(a) Looking duration analysis: target versus distractor
Separate three-way analysis of variances (ANOVAs; model (chimpanzee versus human) × condition (Look versus Reach) × object (target versus distractor)) with looking durations to objects were conducted for each species to examine the effects of social cues. The Neutral condition was not included in these analyses.
For chimpanzees (n = 8), we found a main effect of model (F1,7 = 5.648, p = 0.049) and an interaction among model, condition and object (F1,7 = 7.491, p = 0.029), but no other significant effects. Post hoc paired t-tests (two-tailed) with Bonferroni correction revealed that the chimpanzees looked longer at the target than the distractor in all conditions except Look condition-human model (Look condition-chimpanzee model t7 = 2.527, p = 0.039; Reach condition-chimpanzee model t7 = 2.690, p = 0.031; Reach condition-human model t7 = 2.384, p = 0.049; Look condition-human model t7 = −0.102, p = 0.922) (a). These results indicate that chimpanzees' scanning patterns were engaged to look at the target when they observed another chimpanzee looking at or reaching for it. However, such engagement occurred only when they observed the human model reaching for the target, although the significance of the main effect of model indicates that overall, they looked at objects with the human model for longer than those with the chimpanzee model.
Effect of social cues on looking duration in Look and Reach conditions. (a) Mean (+s.e.m.) looking duration of eight chimpanzees. (b) Mean (+s.e.m.) looking duration of eight humans. Post hoc t-test, *p < 0.05.
For humans (n = 8), only a main effect of the object was found (F1,7 = 12.907, p = 0.009), indicating that humans looked at the target longer than the distractor regardless of the species of the model (b). The absence of any other effects shows that humans did not look at objects differentially as a function of the species of the model. These results suggest that humans were equally sensitive to social cues from both chimpanzee and human models.
For the Neutral condition, two-tailed paired t-tests confirmed that neither species looked at the objects differentially when the stimuli contained no attention-directing actions (chimpanzees: t7 = 1.369, p = 0.213; humans: t7 = −0.156, p = 0.88).
(b) Looking duration analysis: other AOIs (face and hands)
We further analysed whether the duration of looking at other features (i.e. face and hands) changed when participants viewed either the chimpanzee or human models. Two-way ANOVAs were conducted for each species with models (chimpanzee and human) and conditions (Neutral, Look and Reach) as within-subject factors.
Concerning looking time at the face, we found no significant difference for chimpanzee participants (n = 8), whereas in humans (n = 8) the main effect of condition was significant (F2,14 = 7.452, p = 0.006), but not the effect of model and interaction. Post hoc paired t-tests indicate that humans looked at the face significantly longer in the Neutral condition than in the other two conditions (Neutral–Look: t7 = 3.467, p = 0.0038; Neutral–Reach: t7 = 3.203, p = 0.0064). This increased duration of looking at the face in the Neutral condition indicates that directed gaze attracts attention in humans, but not in chimpanzees.
For looking time to hands, there were no significant differences in either species.
(c) First pass gaze duration analysis
Two-way ANOVAs with model (chimpanzees versus model) and condition (Look versus Reach) run for each species separately revealed that chimpanzees (n = 8) looked at the face of the chimpanzee model significantly longer than that of the human model (main effect of model F1,7 = 6.404, p = 0.0392). Additionally, they also looked at the face significantly longer in the Reach condition than in the Look condition (F1,7 = 9.360, p = 0.0183). We found no interaction. In contrast, there were no significant effects for humans. We also analysed the data from the neutral condition with paired t-tests to assess any effect of model, but found no significance in either species. Overall, these results suggest that for chimpanzees, at least when viewing others attending to something, a conspecific face conveys more information.