S2 ) than angry as well as happy or neutral faces, and showed a trend to fixate angry faces more than happy faces (rate: fear vs. anger: χ 2 = 13.69, df = 1, p < 0.001; fear vs. happy χ 2 = 15.92, df = 1, p < 0.001; fear vs. neutral: χ 2 = 20.01, df = 1, p < 0.001, anger vs. happy χ 2 = 3.47, df = 1, p = 0.063; duration: fear vs. anger: χ 2 = 12.09, df = 1, p = 0.001; fear vs. happy χ 2 = 14.46, df = 1, p < 0.001; fear vs. neutral χ 2 = 16.78, df = 1, p < 0.001).
S3 ) than the corresponding distractor emotions, although this comparison was barely or marginally significant in orangutans (rate: humans χ 2 = 21.25, df = 1, p < 0.001, orangutans χ 2 = 3.91, df = 1, p = 0.048; duration: humans χ 2 = 27.85, df = 1, p < 0.001, orangutans χ 2 = 3.37, df = 1, p = 0.066).
Unlike orangutans, humans also fixated emotional faces more often and longer compared to neutral faces, with some of these findings being only marginally insignificant (rate: anger vs. neutral χ 2 = 3.73, df = 1, p = 0.053; happy vs. neutral χ 2 = 7.15, df = 1, p = 0.007; duration: anger vs. neutral χ 2 = 3.60, df = 1, p = 0.058; happy vs. neutral χ 2 = 3.36, df = 1, p = 0.067) (see Tables S3 and S4 ).