In this experiment, we investigated if subjects can remember when and where two types of food were hidden. We presented the subjects with a platform in which three baiting places were available. The experimenter baited two of the three locations. One location contained a favourite but perishable food (frozen juice) and the other location contained a less preferred but non-perishable food (grape). The third location remained empty. After the food was hidden, subjects were allowed to choose one of these items either after a 5-min or 1-h retention interval (RI). After 1 h, the frozen juice melted and became unobtainable, but it was still edible if chosen after 5 min. If apes can remember when and where the two types of food were hidden, they should show a preference for the frozen juice on the 5-min trials because they should expect the frozen juice to be edible. However, this preference should be reversed on the 1-h trials if they can remember that the frozen juice was hidden a relatively long time ago and, therefore, would not be obtainable.
Methods
Subjects
We tested two bonobos (Pan paniscus), seven chimpanzees (Pan troglodytes) and three orangutans (Pongo pygmaeus) housed at the Wolfgang Köhler Primate Research Centre in the Leipzig Zoo (Germany). There were four males and eight females with ages ranging from 6 to 31 years (Table 1). We tested subjects individually (except for Jahaga, who was tested accompanied by a male) and none were food- or water-deprived. Subjects had previously participated or were currently participating in other studies (e.g., tool use, planning), so they were all used to participate in tests.
Table 1 Percentage of choices to the baited container (pre-test) and the frozen juice (experiments 1 and 2) for each subject. A direct comparison between conditions at the individual level in Experiment 1 is also shown
Apparatus and procedure
The apparatus consisted of 36 different opaque containers and two plastic platforms (70 cm × 35 cm). In one of the platforms, we drilled three holes and covered them with a plastic net. This net allowed the liquid to go through it and be collected under the platform inside a hidden cup.
Procedure
The experimenter (E) and the subject sat facing each other on either side of the Plexiglass partition with three equidistant circular holes (6 cm diameter) on its bottom part just above the platform (on E’s side). E placed three containers on the platform about 30 cm apart in front of a vertical Plexiglass, showed one reward (pre-test) or two rewards (what, where and when memory test) to the subject and placed them under one/two of the containers, depending on test. There were three phases:
Memory ability pre-test
In the pre-test, we tested individuals’ general long-term memory. Subjects had to remember the location of a reward placed under one of three cups on a platform after 2-min, 1-, 2- and 24-h RIs. The 2 min, 1, 2 and 24 h trial designations refer to the length of the time that elapsed between baiting and letting the apes to choose one of the containers. Apes received a total of 12 trials, 3 per condition. We used slices of banana as rewards.
Food preference test
Prior to the experiment, we established the apes’ preference for two food items. We placed one piece of each of two foods on the platform. Food included grapes (approximately 3 cm long × 1.5 cm diameter), slices of banana (approximately 0.5 cm) and frozen juice (approximately 3 cm long ×2.5 cm wide × 1.5 cm high). Various food combinations were used (i.e., banana vs. frozen juice, grapes vs. frozen juice) until each subject chose a given food, at least, five times over six trials. The food-locations were counterbalanced and apes could choose only once in each trial. We ranked frozen juice as the high-value food over banana for two chimpanzees and frozen juice as the high-value food over grapes for the rest of the subjects.
It is important to mention that the apes did not have any previous experience with the frozen juice that we used in our experiment. However, they have had experience with snow and in summer, as a part of their enrichment activities, they are sometimes provided with big pieces of ice cubes containing fruit.
What–where–when memory test
There were three possible baiting places on the platform. One was baited with frozen juice, another with the less preferred food and the third site remained unbaited.
Subjects were presented with both 5-min and 1-h trials. The 5-min and 1-h designation refer to the length of the time that elapsed between baiting and letting the apes to choose one of the containers. On the 5-min trials, the frozen juice and the less preferred food were available, whereas on the 1-h trial the frozen juice melted and only the less preferred food was available. The order in which subjects were presented in the two types of trials was counterbalanced. Apes received a total of three 12-trials rounds. To provide a unique baiting location on every trial within each round for each ape, the three baiting sites were covered with three different containers each. Additionally, these containers were different for each trial. In each round and for each trial different baiting places were used and the three possible sites were counterbalanced.
Analyses
We videotaped all trials. Subject’s choice was counted as the first box touched. We used Wilcoxon test to analyse whether individuals’ performance in the memory ability pre-test differed from chance. We also used Wilcoxon test to assess whether subjects chose the frozen juice more often in the 5-min than in the 1-h condition. We used Pearson Chi-square to investigate subject’s choices in the 5-min and 1-h trials at individual level. We analysed the effect of the age using the percentage of correct responses in the memory ability pre-test and the difference between the percentage of choices of the frozen juice at 5 min and 1 h in the what–where–when memory test. To assess the relation between performance and age, we fitted the models expressing performance as a quadratic function of age, i.e. performance = c
0 + c
1 × age + c
2 × age2. We used the ‘glm’ function provided by the R base package (R Development Core Team 2007) to fit the equation. Since the sample sizes were small and potentially included outliers, we used the quadratic term based on permutation (Adams and Anthony 1996; Manly 1997) to test levels of significance. We randomized performance across the subjects and measured the coefficient of the quadratic term (c
2 see equation above) for the randomized data. Finally, we determined the p value of the quadratic term as the proportion of permutations revealing an absolute coefficient being at least as large as the absolute coefficient of the original data. We used 1,000 permutations with the original data included as one permutation. Since we tested a null-hypothesis twice (no relation between age and performance), an error level adjustment was required. We achieved this using Fisher’s Omnibus test. This procedure combines a number of p values into a single c
2-distributed variable with degrees of freedom equalling twice the number of p values (Haccou and Meelis 1994). All statistical tests were two-tailed.
Results
Pre-test
Overall apes remembered the location of the food after each RI above the chance levels in the memory ability pre-test (Wilcoxon test: z = 3.12, p = 0.002, n = 12 in all intervals, Fig. 1) and there was no change in performance across ages (r
s=0.08, p = 0.41, n = 12).
What–where–when memory test
If apes can remember when and where the two types of food were hidden, they should show a preference for the frozen juice on the 5-min trials (frozen juice is still available) but this preference should be reversed on the 1-h trials (the frozen juice is no longer available). Subjects chose the frozen juice significantly more often after the 5-min interval than after the 1-h interval (Wilcoxon test: z = 2.55, p = 0.011, n = 12; Fig. 2). Their choices remained unchanged during testing (Friedman tests: 5-min—χ
2 = 0.16, n = 12, p = 0.92; 1-h—χ
2 = 0.42, n = 12, p = 0.8). Individual analyses showed that 4 out of 12 subjects chose the frozen juice significantly more often than the grape after the short RI and the grape after the long RI (Table 1). Since subjects rarely selected the empty boxes in the pre-test and test (pre-test: less than 12% of the trials, test: less than 3% of the trials), this suggests that their choices of the less preferred food after 1-h delay were not simply due to memory lapses. Unlike general long-term memory in the pre-test (permutation test: c
2 = −0.013, p = 0.375, n = 12, Figure 1), the performance on the www-memory test revealed an inverted U-shaped trend as a function of age (permutation test: c
2 = −0.142, p = 0.0682, n = 12, Fig. 1). Focusing solely on members of the genus Pan (chimpanzees and bonobos) confirmed this result (permutation test: c
2 = −0.026, p = 0.002, n = 9). Subjects younger than 7 and older than 18 years of age showed a much lower performance than adolescents and young adults.
Discussion
Subjects’ performance can only be explained by the recall of three types of information: what types of food (frozen juice and grapes) were hidden, where they were hidden (in which box) and when (5 min or 1 h ago) they were hidden. In terms of purely behavioural criteria, our results provide evidence for episodic-like memory in great apes.
Additionally, apes’ performance in the www-memory task showed an inverted U-shaped curve in which middle-age subjects performed better than the youngest and oldest subjects. In contrast, there was no age effect in the pre-test in which no temporal information was needed to be encoded. During the pre-test, subjects were capable of remembering the location of the food reward after 24 h had elapsed since baiting. In fact, the performance in the 24-h delay condition was as good as that after a 2-min delay condition. Interestingly, longitudinal and cross-sectional studies in human memory have reported an inverted U-shaped curve as a function of age for episodic memory and no age effect for long-term memory (Bialystok and Craik 2006 for a review).