Protocol Test 1: SVDs. The experimenter shows the object, puts it into a box, and shows her empty hand. • Nonrandom series to test Stage 4b abilities


• Separate analysis of the perseveration error in the nonrandom series plus random series to test Stage 5a abilities Test 2: SIDs. The experimenter shows the object, closes her hand, puts the object into a box, and shows her empty hand. • Nonrandom series to test Stage 5b abilities • Separate analysis of the perseveration error in the nonrandom series plus random series to test Stage 6a abilities Test 3: CDs. The experimenter shows the object, closes her hand, puts the object into a box, shows her empty hand, puts her open hand into another box, and shows her empty hand. This test controls for the animal developing a nonrepresentational strategy of searching in the (last) box visited by the experimenter. Using this simple associative strategy, subjects would be able to solve Test 1, Test 2, and Test 4, but would fail Test 3. • Nonrandom and random series Test 4: DIDs. The experimenter shows the object; closes her hand; puts her closed hand into a box but does not place the object into the box; removes her closed hand, puts it into another box, and places the object in it; and then shows her empty hand. • Nonrandom and random series to test Stage 6b abilities We designed our protocol following the work of de Blois et al. (1998) and Call (2001), with some changes. First of all, we did not perform DVD. De Blois et al. said that DVDs (along with triple visible displacements) test Stage 5b object permanence abilities and SIDs test Stage 6a abilities. However, SIDs in a nonrandom order also test Stage 5b, according to Dore´ and Dumas (1987) and Piaget (1937/1954), and SIDs in a randomized fashion are necessary to test Stage 6a (so if a subject can solve SIDs without the perseveration error, it is at Stage 6a). Because time was restricted and gibbons could lose their interest quickly, we tried to include only the most necessary tests to assess their object permanence abilities. That is why we did not perform DVDs and triple displacements. Subjects were randomly assigned to two groups: members of Group 1 (Buci, Goofy, Kaszat, and Hilda) received the tests in the above-mentioned order (SVDnr, SVDr, SIDnr, SIDr, CDnr, CDr, DIDnr, and DIDr), and members of Group 2 (Dodo, Gyurci, Jerry, Kormos, Lenocska, and Todi) received the tests in the reverse order (DIDr, DIDnr, CDr, CDnr, SIDr, SIDnr, SVDr, and SVDnr).

409

We used this procedure to evaluate the effect of passing the supposedly easier Tests 1 and 2 first on performance in Tests 3 and 4. Not all experiments were carried to completion of the full protocol (eight series of 15 displacements each) because 1 gibbon died (Kormos) and some refused to take part in the tests after several series were presented (namely, Dodo, Jerry, Goofy, and Hilda). See Table 2 for all the tests performed with subjects. Testing of individuals lasted for 5 to 8 days, depending on the capacity of the subjects.

Data Analysis Scoring. A trial was scored as correct if the gibbon found the object on the first try (the probability of making a correct response was 0.33). However, on DIDs (Test 4), two distinct responses were scored as correct. These responses were choosing the baited box directly (with a probability of 0.33), which we called first-choice success, or choosing the touched nonbaited box as the first choice (in which case we allowed the subject to open another box) and the touched baited box as the second choice, in other words, following the movements of the human hand (with a probability of 0.333 3 0.5), which we called second-choice success. In sum, success on this test has the combined probability of 0.33 1 0.33 3 0.5 5 0.5. This assumes that subjects infer from the movements of the experimenter’s hand that the object could be in either of the boxes because the subject could not see whether the object was still in the hand during the movement from the first box to the second. However, because in all of the DID trials the object was hidden in the second box, it was possible to learn that in these kind of trials the object was always in the second box and the closed hand during the movement between the boxes meant that the object was still in it. We were curious whether gibbons had a tendency to follow the movements of the human hand or rather would choose the second box on the first try. This assesses the inferential, rather than the perceptual and representational abilities of subjects. The perseveration error. The A-not-B error, or perseveration, error occurs when the subject persists in searching the location where it was previously successful (e.g., de Blois et al., 1998). We

Table 2 Performance of Subjects (Proportion of Correct Choices) in Tests Subject Buci Goofy Hilda Kaszat

SVDnr

SVDr

SIDnr

0.93 0.87 0.47 0.67

0.87 1.00 0.47 0.53

0.73 0.87 0.27 0.47

SIDr

CDnr

Group 1 0.73 0.60 0.90 0.93 0.27 0.33 0.40

CDr

DIDnr

DIDr

0.40 0.46

0.60 0.73

0.60

0.33

0.73

0.67

0.53 0.73 0.40 0.40 0.60

0.73 0.53 0.69 0.67 0.60

0.79 0.80 0.27 0.46 0.60 0.53

Group 2 Kormos Dodo Gyurci Jerry Lenocska Todi

0.93

0.93

0.93

0.33 0.60

0.33 0.80

0.33 0.80

0.53 0.47

0.47 0.80

0.53 0.67 0.67 0.60

Note. SVD 5 single visible displacement; SID 5 single invisible displacement; CD 5 control displacement; DID 5 double invisible displacement; nr 5 nonrandom series; r 5 random series.


410

used the nonrandom series to test whether gibbons committed this type of error. When the gibbon persisted in searching the previous box after the experimenter changed the hiding place of the object, this counted as a perseveration error (as a proportion of the changes made by the experimenter). The chance level of committing this type of error is 0.33. Statistics. We used the Kolmogorov–Smirnov normality test to test whether our data conformed to a Gaussian distribution. We used t test and binomial test to determine whether gibbons performed better than chance by comparing the subjects’ mean score on each test to the mean score expected by chance. We compared the performance on different tests by paired t test. We analyzed the effects of different factors on performance by means of one-way analysis of variance. For the analysis of individual performance, we used binomial tests. All confidence intervals reported are 95%, and all statistical tests used were evaluated at p , .05 (two-tailed).

Results Motivation and Attentiveness Of the trials, 8% had to be repeated (considering all tests of all subjects) because of the subjects’ inattentiveness, which shows their relatively low motivation and attentiveness to the task compared with great apes (e.g., Call, 2001; de Blois et al., 1998). However, there was great variance across subjects, with the extremes of 0% (Gyurci and Goofy) and 16% (Jerry). It seems that subjects were more attentive to single displacements (all trials of Tests 1 and 2 unified) than to double displacements (all trials of Tests 3 and 4 unified): 6% of the single displacements and 11% of the double displacements were invalid (see Figure 2), and the difference was nearly significant, t(6) 5 2.303, p 5 .0608. This could be explained by gibbons’ relatively short attention span:

Single Displacements

Double Displacements

Mean Percentage of Invalid Trials/A-not-B errors

60

50

40

30

20

10

0 Invalid Trials

A-not-B error

-10

Figure 2. Mean percentage (6SD) of invalid trials and A-not-B errors in single displacements (Test 1 and Test 2) and in double displacements (Test 3 and Test 4).

Because double displacements take longer to perform than single displacements, it is more probable that subjects become inattentive during the trial. We intended to wait 5 min for subjects to respond (see Protocol section); however, we found that that gibbons never responded after more than 1 min. Even if we had waited longer, subjects that did not give a response within 1 min did not respond later either. In approximately 1% of the trials, subjects gave a response by between 30 and 60 s; in the remaining trials, they either gave a response more quickly or did not respond at all.

Average Performance According to the Kolmogorov–Smirnov normality test, all of our data were from a Gaussian distribution (SVDnr, p 5 .908; SVDr, p 5 .900; SIDnr, p 5 .938; SIDr, p 5 .906; CDnr, p 5 .747; CDr, p 5 .785; DIDnr, p 5 .911; and DIDrm, p 5 .995). Table 2 shows the performance of subjects (the proportion of correct choices) on the different tests, and Figure 3 shows the results of the t tests. Gibbons performed significantly above chance in SVDs, SVDnr, t(6) 5 4.309, p 5 .005, and SVDr, t(6) 5 3.834, p 5 .009; in SIDs, SIDnr, t(6) 5 3.087, p 5 .021, and SIDr, t(7) 5 2.643, p 5 .033; in CDs, CDnr, t(6) 5 4.870, p 5 .003, and CDr, t(7) 5 3.247, p 5 .014; and in DIDnr, t(7) 5 6.002, p 5 .001. However, they performed below chance in DIDr, t(7) 5 1.457, p 5 .188. Because they succeeded in CDs we can rule out the possibility that gibbons solved Test 2 and Test 4 by using the strategy of “search in the (last) box visited by the experimenter.” However, there is another possible strategy: “Search in the (last) box visited by the experimenter with closed hand,” that is, that the gibbons used the manipulations performed by the experimenter combined with the open or closed position of the hand as a cue. To test this, another control would have been useful: After invisibly hiding the object in one of the boxes, the experimenter shows her empty hand, closes it, and then visits another box with the hand in a closed position. However, a more likely explanation is that the gibbons tried to solve Test 4 by place learning, which would explain why they succeeded in the nonrandom series but not in the random series. Although t tests are often used in the literature to analyze the results of object permanence tests, binomial tests would be statistically more appropriate. Reanalyzing our data using binomial tests, we had the same test results, that is, all tests being significantly above chance except for the random series of DIDs ( p , .0005 for all series of SVDs, SIDs, and CDs; p 5 .002 for DIDnr; and p 5 .128 for DIDr). To examine the possible effect of the human-reared subject on average performance, we repeated all t tests excluding Gyurci. Almost all p values increased, although all but one (the one for the SIDr test) remained significant: SVDnr, t(5) 5 3.608, p 5 .015; SVDr, t(5) 5 3.158, p 5 .025; SIDnr, t(5) 5 2.611, p 5 .048; SIDr, t(6) 5 2.228, p 5 .067; CDnr, t(5) 5 4.046, p 5 .010; and CDr, t(6) 5 3.328, p 5 .016. For DID tests, p values decreased because in these tests Gyurci performed worst among all subjects: DIDnr, t(6) 5 8.086, p 5 .000, and DIDr, t(6) 5 2.830, p 5 .030. In sum, excluding the human-reared subject from the analysis did not alter the results to a great degree.


411

Mean Percentage of Correct Responses

100.00 90.00 80.00 70.00 60.00

**

** **

**

* *

NS

*

50.00 40.00 30.00 20.00 10.00 0.00 SVDnr

SVDr

SIDnr

SIDr

CDnr

CDr

DIDnr

DIDr

Test

Figure 3. Mean percentage (695% confidence interval) of correct trials as a function of the type of displacement. Gray columns indicate single displacements, white columns indicate double displacements. SVD 5 single visible displacements; SID 5 single invisible displacements; CD 5 control displacements; DID 5 double invisible displacements; nr 5 nonrandom series; r 5 random series; NS 5 nonsignificant. *p , .05. **p , .01.

Effect of Different Factors

On average, gibbons committed the A-not-B error in 33% of the changes made by the experimenter, which is considered to be equal to chance, t(9) 5 0.514, p 5 .621, ns. This means that although they have occasionally committed this type of error, it is not the case that they systematically used the strategy of choosing the previously baited box. However, individual performances were highly variable, with the extremes of 12.5% (Gyurci) and 75% (Dodo), which suggests that some of the subjects could not inhibit choosing the previously baited box. This happened more often in difficult situations: The percentage of A-not-B errors was higher in double displacement tasks (all trials of Test 3 and 4 unified; M 5 41%, SD 5 16%) than in single displacement tasks (all trials of Test 1 and 2 unified; M 5 21%, SD 5 9%), t(5) 5 2.871, p 5 .0349 (Figure 2). Indeed, all subjects committed the perseveration error only once out of four changes in SVDs and SIDs, with the exception of Gyurci, who did not commit it at all. This means that they do not rely on place learning when solving sequential invisible displacements.

According to the one-way analysis of variance, the following factors had no significant effect on the average performance of the subjects: order of tests F(9) 5 0.069, p 5 .799; sex F(9) 5 0.230, p 5 .644; age F(9) 5 0.205, p 5 .663; genus F(9), 5 0.018, p 5 .983; and birthplace F(9) 5 0.012, p 5 .915.

Mean Percentage of Correct Responses

The Perseveration Error

Further Analysis of DIDs According to previous experiments, the performance of subjects (e.g., orangutans, de Blois et al., 1998; orangutans, chimpanzees, and human children, Call, 2001) depends on the trajectory of displacements: Nonadjacent invisible displacements (in which the experimenter visits the extreme [i.e., left and right] boxes) seem to be the hardest ones to solve. We have found the same for gibbons in the three different trajectories of displacement: extreme–middle, middle– extreme, and extreme– extreme. To rule out place learning, we investigated the random series separately (see Figure 4): The results were significant for the extreme–middle displacements,

100.00 90.00 80.00

*

70.00

NS

60.00 50.00

NS

40.00 30.00 20.00 10.00 0.00 Extreme-Middle

Middle-Extreme

Extreme-Extreme

Figure 4. Mean percentage (695% confidence interval) of correct trials in the random series of double invisible displacements as a function of the trajectory of displacements. NS 5 nonsignificant. *p , .05.


412

t(7) 5 2.518, p 5 .0399, but not for the other two trajectories, middle– extreme, t(7) 5 1.702, p 5 .1326, ns, and extreme– extreme, t(5) 5 0.8277, p 5 .4455, ns. This is highly similar to the performance of orangutans, chimpanzees, and children in the above-mentioned studies. Success in these tests can be either first-choice success, when the subject chooses the baited box first, or second-choice success, when the subject chooses the touched nonbaited box first, then the baited box. Analysis of successful trials showed that there were more than two times more first-choice successes than secondchoice successes; in other words, subjects tended to choose the last box touched by the experimenter. The proportions of first- and second-choice successes in the DIDnr test were 0.46 and 0.20, respectively; in the DIDr test, they were 0.41 and 0.18, respectively. There is the possibility that gibbons solved Tests 1–3 by a simple nonrepresentational strategy: selecting the first box manipulated by the experimenter. Their performance was significantly above chance level in both the random and the nonrandom series of SVDs, SIDs, and CDs, when the object was always in the first box, whereas they were not successful in the random series of Test 4, when the object was in the second box. The gibbons might have solved Tests 1–3 on the basis of the simple association between the first touched box and the food, and they might have used spatial association (i.e., selecting a previously baited box) in Test 4 because the above strategy did not work (so that is why they were successful in the nonrandom series, but not in the random series). In the random series of Test 3, gibbons chose the first-touched box significantly more times than expected by chance because their performance was also above chance and the reward was always in the first box. In contrast, in the random series of Test 4, they did not chose the first box more than expected by chance (M 5 0.35),

succesful

t(7) 5 0.2081, p 5 .8411. So either they did not use the strategy of choosing the first box touched by the experimenter or they switched to another strategy when encountering Test 4.

Individual Analyses There is a possibility that subjects changed strategies when they encountered new types of displacements; however, it is very unlikely that this happened on the first trial of tests. We analyzed first trials and possible strategies individually to get a better understanding of the gibbons’ choices. An interesting pattern emerged: It seems that when they found a task unsolvable, gibbons did not develop a particular strategy but became biased toward one of the boxes, or started to neglect one. According to the binomial distribution, the performance of a subject is above chance in SVD, SID, and CD tests (with a 5 5%) if it found the object in a minimum 10 of the 15 trials of a series, whereas in DID tests a minimum of 12 successful trials are needed to meet this criteria. Figure 5 shows the results for all subjects. First-trial error is an error committed by a subject on a trial when first encountering a particular type of displacement. This trial was the first trial of every nonrandom series for Group 1 subjects and the first trial of every random series for Group 2 subjects. Successful performance according to the binomial distribution together with first-trial success (see Table 3) indicates that the subject solved the task without learning. If the subject also succeeded similarly on control trials, this means that it did not use the ad hoc strategy of choosing the last box touched by the experimenter. Only 1 gibbon, Dodo, was successful in DID trials; moreover, it was in the random series and even the first trial was successful. Note that it was the first series of his very first session. Later on,

not successful

9

Number of subjects

8 7 6

2

3

5

4

5

4

7

4

8

7

3

5 2

4

3

3

3

1

1 0 SVDnr

SVDr

SIDnr

SIDr

CDnr

CDr

0 DIDnr

1 DIDr

Test Figure 5. Individual performances according to the binomial distribution. The performance of a subject is above chance, (i.e., successful) with an alpha of 5% if it found the object in a minimum 10 of the 15 trials of a series except in DID tests, in which a minimum 12 successful trials are needed. Black parts of the columns represent individuals successful in the test; white parts of the columns represent individuals not successful in the test. SVD 5 single visible displacements; SID 5 single invisible displacements; CD 5 control displacements; DID 5 double invisible displacements; nr 5 nonrandom series; r 5 random series.


413

Table 3 Analysis of Individual Performance According to the Binomial Distribution (p values), First Trial Data, and Bias Subject and Criteria

SVDnr

SVDr

SIDnr

SIDr

CDnr

CDr

DIDnr

DIDr

Group 1 Buci p First trial Bias Goofy p First trial Bias Hilda p First trial Bias Kaszat p First trial Bias

0.000*

0.000*

0.004*

fail — 0.000*

—

—

0.000*

0.000*

pass — 0.404

0.017*

—

—

0.404

1.580

0.761

0.607

fail —

—

0.001*

0.000*

— 0.481

0.607 fail left

0.118

pass —

—

—

pass left

left

1.580 fail

middle

—

0.176

0.404

pass —

0.061

pass

fail middle

0.004* pass

middle 1.190

0.761

fail —

left

1.190

0.118

fail left

left

0.302 pass

middle

left

0.176

0.118

right ig.3

Group 2 Dodo p First trial Bias Gyurci p First trial Bias

0.000*

0.000*

0.000*

pass —

1.190 fail left

0.176

0.061

0.017*

pass left

fail —

—

left 0.004*

left

—

Jerry p First trial Bias

Todi p First trial Bias

1.965 fail

— 0.761

right ig.3

—

0.267

fail

1.419 fail

left

left

Kormos p First trial Bias Lenocska p First trial Bias

—

1.000

pass left

0.035* pass

left 0.057 pass middle

1.190

1.190

0.176

fail middle 0.001*

middle

middle

0.001*

0.404

pass —

0.404

0.017*

fail middle

middle

0.001*

0.061

pass —

—

0.761

0.302

pass middle

middle

0.061

0.607

pass middle

middle

0.607 pass middle 1.000 pass

middle

middle

middle

Note. SVD 5 single visible displacements, SID 5 single invisible displacements, CD 5 control displacements, DID 5 double invisible displacements; nr 5 nonrandom series; r 5 random series; 3 right ig. 5 ignoring the right box without a bias to either of the other boxes. * p value according to the binomial distribution is smaller than .05; performance is considered significantly above chance according to the binomial distribution.

he developed left bias and in addition neglected the right box. In the DIDnr, CDr, and CDnr series, he did not choose the right box more than once per series, and his performance was below chance. He was not successful even in SID trials. In DIDr trials, most of his correct responses were first-choice successes, which is possible through the simple strategy of choosing the second box touched by

the experimenter. However, with this strategy he would have succeeded in the DIDnr test, too. We do not know what caused this sudden drop in his performance because the nonrandom series was performed right after the random one. Four gibbons were not successful in any of the tests (Kormos, Jerry, Hilda, and Lenocska). We do not know much of the

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abilities of Kormos because she completed only the most difficult test, the random series of DID. In this test she succeeded in 6 trials, out of which 5 can be attributed to her bias for the middle box, which she chose 10 times. The other 3 subjects had very strong biases from the beginning, which supposes that they did not understand the tasks. Jerry almost always chose the left box; in fact, he chose the other two boxes only 4 times during the three tests completed. However, because single displacements were not performed with him, we cannot draw conclusions about his object permanence abilities either. Hilda never chose the right box but chose the middle box 12, 8, 11, and 13 times in SVDnr, SVDr, SIDnr, and SIDr tests, respectively. Lenocska had middle bias from the beginning, and it even increased with time. She had successful first trials in DIDr and DIDnr tests, but in both the middle box was baited so the success was because of her bias. Her performance was above chance on the CDnr test, but only marginally, which we think could be due to chance (10 successful trials). Kaszat succeeded in SVDnr in the first trial, and even the test result was above chance, however only marginally (10 successful trials), so it might be that this was only due to chance or place learning because he committed the perseveration error and performed below chance in the random series of the same type of displacements. He gradually developed a bias for the left box and neglecting of the right box. From SIDnr to DIDr, he chose the right box only twice. According to the Piagetian scale, Kaszat reaches Stage 4b of object permanence, but Hilda and Lenocska do not. However, it is very improbable that they do not understand simple visible displacements; it might be more possible that they lack other abilities necessary to succeed in an interactive task. Buci succeeded on SVD and SID tests in both a random and a nonrandom order. She found the reward in the first trial of SID, but not in SVD; however, this was the very first trial of her first session, and she did not make any more errors in this series. Because she did not succeed in the control test and had left bias in the nonrandom series of DID test (she chose the left box 13 times), it is possible that she solved SVD and SID tests by an ad hoc strategy of choosing the box manipulated by the experimenter. For Goofy, all tests and first trials were successful from SVDnr to CDnr. Because he was not successful in the random series of CDs, it is possible that he solved CDnr by place learning (he committed the perseveration error once) and the other tests by ad hoc strategies. He developed left bias from the CDr test, and he did not chose the right box in CDr and chose it only once in DIDnr. Todi’s performance was similar; thus, he also succeeded in most SVD and SID tests and almost succeeded in CD tests— he had 9 correct choices in both series and the first trial was correct. His performance was below chance in SIDnr; however, his 12 correct trials in the random series of the same displacements that was performed to him earlier suggests that this must be due to some other distracting factors. He chose the left box only once in the first three tests performed to him (DIDr, DIDnr, and CDr) and had middle bias in the beginning, but this disappeared on proceeding to easier tasks. Gyurci, the human-reared subject, performed the best among all subjects. He performed above chance and had correct choices on the first trials of SVD, SID, and CD tests. The only exception was

the SIDr test, in which he had left bias and chose the right box only once (he was not biased in the other tests). However, he still had nine correct choices, which is nearly significant. A reason for this could be that one of the zookeepers entered the room during this test, and this might have distracted him. However, this pattern of performance suggests that he did not learn to solve the tasks. He committed the perseveration error the least among all subjects. It is interesting that the 2 most motivated subjects (Goofy and Gyurci) performed the best in the tests. Were they more successful than other subjects because they were more motivated and were more attentive to the tasks? Or were they more motivated because they understood the tasks (although this was not the case in DIDs)? Because they apparently did not understand DID trials but always gave responses in these tests, too, it is more likely that their motivation did not depend on their understanding of the tasks, and they were more successful than others because they were more motivated. The other question is about developing biases. Did subjects develop biases because they were confused by the task and did not know which box to choose? Or did they have biases for some other reason that then pushed them toward choosing one of the boxes in spite of their understanding of the place of the reward? We think that in the case of Gyurci, the second explanation is more probable because he was biased in only one test; however, for other subjects that developed or lost biases gradually according to the difficulty of the tasks, the first explanation could be true.

Discussion Summary The main goal of our study was to assess the object permanence abilities of the small apes because no conclusive experiments have been done so far. This study is not only useful to gain more knowledge about the cognition of this little-studied family of apes but also to make comparisons across species because we used the widely used Piagetian object permanence tasks. Gibbons as a group were successful in SVDs and SIDs and did not commit the perseveration error more than expected by chance. The result of the CDs was also significant, which shows that they did not solve invisible displacements by using the simple associative strategy of searching in the last box touched by the experimenter. However, it is still possible that they used other associative strategies. All these results suggest that gibbons perform at the Piagetian Stage 6a of object permanence, assuming that they are able to solve sequential visible and invisible displacements. They were also successful in the nonrandom series of the DIDs; however, this performance could be the result of place learning. Moreover, they failed in DIDs in the random order, so it is likely that their object permanence ability does not reach Stage 6b. The other possibility is that they can mentally reconstruct the invisible movements of an object, but their abilities in mental representation are not robust enough to inhibit convenient sequential searches because they were successful in extreme–middle displacements but not in the other two trajectories. This limitation was not true for 1 subject, whose individual performance in the random series of DIDs was better than expected by chance. Analyzing the results individually, it turned out that 2 subjects could not solve even SVDs (Hilda and Lenocska), and 1 could


solve these only with committing the perseveration error (Kaszat). However, we do not think that these subjects are below Stage 5 of the Piagetian scale of object permanence but that other factors could be blamed for their poor performance. We could not draw conclusions about the individual performance of 2 other subjects (Kormos and Jerry), to whom only double displacements were performed in which they were not successful. Four gibbons succeeded in SVDs in both random and nonrandom order. Two of them (Buci and Goofy) were also successful in both series of SIDs, too, which marks the beginning of Stage 6a of object permanence. The other 2 failed either in the random series (Gyurci) or in the nonrandom series (Todi). Only 1 (Gyurci) of these 4 subjects was clearly successful in control displacements, performing above chance in both series and successful even in the first trial. Because his failure in the nonrandom series of SIDs was marginal, we assume that he understood these three types of displacements and that he did not use the simple strategy of searching in the last box touched by the experimenter (however, other ad hoc strategies are possible). The other 3 subjects that were successful in single displacements showed marginal performance in CDs, which does not exclude the possibility of their using the above-mentioned strategy. Surprisingly, 1 of the gibbons (Dodo) could solve DIDs in random order, but was unsuccessful in all other tests. If he had used a local rule to solve this task, he would have succeeded in the nonrandom series, too, which is not the case. It might be that other distracting factors caused his low performance in this task. In sum, we did not find strong evidence that gibbons understand SIDs; however, we assume that at least 1 of our subjects does. As regards the preference for certain boxes, it seems that subjects develop biases when finding a task too difficult. Four of our subjects had a clear bias toward the left box, and 4 other subjects preferentially chose the middle box. In addition to this, 5 of our subjects inhibited searching in the right box. No subject was biased toward the right box or against the left box. In general, we can say that our subjects were biased toward the left side of the apparatus, which seemed to be a common pattern in all of the zoos. We could not find any logical reason for this pattern of behavior because all the experimental environments were different from place to place.

Comparison With Other Species In Call’s (2001) study, orangutans, chimpanzees, and 19- and 26-month-old children could solve invisible adjacent displacements, but were not successful in invisible nonadjacent displacements. De Blois et al. (1998) had similar results with orangutans when using a cup as a displacement device (when they used a hand, even performance on middle– extreme adjacent trials was below chance). According to de Blois et al., orangutans have memory problems and after searching in the first box visited by the experimenter they forget which was the second and therefore they search in an adjacent one. In the case of extreme– extreme displacements, the first one is either of the extreme boxes and the adjacent one is the middle box, which is empty. This hypothesis also explains why they succeed in extreme–middle displacements because they automatically found the object on their second choice provided that they remembered the first box visited by the experimenter. According to Call, a response bias–inhibition problem is responsible for this pattern of performance. In this account, apes

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do not forget the second box, but they cannot inhibit searching in the adjacent one after visiting the first. For the same reasons as in de Blois et al.’s study, apes succeeded in the extreme–middle displacements and performed worst in extreme– extreme displacements. Call (2001) tested de Blois et al.’s (1998) memory problem hypothesis with a set of nondisplacement trials. He argued that if apes forgot the second place, then they would search randomly after visiting the first box. However, de Blois et al. explained the performance of apes with memory problems plus a tendency to search in sequence. Call did not take into consideration the tendency to search in sequence, only memory problems, therefore his nondisplacement trials did not prove that the memory problem hypothesis was wrong. In summary, the only difference in the two hypotheses is that according to Call, apes do not forget the second box, only they cannot inhibit searching in sequence even if the displacements were visible. The main point of both is that apes tend to search in sequence for some reasons (memory or inhibition). We cannot tell what the reason for this tendency is: inhibition or memory problems. But we raise another possibility: It is simply more convenient to search in sequence, at least in the case of our subjects. When gibbons sit facing the experimenter in front of the apparatus, they are not able to reach all of the boxes from the same position with equal ease. If after choosing the left box, they wanted to choose the right one, they would have to stand up and go to the other end of the board and vice versa. If they were hanging on the mesh with one hand, they could not reach all of the boxes with the same hand, but would have to change hands. This hypothesis could be tested by another experimental setup in which all of the boxes are at the same distance from the subjects (e.g., boxes arranged in a triangle on a vertical board in front of the animals, or in the form of a semicircle suggested by Call, 2001). Collier-Baker & Suddendorf (2006) used four hiding boxes in a diamond-shaped array on a vertical plane to test chimpanzees and children. The problem with this arrangement with gibbons is that it would be difficult to prevent them from climbing up and looking into the boxes from above. In sum, we assume that only 1 of our subjects solved single visible and single invisible displacements without learning and, based on our control, it happened not by using the strategy of choosing the (last) box touched by the experimenter. This performance is similar to that of great apes and superior to that of most monkey species; however, great individual differences prevent us from drawing general conclusions on the level of Hylobatidae. Further experiments are needed to test more subjects and to exclude other associative strategies by using stringent controls like those in Collier-Baker et al. (2004, 2006). It would be also useful to use a different experimental setup, in which all boxes are reachable from the same position with equal ease. Another problem to solve is to better motivate subjects and to assess why they develop biases.

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Received April 20, 2007 Revision received February 19, 2008 Accepted February 19, 2008 n

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