(Gorilla gorilla gorilla) Spatial Memory in a Foraging ...

Copyright 1994 by the American Psychological Association, Inc. 0735-7036794/$3.00

Journal of Comparative Psychology 1994, Vol. 108, No. 2, 107-113

Gorillas' (Gorilla gorilla gorilla) Spatial Memory in a Foraging Task Suzanne E. MacDonald The spatial memory of 2 gorillas (Gorilla gorilla gorilla) was explored in a simulated foraging task. Trials consisted of 2 parts separated by a delay. In the 1 st part, half of the total number of food sites were baited with a highly preferred food, and the subject was allowed to search, find, and consume these items (search phase). During the delay the same locations were again baited. After the delay the animal was reintroduced to the test enclosure and allowed to search through the sites again (re-search phase). In Experiment 1, an adult gorilla was very accurate in remembering locations that had previously contained food at delay intervals of 24 hr or more. In Experiment 2, a juvenile gorilla was also accurate in remembering locations that had previously contained food at delays up to 10 min. The adult gorilla appeared to use a counting strategy during the search phase to minimize the number of sites visited.

very accurate at later retrieving that food, as compared with control animals that had not seen the baited sites. Furthermore, the test chimpanzees tended to use a least distance strategy by minimizing the total distance travelled to retrieve the hidden food. One similarity between Tinklepaugh's early work and Menzel's later study is that the animals were shown the location of food by the experimenter and then tested for their ability to recall the location. In nature, of course, animals discover the location of food while foraging and then may return to those locations at a later time. Most investigations of spatial memory typically involve variations on the radial arm maze (Olton & Samuelson, 1976), in which the animal freely searches for and retrieves food. Andrews (1988) tested two species of New World monkeys in a radial arm maze adapted for primates. The elevated maze had a central starting platform from which radiated eight runways, each ending in a goal box with food. The monkeys did not avoid previously visited goal boxes; in fact, the number of runway arms selected by the monkeys did not differ from chance performance. However, the low accuracy demonstrated by the monkeys in Andrews' study may have been due to the constricting nature of the radial arm maze, which is much different from what monkeys encounter while foraging. To address this issue MacDonald and Wilkie (1990) used a simulated foraging task, which was a variation of the standard radial arm maze, with yellow-nosed monkeys (Cercopithecus ascanius whitesldei). Eight food sites were scattered throughout a large enclosure; only four of the eight sites contained food. The location of the food sites remained constant throughout testing, but different sites were baited with food for each trial. Individual monkeys were allowed to explore all the food sites and to retrieve food from the baited sites. After all eight sites were visited, the monkey was removed from the enclosure for a delay period. During the delay, the food sites were rebaited. Either the same four sites that had previously contained food were baited again (win-stay task) or the four sites that had not previously contained food were baited (win-shift task). Results from this experiment showed that monkeys were extremely ac-

The ability to remember where food is located appears to be an important cognitive skill for animals, as evidenced by the increasing body of research that has demonstrated excellent spatial memory in many species. Memory for the spatial location of food has been extensively studied in rodents (e.g., Jones, McGhee, & Wilkie, 1990; Olton & Samuelson, 1976; Roberts, 1979; Wilkie & Slobin, 1983), birds (e.g., Sherry, 1984; Shettleworth & Krebs, 1982; Spetch & Edwards, 1986; Wilkie, Spetch, & Chew, 1981), and insects (e.g., Gould, 1987). Primates provide an excellent opportunity to study spatial memory in a broad range of closely related species, ranging from solitary tree-dwellers to group-living terrestrial animals. However, primate spatial memory has been understudied in comparison with research on spatial memory in other species. Tinklepaugh (1928, 1932) looked at monkeys' (Macaco spp.) and chimpanzees' (Pan troglodytes) ability to discriminate between pairs of cups located in different spatial locations, after seeing one cup of each pair baited with food. Monkeys were very accurate at this task, but their accuracy declined when the task was made more complex by increasing the number of cup pairs. In contrast, chimpanzees were very accurate on both the simple and the complex task, even when delays of up to 24 hr were interpolated between baiting and recovery. Similarly, Menzel (1973) found that chimpanzees that had seen food hidden in 18 locations were Portions of this research were presented at the annual meeting of the Canadian Society for Brain, Behavior and Cognitive Science, Quebec, Quebec, Canada, June 1992. This research was supported by a Natural Sciences and Engineering Research Council Operating Grant to Suzanne E. MacDonald. The encouragement, cooperation, and assistance of the staff at the Metropolitan Toronto Zoo was greatly appreciated. I extend special thanks to Dianne Devison, Marilyn Cole, and Vanessa Phelan. Correspondence concerning this article should be addressed to Suzanne E. MacDonald, Department of Psychology, York University, Atkinson College, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada. Electronic mail may be sent to suzmac @ vm 1 .yorku.ca.

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SUZANNE E. MACDONALD

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curate at recalling the location of food. In the win-stay task, the monkeys returned to the sites that had previously contained food; in the win-shift task, the monkeys avoided sites that had previously contained food and instead visited sites that had not previously contained food. The monkeys tended to use a least distance strategy similar to that used by the chimpanzees in Menzel's (1973) study. The fact that the monkeys tended to minimize the total distance travelled in retrieving food is interesting, given that the monkeys were in a free-foraging situation, with no time constraints placed on food retrieval. The finding that both chimpanzees and an Old World monkey species used similar retrieval tactics in a spatial memory task suggests that this may be a common primate strategy. The purpose of these experiments was to explore spatial memory and retrieval strategies in another primate, the gorilla (Gorilla gorilla gorilla). Gorillas, like chimpanzees, are apes. Unlike chimpanzees, the cognitive capabilities of gorillas have not been extensively studied, and spatial memory in particular has not been investigated. A foraging task provides a natural test for spatial memory skills and also provides direct comparison with spatial memory data from other primate species (e.g., MacDonald & Wilkie, 1990; Menzel, 1973). In my experiments, different locations in a large outdoor enclosure were baited with food, which the gorilla subjects attempted to find. Like the monkeys tested by MacDonald and Wilkie (1990), both experimental subjects were zoo animals, and testing was conducted entirely within the confines of a zoo.

Experiment 1 The purpose of this experiment was to assess spatial memory in a foraging task in an adult gorilla. Gorillas are predominantly terrestrial herbivores (Spinozzi & Natale, 1989). They spend most of their waking hours moving slowly through the forest, eating leaves, vines, and branches as they travel (Bramblett, 1976). It is unlikely that a gorilla group would return to the same site in their large home range soon after leaving that site. In order to more closely approximate conditions that a gorilla may encounter in the wild, a delay period of at least 1 day separated the initial search phase from the memory test.

Method Subject Charles, an experimentally naive 20-year-old lowland gorilla (Gorilla gorilla gorilla) served as the subject. Charles weighed approximately 400 Ib (182 kg) and was approximately 6 ft (2.8 m) tall. Charles was born in the wild in Africa but had resided at the Metropolitan Toronto Zoo in Toronto, Ontario, Canada since he was 2 years old.

Testing Area The experiment was conducted at the Metropolitan Toronto Zoo, in the large outdoor gorilla enclosure shown in Figure 1. The

X Observation point

Figure I. Schematic overhead enclosure.

view

of gorillas' outdoor

outdoor enclosure measured 10 m wide X 10 m high X 20 m long. The enclosure was surrounded by steel fencing and two Plexiglas observation booths through which the public viewed the animals. Two doors provided access to the outdoor enclosure from the indoor holding area; the doors could be opened or closed remotely by the zookeepers. The floor of the enclosure was natural grass. As shown in Figure 1, several climbing trees and elevated platforms were scattered throughout the enclosure, and a circular wading pool was located in the center of the area. The gorillas normally had free access to the outdoor enclosure but could be kept in the indoor holding area if the keepers closed the sliding steel doors at the entrance. Observations were made by the experimenter from a site located behind dense shrubbery, shown in Figure 1. Eight blue plastic containers with tightly fitted lids served as food sites. The food site containers remained in the same locations, shown in Figure 1, throughout the experiment. Each of the food sites was baited with 40 g of raisins, a highly preferred food. The outdoor enclosure was only illuminated by natural light. Testing took place at approximately 0900 hr, before zoo opening hours. Test sessions were usually conducted 2 days per week. Charles was fed regular gorilla rations at 0730 hr every morning by the zookeepers and had free access to the food throughout the day.

Procedure Charles was initially shown a baited food container, and removal of the lid was demonstrated by the experimenter while Charles watched through the fencing of the outdoor enclosure. His first direct contact with the food containers was during the experimental testing. The experiment consisted of two phases, walking maze and win-stay.

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GORILLA SPATIAL MEMORY Walking maze. Charles received 5 days of testing with all eight sites baited. On these trials, Charles was released into the outdoor enclosure and allowed to explore the eight food sites and to remove the raisins from each. The order in which the sites were visited was recorded. Win-stay phase. On each of these trials, Charles was released into the outdoor enclosure and allowed to freely explore all eight food sites. As before, the order of visits to the food sites was recorded. However, in this phase only four randomly determined food sites were baited during each trial. When the gorilla had visited all eight sites and eaten the food from all four baited sites (search phase), the test was finished for the day. Charles was then free to either leave or stay in the outdoor enclosure. The empty food sites remained in the enclosure throughout the day. The remaining gorillas were allowed access to the enclosure during the afternoon, and they used the empty food containers as toys. After a delay, which ranged from 24 to 48 hr, the experimenter reentered the outdoor enclosure, replaced the containers in their original position, and rebaited the four food sites that had been baited the during the search phase. Charles was then reintroduced into the outdoor enclosure and allowed to search through the food sites again (re-search phase). (A visit to a food site was defined as the gorilla's going to a site and picking up the container but not opening it. Charles quickly learned to shake a container to determine if it contained food, thus avoiding the difficult task of opening the lid.) The order of visits and the number of choices taken to recover the food from all four baited sites were recorded. Only one search-re-search trial set was conducted per week, and the trial set was usually conducted on the same days of the week. Because of logistical constraints placed on data collection by the zoo environment, testing was discontinued after a total of 12 search-re-search trials run over 6 months.

Results Walking Maze During this phase all eight sites were baited, and the gorilla was free to retrieve the food from all the sites. Charles acquired this task immediately: He always visited each of the eight sites, and he never revisited a previously

depleted site, which suggests that he remembered the sites. On each of the five test trials, Charles exhibited a strong tendency to visit adjacent sites. On each of the trials, he literally galloped into the center of the enclosure, began his search at the site closest to him (usually Sites B, C, or D; see Figure 1), and moved from that site in a clockwise direction around the enclosure. Depletion of each food site took approximately 30 s, while Charles opened the tightly closed container, ate the raisins inside, and then dropped the container and lid on the ground before moving to the next site. Although the gorilla never revisited a depleted site during this phase, the fact that the empty food container and lid were always dropped in a different position than where they were originally may have served as a cue to avoid revisits. Thus, although results from the walking maze test suggest that the gorilla used spatial memory to search the baited sites, the possibility that he used overt cues from the food cups necessitated further testing with the win-stay task. Win-Stay Task Recovery accuracy during the re-search phase of the trials was extremely high (Figure 2). The minimum number of visits required to recover all of the food was four; the overall mean number of sites visited by Charles in the re-search phase was 5.5. The chance probability of obtaining four items in five choices, if one assumes that a site is never revisited, is .07. As shown in Figure 2, Charles obtained all four items in his first five choices on 58% of the trials, which is much better than predicted by chance, z(l 1) = 6.93, p < .0001, for proportions. It is important to note that this level of accuracy was obtained after an extremely long delay between search and re-search. The minimum delay was 24 hr, but four of the sessions (Trials 4, 6, 7, and 11) had a 48-hr delay interpolated between the search and re-search phases. Testing was completed in early summer, when Charles began mating with the female gorillas;

12

Figure 2. An adult gorilla's accuracy in finding hidden food in the re-search phase of the win-stay problem as a function of trials.

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this may explain his decreased accuracy for the final few trials. In addition to accuracy, movement patterns between the different food sites were examined. In the search phase 83.6% of Charles' choices were made to an adjacent site. He typically started at one site and moved in a clockwise direction around the enclosure. This pattern did not transfer to the re-search phase, in which Charles's choices depended primarily on which four sites were to be visited. Although the gorilla made few errors in the re-search phase after the third trial, the type of errors that he did make were consistent: 87.5% of his errors were made to a site adjacent to a correct site. For example, on Trial 8, Charles incorrectly chose Site B rather than Site A during the re-search phase. Incorrect choices were always followed by an aggressive display and often the empty food container was hurled across the enclosure. After making an adjacent error, Charles always moved to the adjacent correct site. There was no evidence that the gorilla used a least-distance strategy in the re-search phase, although the number of trials were limited, so it is difficult to determine if he would have used this strategy with more testing. The most interesting aspect of Charles' behavior during the search part of the trials was that he consistently terminated his search after finding the fourth baited site. In the search phase the gorilla did not know which of the eight sites contained food. As shown in Figure 3, he sometimes had to search all eight sites before finding the four baited locations (e.g., Trial 4); sometimes he found the four baited locations on his first four tries (e.g., Trial 9). As the figure illustrates, from Trial 3 to the end of testing, Charles searched only until he found the fourth baited site. (On Trial 7, Charles visited an additional site, but this may have been because the container at that site was new, a substitute for a missing container). These data are particularly interesting because there were no experimental time constraints placed on the gorilla—he was free to search through the sites all day if necessary. Although the experimenter sat behind dense shrubbery while recording the data, it was possible

that Charles picked up subtle movements made by the experimenter to cue him when to terminate his search. To eliminate that possibility, naive observers were used for several of the trials. The observers had not been told how many sites were baited, so they had no idea when Charles ought to have stopped searching. Charles continued to use his counting strategy and ended each search after finding the fourth baited site.

Experiment 2 In Experiment 2, the task was modified slightly to allow a juvenile gorilla to participate. The young gorilla was 1 year old at the start of testing; he had just begun to forage independently but had not yet been completely weaned. Because of the small size and very young age of the gorilla, the number of food sites was reduced from eight to six, the sites were located much closer together, and the delay was reduced from 24 hr to 10 min. The proportion of baited to unbaited sites remained the same as that used in Experiment 1; three of the six sites were baited on every trial. As before, the location of the food sites remained the same throughout testing; baited sites varied randomly from trial to trial. The data from this experiment address the previously uninvestigated issue of how spatial skills and foraging strategies develop in primates. Does a young gorilla remember spatial locations as accurately as an adult gorilla does? Are the strategies used by an adult gorilla the same as those used by a juvenile gorilla?

Method Subject Jabari, an experimentally naive 1-year-old lowland gorilla (Gorilla gorilla gorilla) served as a subject. Jabari weighed 22 Ib (10 kg) at the time of testing and was approximately 2 ft (60 cm) tall. Jabari was bom at the Metropolitan Toronto Zoo and had been

8

I Number of choices to obtain four items

9

10

11

1Z

i_i Total number of choices in search phase

Figure 3. The number of choices made by an adult gorilla in the search phase of the win-stay problem.

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GORILLA SPATIAL MEMORY hand-reared by the zoo staff. He was not taught any special skills and was encouraged to be as gorillalike as possible by his keepers.

Testing Area The testing area was the outdoor enclosure used in Experiment 1. Observations were made by the experimenter from a site located behind dense shrubbery, shown in Figure 1. Six opaque white containers with fitted lids served as food sites. The containers were placed in a circle, 2.5 m in diameter, 1.3 m apart and were placed in the same locations throughout the experiment. Each of the food sites was baited with a single grape, a highly preferred food. A zookeeper was present for each of the sessions; Jabari could not be left alone because of his age. The zookeeper was unaware of the baited locations and simply sat in the comer of the enclosure while Jabari searched through the sites. The outdoor enclosure was only illuminated by natural light. Testing took place at approximately 0900 hr, before zoo opening hours. Test sessions were conducted whenever access could be obtained, but never more than 3 days each week. Jabari was fed his usual food rations at 0730 hr every morning by the zookeepers and had free access to the food throughout the day.

Procedure Jabari received preliminary training with the food containers until he could open the lids and extract the grape inside. As in Experiment 1, this experiment consisted of two phases, walking maze and win-stay. Walking maze. Jabari received 5 days of testing with all six sites baited. On these trials he was brought into the outdoor enclosure and allowed to explore the six food sites and to remove the grapes from each. The order in which the sites were visited was recorded. Win-stay phase. On each of these trials, Jabari was brought into the outdoor enclosure, placed in the center of the circle of food containers by the zookeeper and allowed to freely explore all six food sites. As before, the order of visits to the food sites was recorded. However, in this phase only three randomly determined food sites were baited during each trial. In the search phase of the

trial, the gorilla searched all six sites, removing the food from the three baited sites. The experimenter then reentered the outdoor enclosure, replaced the containers in their original position, and rebaited the three food sites that had been baited during the search phase. During this delay period Jabari played with the zookeeper in another part of the enclosure. The length of the delay interval was short (30 s or less) for the first few trials and was gradually increased to 10 min. After the delay Jabari was again placed in the center of the circle of containers and allowed to search through the food sites (re-search phase). The order of visits and the number of choices taken to recover food from all three baited sites were recorded. Only one search-re-search trial set was conducted per day. A total of 15 trials were conducted over 4 months.

Results Walking Maze During this phase all six sites were baited, and the gorilla was allowed to retrieve the food from all of them. Jabari immediately learned to visit and deplete each site, and he never revisited a previously depleted site. He also showed a strong tendency to visit adjacent sites in a clockwise direction. However, as in Experiment 1, the fact that the empty food container and lid were always dropped in a different position than where they were originally may have served as a cue to avoid revisits. Jabari often carried the empty containers with him to the next site, which made revisits impossible. Thus, although results from the walking maze test suggest that Jabari may have used spatial memory to search the baited sites, further testing with the win-stay task was necessary.

Win-Stay Task Recovery accuracy during the re-search phase of the trials was quite high (Figure 4). The minimum number of visits required to recover all of the food was three; the overall

10

11

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Figure 4. A juvenile gorilla's accuracy in finding hidden food in the re-search phase of the win-stay problem as a function of trials.

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mean number of sites visited by Jabari was 4.47. The chance probability of obtaining three items in four choices, if one assumes that a site is never revisited, is .20. As shown in Figure 4, Jabari obtained all three items in his first four choices on 53% of the trials, which is much better than predicted by chance, z(14) = 3.30, p < .001, for proportions. A comparison of accuracy at short delays (5 min or less) with that at longer delays (10 min) showed that the length of the interval did not significantly affect accuracy (t test for dependent samples, p > .05). These data are probably underestimates of his true spatial memory, because Jabari was often distracted by the food containers and spent much of his time playing with them rather than searching the food sites. In addition to accuracy, movement patterns between the different food sites were examined. In the search phase, 82.7% of Jabari's choices were made to the site adjacent to the one just visited. He typically started searching at one site and then moved around the circular configuration opening containers in quick succession. However, in the re-search phase, only 26.7% of his choices were incorrectly made to adjacent sites, which indicates that Jabari was not simply re-searching the sites with an adjacent strategy. No systematic retrieval strategy was evident from his pattern of choices in the re-search phase. Unlike Charles in Experiment 1, Jabari showed no evidence of counting the sites in the search phase. On every trial he opened each of the six containers and continued to search after finding the third baited site.

Discussion The results from Experiments 1 and 2 strongly suggest that gorillas can remember the spatial location of food. In the win-stay foraging task used in both experiments, the gorilla subject initially searched for baited food sites and then, after a delay interval, was allowed to re-search the food sites. Both the adult (Experiment 1) and juvenile (Experiment 2) gorillas were accurate at choosing sites in the re-search phase that had contained food in the search phase. The adult gorilla made extremely accurate choices after delays of 24 hr between the search and re-search phase. Both gorillas tended to use an adjacent strategy in the search phase, choosing the next closest site and moving in a circular pattern around the test enclosure. Use of an adjacent strategy has been reported occasionally in rats in the radial arm maze (e.g., Roberts & Dale, 1981) and in pigeons in a free-foraging environment (e.g., Spetch & Edwards, 1986). The placement of the food sites in a roughly circular arrangement in both Experiments 1 and 2 may have facilitated the use of this strategy. Unlike chimpanzees (Menzel, 1973) and yellow-nosed monkeys (MacDonald & Wilkie, 1990), these gorillas did not use a least distance strategy in retrieval. A least distance strategy minimizes the energy expended in foraging by reducing the distance travelled to obtain food. However, the data from the search phase of Experiment 1 suggest that the adult gorilla used a counting strategy to minimize foraging

effort. Charles always stopped searching the food sites after finding the fourth baited site. By counting the baited sites, the gorilla reduced his foraging effort, given that further searching after visiting the four baited locations never resulted in more food. This search strategy has not been observed in monkeys (e.g., Andrews, 1988; MacDonald, Pang, & Gibeault, in press; MacDonald & Wilkie, 1990) or chimpanzees, although chimpanzees have shown an ability to count (Boysen & Berntson, 1989). Counting by gorillas has not been previously demonstrated, so the spontaneous use of a counting strategy is particularly exciting. An additional important finding from these experiments was that a very young gorilla exhibited above-chance accuracy in the foraging task. Although comparisons between the adult gorilla (Experiment 1) and the juvenile gorilla (Experiment 2) are limited because the task difficulty varied, these results do parallel those obtained with human children. Even very young human children show some evidence of memory for spatial location (e.g., Aadland, Beatty, & Maki, 1985; Cornell & Heth, 1983; Foreman, Arber, & Savage, 1984; Foreman, Warry, & Murray, 1990). Spatial memory may be present in a rudimentary form in very young primates, both human and nonhuman. The development of spatial memory and retrieval strategies in nonhuman primates remains to be explored. References Aadland, J., Beatty, W. W., & Maki, R. H. (1985). Spatial memory of children and adults assessed in the radial maze. Developmental Psychobiology, 18, 163-172. Andrews, M. W. (1988). Selection of food sites by Callicebus moloch and Saimiri sciureus under spatially and temporally varying food distribution. Learning and Motivation, 19, 254268. Boysen, S. T., & Berntson, G. G. (1989). Numerical competence in a chimpanzee (Pan troglodytes). Journal of Comparative Psychology, 103, 23-31. Bramblett, C. A. (1976). Patterns of primate behavior. Palo Alto, CA: Mayfield Publishing. Cornell, E. H., & Heth, C. D. (1983). Spatial cognition: Gathering strategies used by preschool children. Journal of Experimental Child Psychology, 35, 93-110. Foreman, N., Arber, M., & Savage, J. (1984). Spatial memory in preschool infants. Developmental Psychobiology, 17, 129-137. Foreman, N., Warry, R., & Murray, P. (1990). Development of reference and working spatial memory in preschool children. Journal of General Psychology, 117, 267-276. Gould, J. L. (1987). Landmark learning by honeybees. Animal Behaviour, 35, 26-34. Jones, C. H., McGhee, R., & Wilkie, D. M. (1990). Hamsters (Mesocricetus auratus) use spatial memory in foraging for food to hoard. Behavioural Processes, 21, 179-187. MacDonald, S. E., Pang, J. C., & Gibeault, S. (in press). Marmoset (Callithrix jacchus jacchus) spatial memory in a foraging task: Win-stay versus win-shift strategies. Journal of Comparative Psychology. MacDonald, S. E., & Wilkie, D. M. (1990). Spatial memory in yellow-nosed monkeys (Cercopithecus ascanius whitesidei) in a simulated foraging environment. Journal of Comparative Psychology, 104, 382-387.

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GORILLA SPATIAL MEMORY Menzel, E. W. (1973). Chimpanzee spatial memory organization. Science, 182, 943-945. Olton, D. S., & Samuelson, R. J. (1976). Remembrance of places passed: Spatial memory in rats. Journal of Experimental Psychology: Animal Behavior Processes, 2, 97-116. Roberts, W. A. (1979). Spatial memory in the rat on a hierarchical maze. Learning and Motivation, 10, 117-140. Roberts, W. A., & Dale, R. H. I. (1981). Remembrance of places lasts: Proactive inhibition and patterns of choice in rat spatial memory. Learning and Motivation, 12, 261-281. Sherry, D. F. (1984). What food-storing birds remember. Canadian Journal of Psychology, 38, 304-321. Shettleworth, S. J., & Krebs, J. R. (1982). How marsh tits find their hoards: The role of site preference and spatial memory. Journal of Experimental Psychology: Animal Behavior Processes, 8, 354-375. Spetch, M. L., & Edwards, C. A. (1986). Spatial memory in pigeons (Columba livia) in an open-field feeding environment. Journal of Comparative Psychology, 100, 266-278. Spinozzi, G., & Natale, F. (1989). Early sensorimotor development

in Gorilla. In F. Antinucci (Ed.), Cognitive structure and development in nonhuman primates (pp. 21-38). Hillsdale, NJ: Erlbaum. Tinklepaugh, O. L. (1928). An experimental study of representative factors in monkeys. Journal of Comparative Psychology, 8, 197-236. Tinklepaugh, O. L. (1932). Multiple delayed reaction with chimpanzees and monkeys. Journal of Comparative Psychology, 13, 207-243. Wilkie, D. M., & Slobin, P. (1983). Gerbils in space: Performance on the 17-arm radial maze. Journal of the Experimental Analysis of Behavior, 40, 301-312. Wilkie, D. M., Spetch, M. L., & Chew, L. (1981). The ring-dove's short-term memory capacity for spatial information. Animal Behaviour, 29, 639-641. Received March 22, 1993 Revision received July 7, 1993 Accepted August 11, 1993

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