Supplemental materials
Supplemental experimental procedure
Apparatus
Bonobos were separated from the experimenter and eye-tracker by a transparent acrylic panel. To keep their heads relatively still, we implemented a nozzle and tube attached to the acrylic panels, which produced a dripping of juice, and let the apes suck the nozzle during the recordings (Movie 2). As chimpanzees were able to stay with an experimenter in the testing room, one of the experimenters stayed inside the room, sat beside the chimpanzees, and lightly held their heads during the recording (Movie 1). The other experimenter and the eye tracker stayed outside the room and recorded the participants’ eyes through the transparent acrylic panel.
Ethics statement
All participants were tested in the testing rooms prepared for each species, and their daily participation in this experiment was voluntary. They were given regular feedings, daily enrichment, ad lib water. Animal husbandry and research protocol complied with the local guidelines which are strictly adhered to the international standards (the Weatherall report “The use of non-human primates in research”) and the national laws of Japan [KS: Wildlife Research Center “Guide for the Animal Research Ethics” (No. WRC-2014KS001A)] [1].
Calibration
Two-point automated calibration was conducted for the apes by presenting a small object or movie clip on each reference point. A relatively small number of reference points was adopted for apes because they tended to view these reference points only briefly and no training procedure was adopted for them. However, we manually checked the accuracy after the initial calibration and repeated the calibration if necessary. As a result, our validation session confirmed the comparable accuracy between apes and humans (see [2]; the calibration errors were 0.5-1 degree in both apes and humans).
Species difference
Inclusion of Species factor (bonobo or chimpanzee) into the main ANOVAs did not change our main results [i.e. an interaction effect between AOI and Day; F(1, 7) = 11.77, p=0.011, partial ŋ2=0.67, in experiment 1; F(1, 7) = 6.42, p=0.039, partial ŋ2=0.47 in experiment 2].
Supplemental table
Table S 1. Participant information.
Subject / Species / Age class / Sex / Rearing history / Excluded* (exp.1) / Excluded* (exp.2)Ikela / Bonobo / Adult / Female / Nursery / No / No
Junior / Bonobo / Adult / Male / Mother / No / No
Lenor / Bonobo / Adult / Female / Nursery / Yes / No
Loise / Bonobo / Adult / Female / Nursery / No / No
Lolita / Bonobo / Adult / Female / Nursery / Yes / Yes
Vijay / Bonobo / Adult / Male / Nursery / No / No
Jamba / Chimp / Adult / Male / Mother / No / Yes
Hatsuka / Chimp / Juvenile / Female / Nursery / No / No
Iroha / Chimp / Juvenile / Female / Mother / No / No
Misaki / Chimp / Adult / Female / Mother / Yes / Yes
Mizuki / Chimp / Adult / Female / Nursery / No / No
Natsuki / Chimp / Adult / Female / Mother / No / No
*Excluded due to the lack of explicit looks to the target/distractor during the pre-event period.
Supplemental figures
Figure S 1. Number of participants viewing the target and distractor on each testing day, as a function of time, sampled for each eye-tracking sample (300 Hz).
Figure S 2. Viewing times (ms) to each scene element as a function of time relative to critical events (set to the zero points) on each testing day, averaged across participants, for each 3-sec time bin.
1. Morimura, N., Idani, G., and Matsuzawa, T. (2010). The first chimpanzee sanctuary in Japan: an attempt to care for the “surplus” of biomedical research. Am. J. Primatol. 73, 226-232.
2. Kano, F., Hirata, S., Call, J., and Tomonaga, M. (2011). The visual strategy specific to humans among hominids: A study using the gap-overlap paradigm. Vision Res. 51, 2348-2355.