In the first experiment, we investigated whether different ape species (chimpanzees, bonobos, and orangutans) would use mirror images and shadows to locate hidden food. We compared these cues to a baseline condition in which apes could see where the food was hidden and a control condition in which the apes did not receive any cue about the food location. We hypothesized that apes would perform significantly better in the mirror and shadow condition compared to the control condition if they were sensitive to the relation between these optical effects and their physical referents (i.e., the desired food reward). Moreover, we expected a similar performance between the mirror and shadow conditions and the visible displacement baseline condition.

Materials and methods

Subjects

Eleven chimpanzees (P. troglodytes), six orangutans (P. abelii), and eight bonobos (P. paniscus) participated in this experiment. The subjects were housed at the Wolfgang Köhler Research Centre, Leipzig Zoo (Leipzig, Germany). The subjects were not deprived of food, and water was available ad libitum during testing. One of the 11 chimpanzees (Jeudi) stopped approaching the test site after two sessions and was, therefore, excluded from data analysis. Another chimpanzee (Frodo) only completed the mirror condition but was not available for the shadow condition. Our final sample consisted of 16 females and 8 males aged between 5 and 41 years (M Age 20.3 years). Subjects had participated in various cognitive tasks prior to the study, none of which involving shadows or mirrors as source of information. All chimpanzees and orangutans (but not the bonobos) had previous experience with shadows in a pilot experiment in which they failed to use the shadows as a cue (except for two chimpanzees, Sandra and Jeudi, who scored significantly above chance at the individual level). In the pilot experiment, the hidden food reward was located on a transparent Plexiglas table and its shadow was projected to a surface underneath this table. In the current setup, the shadow of the reward was projected to a screen behind the hidden food reward in the apes’ line of sight. The individual performance in the shadow condition of Experiment 1 and 2 was not correlated with the performance in the pilot experiment (Spearman’s correlation: r s = 0.240, N = 23, p = 0.267). Additionally, the bonobos who did not participate in the pilot experiment performed at similar levels compared to the other apes. Consequently, carry-over effects from the pilot experiment appear unlikely.

Apparatus

The experimenter (E) sat behind the sliding platform (78.5 × 34 cm) facing the subject who was behind a transparent acrylic glass panel. This panel contained two or three horizontally aligned, circular holes (6 cm). The sliding platform was mounted 10.5 cm below the Plexiglas panel to ensure that the subjects could see all relevant parts of the platform, particularly the mirrors and projection areas for the shadows. On top of the sliding platform, there was a movable displacement device, a piece of PVC plastic material (7 × 3 cm), that served to displace the food placed on top of it (see Fig. 1). E could move the displacement device to the left or to the right side of the platform via a thin, transparent string (diameter 1 mm) attached to it. The two ends of the string were connected underneath the platform. E displaced the food by pulling the string underneath the platform, occluded from the subjects’ view. Thereby, the experimenter remained in a central position without leaning to the left or to the right. At the experimenter side of the platform, there was a screen mounted vertically to the platform (78 × 17 cm) onto which the shadows were cast (shadow condition) or the mirrors (15 × 15 cm) were attached (mirror condition). Three covers occluded the location of the food at the left, central, and right side of the platform (height × depth × width: 6.5 cm × 24.5 cm × 22/26 cm). In the shadow condition, two battery-powered bicycle lights (single LED, 0.5 W, 12 lx) were located under the left and right cover facing toward the screen onto which the shadows were cast. When the covers were in place, the subjects could still see the screen with the silhouette/mirror image in the back of the platform but not the food or the displacement device.

Fig. 1 Experiment 1 and 2: illustration of the different conditions from the subjects’ point of view. Experiment 1 includes conditions (a–d); Experiment 2 includes conditions (b–e). A piece of banana (yellow oval) is placed on a displacement device. In all conditions except for the baseline condition, the food reward is then concealed. A silhouette (shadow condition), mirror image (mirror condition), or a black rubber patch (arbitrary condition) indicates the location of the concealed food reward. The rubber patch is displaced by the experimenter (tan shape = experimenter’s hand). (Color figure online) Full size image

Design

We administered four different conditions: mirror, shadow, control, and baseline. In the mirror and shadows conditions, apes could use the mirror image and the shadow of the food, respectively, to locate the food (see Fig. 1a, b and Online Resource 2). In the control condition, apes did not receive any information about the location of the food (controlling for inadvertent cues that the subjects might pick up; see Fig. 1c) and in the baseline condition apes could directly observe how the food was moved under one of the two lateral covers (visible displacement baseline; see Fig. 1d).

We divided our sample into two groups according to the order in which they received the mirror and shadow condition. The mirror-first group received the mirror condition in the first four sessions and the shadow condition in the following four sessions, for the shadow-first group the order of conditions was reversed. Assignment to the groups was random except that we balanced the groups for age, sex, and species as much as possible (mirror-first group: M Age : 22.6 years, 7 females, 4 males; shadow-first group: M Age : 17.3 years, 9 females, 3 males). Three orangutans who were in the mirror-first group received a different experimental setup in the mirror condition (the sliding platform was mounted at a greater height from the floor). Due to this change in the setup, apes could barely see the food in the mirror when sitting in front of the Plexiglas panel. We repeated the mirror condition for these individuals after the shadow condition. Due to the deviation in procedure, we discarded the data of their first four sessions in the mirror condition and re-assigned them to the shadow-first group.

We administered 12 trials per session, each including 6 trials of the experimental condition (mirror or shadow), and 3 trials of the control and baseline condition. The reward was moved to the left in half of the trials and to the right in the other half. We pseudo-randomized the order of trials within a session with the restriction that the food was moved to the same side in no more than three consecutive trials. Moreover, subjects did not receive the same condition in more than four consecutive trials. All subjects completed 8 sessions, for a total of 24 trials per condition.

Procedure

We tested subjects individually. At the beginning of each trial, E moved the displacement device in the centre of the platform. In the mirror condition, E then mounted two mirrors (via magnets) at the screen on the left and right side of the platform facing toward the subject. In the shadow condition, E placed two lights on the platform in front of the left and right hole of the Plexiglas panel facing towards the vertical partition in the back. In the baseline and control condition, there were neither lights nor mirrors. Then E put the left and right covers on the platform (covering the two lamps in the shadow condition) and positioned a piece of banana on top of the displacement device in the centre. E covered the reward (except for the baseline condition) by putting the central cover on the platform and pulled the reward either to the left or to the right side via a string underneath the platform. E then lifted the central cover revealing that the food reward was gone. E pushed the platform forward allowing subjects to make a choice by inserting their hands into one of the two outer holes in the panel (for a picture of the setup, see Online Resource 1). E looked straight to the ground to avoid inadvertent cueing, while displacing the food until the subjects made their choice. Once the subjects had made their choice, E lifted the selected cover. If the choice was correct, the subjects received the piece of banana underneath the chosen cover. If the choice was incorrect (i.e., no food under the cover), E showed the apes where the food was hidden and discarded the food in the food bucket underneath the platform. In a few trials, apes’ selection was equivocal because they inserted their hands in both holes or they switched rapidly between the two options. In those cases, E pushed the platform back and forth again until the subject made an unambiguous decision.

Scoring and analysis

We scored the first choice of the subjects after E had pushed the sliding table to the subject. A second coder naïve to the hypotheses and theoretical background of the study scored 20% of all trials to assess inter-observer reliability, which was excellent (Κ = 0.99, N = 456, p < 0.001).

We used a generalized linear mixed model (GLMM; Baayen 2008) with binomial error structure and logit link function to analyse the effects of condition, sex, age, species, order of experimental conditions, and session on apes’ choices (correct/incorrect). We included these factors as fixed effects and subject ID as random effect. Additionally, to keep type I error rate at the nominal level of 5% (Barr et al. 2013; Schielzeth and Forstmeier 2009), we included all random slope components of condition (dummy coded), order of condition, and session. As an overall test of the effect of the predictor variables we compared the full model with a null model lacking the fixed effects but comprising of the same random effects structure as the full model (Forstmeier and Schielzeth 2011) using a likelihood ratio test (Dobson 2002). p values for the individual effects were based on likelihood ratio tests comparing the full with the respective reduced models (Barr et al. 2013; R function drop1).

We assessed model stability by comparing the estimates derived by a model based on all data with those estimates obtained from models with individual subjects excluded one at a time. This revealed the model to be stable with regard to the fixed effects. Overdispersion appeared to be no issue (dispersion parameter: 0.86).

Additionally, we used Wilcoxon signed-ranks tests to examine whether performance deviated significantly from the hypothetical chance level (p = 0.5). At the individual and for first trial analysis, we conducted binomial tests with a hypothetical probability of p = 0.5. All p values reported throughout this study are exact and two tailed.

Results

We found evidence that apes used the mirror and shadow cues spontaneously to locate the food. Apes performed significantly better in the shadow and mirror condition compared to the control condition. Additionally, their performance in the shadow and mirror conditions was above chance levels. Analyses of the individual performances confirmed this result.

A GLMM comprising of the factors condition, order of mirror and shadow conditions, session, species, age, and sex was significant compared to a null model lacking these factors (likelihood ratio test: χ2 = 73.6, df = 9, p < 0.001; see Online Resource 1, for the model output). Condition had a significant effect on performance (χ2 = 67.7, df = 3, p < 0.001; see Fig. 2). Apes performed significantly better in the baseline (χ2 = 38.7, df = 1, p < 0.001), mirror (χ2 = 18.0, df = 1, p < 0.001), and shadow condition (1.08 ± 0.23, χ2 = 17.1, df = 1, p < 0.001) compared to the control condition. Moreover, apes performed better in the baseline condition compared to the shadow condition (χ2 = 8.1, df = 1, p = 0.004) but not compared to the mirror condition (χ2 = 1.8, df = 1, p = 0.180). There was no significant difference between shadow and mirror condition (χ2 = 2.95, df = 1, p = 0.086). We found no significant effect of species (χ2 = 2.3, df = 2, p = 0.322), the order of conditions, (χ2 = 0.1, df = 1, p = 0.816), session (χ2 = 1.0, df = 1, p = 0.328), age (χ2 = 0.002, df = 1, p = 0.964), or sex (χ2 = 1.7, df = 1, p = 0.195).

Fig. 2 Experiment 1: proportion of correct trials (mean ± SE) as a function of condition Full size image

Apes performed significantly better than expected by chance in the baseline (mean ± SE: 0.86 ± 0.02; Wilcoxon signed-ranks test: T+ = 300, N = 24, p < 0.001), shadow (0.71 ± 0.04; T+ = 218, N = 21, p < 0.001), and mirror condition (0.76 ± 0.04; T+ = 209, N = 20, p < 0.001) but not in the control condition (0.51 ± 0.02; T+ = 129, N = 20, p = 0.380). This pattern of findings was already present in the first session of each experimental condition (shadow: T+ = 141, N = 18, p = 0.010; mirror: T+ = 132, N = 16, p < 0.001, mirror analysis without the data of the three orangutans that received another experimental setup initially). First trial analysis showed that apes performed above chance in the mirror condition (16 of 21 individuals chose correctly, binomial test: p = 0.027) but not in the shadow condition (15 of 23 individuals chose correctly, p = 0.210).

At the individual level, five out of eight bonobos, eight out of ten chimpanzees, and all six orangutans performed above chance in the baseline condition (binomial test, p < 0.05). In the shadow condition, four out of eight bonobos, four out of nine chimpanzees, and three out of six orangutans performed significantly above chance. In the mirror condition, three out of eight bonobos, six out of ten chimpanzees, and five out of six orangutans (for three of these orangutans the mirror condition was repeated) performed significantly above chance. In the control condition, none of the apes performed significantly above chance.

Discussion

We found that some individuals of all examined ape species used shadows and mirror images of hidden food items as a cue to locate them. Other individuals did not use these cues. The reasons for the observed individual differences are unclear but attention to the problem situation, food motivation, and more specific cognitive differences might play a role here. We found no evidence that apes’ performance improved across sessions. On the contrary, we found that they used both cues already in their first session (i.e., within the first six trials with these cues), and mirror images already in their first trial.

However, these findings leave a number of open questions regarding apes’ understanding of these optical effects. With regard to the shadows, did they associate the silhouette with the location of the food based on rapid reinforcement learning? Or did the apes make use of the silhouette because they inferred location of the food reward as the physical referent of the shadow? If reinforcement learning was sufficient to explain apes’ performance, we expected that they would learn to use a perceptually similar and equally deterministic cue to locate the food within the same number of trials (see Experiment 2). With regard to the mirror images, first trial performance showed that reinforcement learning was not a viable explanation. However, in the latter case, apes might just have confused the mirror reflection with its physical referent. It is possible that they pointed toward the food they saw (in the mirror) without taking into account that it was just a reflection of the food reward. Thus, the question is whether apes discriminated between appearance and reality in the case of the mirror image.

Moreover, motion cues were available in both experimental conditions. The moving shadow or mirror reflection of the food reward was visible before the apes were allowed to choose. These motion cues may have directed their attention to the correct side which could potentially explain apes’ performance without invoking any higher-level processing of these optical effects. Were such motion cues necessary and/or sufficient to allow apes to exploit these optical cues?

The remaining experiments addressed these questions one by one. In the next experiment, we introduced a control condition, which shared perceptual features (including motion cues) and the reinforcement regime with the shadow condition but lacked the causal relation between cue and food location.