Chimpanzees (Pan troglodytes) detect strange body parts: an eye-tracking study

This study investigated chimpanzee body representation by testing whether chimpanzees detect strangeness in body parts. We tested six chimpanzees with edited chimpanzee body pictures in eye-tracking tasks. The target body parts were arms or legs. For either target, there were four conditions: “normal” condition as control, where all bodies were normal; “misplaced” condition, where one arm or one leg was misplaced to an incorrect body location in each picture; “replaced by a chimpanzee part” condition, where one arm or one leg was replaced by a chimpanzee leg or arm, respectively, in its original place in each picture; and “replaced by a human part” condition, where one arm or one leg was replaced by a human arm or leg in each picture. Compared to the looking times toward the normal parts, chimpanzees had significantly longer looking times toward the human arms or legs. The looking times toward the misplaced parts were also longer than the normal parts, but the difference just failed to meet significance. These results indicate more interests toward strange body parts, compared to typical parts, suggesting that chimpanzees might have a body representation that is sufficiently sensitive to detect these aspects of strangeness. Supplementary Information The online version contains supplementary material available at 10.1007/s10071-021-01593-2.


Supplementary Material I Additional data analyses of fixation duration
In eight trials of the "arm" data, the time to first fixation was zero. This suggests that the participants were already looking at the AOIs at the onset of the stimuli, which might make the fixation duration in these trials longer than they should be. Therefore, we deleted the fixation duration data from these eight trials and did the analysis again (data from 318 trials).
The full model with the fixed effects being condition, body part and their interaction was significantly different from the null model, which had no fixed effects and only random effects (χ 2 (7, N=6) = 17.66, p = .014). For the full model, an analysis of variance based on mixed gamma regression indicated a significant effect of condition (χ 2 (3, N=6) = 9.41, p = .024), but the effects of body part (χ 2 (1, N=6) = 0.35, p = .56) or the interaction of condition and body part (χ 2 (3, N=6) = 1.81, p = .61) were not Chimpanzees detect strange body parts 2 significant. Simultaneous pairwise comparisons based on the effect "condition" using Tukey's HSD test indicated that the "normal" condition had significantly shorter fixation duration on AOIs than the "replaced by a human part" condition (Z = 3.58, p = .002).
Other pairs did not show significant difference (Table S1; Figure S1).
The comparison between the full model and the model with "participant ID" dropped from the full model showed a significant random effect of participants ID (χ 2 (1, N=6) = 38.09, p < .001; Figure S2). The comparison between the full model and the model with "picture ID" dropped from the full model showed a significant random effect of picture ID (χ 2 (1, N=6) = 13.81, p < .001; Figure S3).

Table S1
Results of post hoc pairwise comparison of fixation-duration data, based on condition .14 † The comparisons are shown in the format of "item A -item B". Each item shows the condition. "Replaced-C" is short for "replaced by a chimpanzee part", and "replaced-H" is short for "replaced by a human part".
Chimpanzees detect strange body parts Chimpanzees detect strange body parts 4

Fig. S2
Mean fixation duration on AOIs of each chimpanzee participant in all conditions. Error bar: SD.

Fig. S3
Mean fixation duration on AOIs of each picture in all conditions. Error bar: SD.
Regarding "picture ID", "A" represents arm manipulations, "L" represents leg Chimpanzees detect strange body parts 5 manipulations, and numbers refer to different stimulus series. For example, data at "1A" included data from the "1A" picture series: they all had the same chimpanzee individual in the pictures, but in the "normal" condition, the body was intact; in the "misplaced" condition, one arm of the body was misplaced; in the "replaced by a chimpanzee part" condition, the same arm of that body was replaced by a chimpanzee leg; and in the "replaced by a human part" condition, the same arm of that body was replaced by a human arm.