Phase 1: Face-on vs. side-on views
The overall mean frequency of looking at the two screens was similar: 51.0 and 56.7 times per session for face-on and side-on screens, respectively. Concerning total duration of looking, the main effect of days was significant, F9, 36 = 15.19, P < 0.001, as was the screens × days interaction, F9, 36 = 2.76, P = 0.015 (see ). After looking for a long time at both screens on Day 1, the monkeys habituated to the side-on view rapidly, whereas habituation to the face-on view was more erratic.
Mean total duration of looking at real-time face-on and side-on screens across days.
Regarding the mean duration of individual looks, the longest occurred on Day 1 (mean: 38.1 frames), the shortest on Day 7 (14.8), with a slight recovery on Day 10 (23.1), F9, 36 = 7.88, P < 0.001. The screens × days interaction was significant, F9, 36 = 3.89, p = 0.002. Both screens received their mean longest looks on Day 1 (face-on: 39.8 frames, side-on: 36.3). Looks towards both screens then became shorter, with face-on looks lasting slightly longer than side-on looks until Day 8, when the trend was reversed. The dominant adult male, Heiji, looked more frequently at the face-on than the side-on image (means: 33.7 and 20.9 respectively), t = 2.91, df = 9, P = 0.017, d = 10.8. Heiji also looked longer at the face-on screen (means: 926.2 vs. 571.6 frames, respectively), t = 2.21, df = 9, P = 0.054, d = 0.59. For the younger adult male, Pigmon, individual looks toward the face-on screen were longer (means: 21.7 vs. 15.1 frames), t = 2.52, df = 9, P = 0.033, d = 0.70. The adult female Kiki looked longer overall at the side-on screen than the face-on screen (means: 973.6 and 691 respectively), t = −2.91, df = 9, P = 0.018, d = 0.64; the corresponding difference for frequency of looking (means: 39.9 vs. 30.2) was not significant, t = −2.06, df = 9, P = 0.069, d = 0.73. Zilla looked at the side-on screen on average 38.7 times per session and at the face-on screen 29.7 times, but this difference failed to reach significance, t = −2.17, df = 9, P = 0.058, d = 0.61.
Only the males displayed facial expressions, directed either preferentially or exclusively towards the face-on image (). Four monkeys showed lateral head movements/head-cocking, and with one exception all of these occurred while looking at the side-on image. Two other behaviours are noteworthy: On 21 occasions Heiji emitted “whistle series” vocalizations while looking at his face-on image, compared to only 4 times while looking at his side-on image. The adult female Theta turned to look back at the face-on image (occasionally under her arm, or through her legs) 38 times, whereas she did this only 4 times with her side-on image.
Individual total frequencies of facial expressions and lateral movements/head-cocking towards the two screens in Phases 1 and 2.
Phase 2: Real-time vs. delayed images
In Phase 2 the group as a whole looked at the real-time and delayed images on average 35.7 and 29.3 times per session, respectively, a non-significant difference. Mean total duration of looking was longer for the real-time image (915.3 vs. 603.9 frames), F1, 36 = 13.84, P = 0.02, d = 1.16. The main effect of days on mean bout length was significant, F9, 36 = 2.15, P = 0.051: on Day 1 the mean bout was 24.0 frames; this measure peaked at 28.2 on Day 3 and declined gradually thereafter. There were no significant interactions.
All adult females looked preferentially at the real-time screen; frequencies are shown in along with those of the males (Zilla: t = 3.82; Kiki: t = 7.05; Theta: t = 3.56, all df = 9, P = 0.004, d = 1.46, P < 0.001, d = 2.40 and P = 0.0065 d = 1.44, respectively). Furthermore, individual looks at the real-time image were usually longer than towards the delayed image (Zilla: 25.4 frames vs. 15.4, t = 2.31, df = 9, P = 0.046, d = 1.04; Kiki: 24.0 vs. 18.2, t = 5.56, df = 9, P = 0.0004, d = 0.93; Theta: 22.9 vs. 17.4, t = 2.86, df = 9, P = 0.019, d = 0.89), and total duration of looking was longer (Zilla: 425.6 vs. 126.6, t = 5.31, df = 9, P = 0.0005, d = 1.94; Kiki: 794.4 vs. 286.7, t = 7.58, df = 9, P < 0.0001, d = 2.24; Theta: 1136.3 vs. 645.8, t = 5.81, df = 9, P = 0.0003, d = 1.48). The male Heiji showed no clear bias for either image; he looked at the real-time image for 440.1 frames vs. 267.9 frames for the delayed image, but this was not a significant difference, t = 1.92, df = 9, P = 0.088, d = 0.93. Like the adult females, the juvenile male Zinnia looked more frequently at the real-time image, t = 3.00, df = 9, P = 0.015, d = 0.80 (see ), although duration measures were not significantly different between screens. Finally, Pigmon looked at the delayed image more frequently, t = −3.5, df = 9, P = 0.008, d = 1.15, and for longer overall (698.6 vs. 1538.7), t = −3.98, df = 9, P = 0.041, d = 1.45.
Individual monkeys’ mean frequencies of looking at real-time and 1-sec-delayed images of self.
The two screens did not differentially elicit facial expressions, lateral movements/head-cocking (), vocalizing, or scratching, but there were some notable individual responses. In this phase all 3 females showed some facial expressions. The juvenile Zinnia was again highly responsive, especially to the real-time image. Heiji and Kiki performed 5 and 6 lateral movements/head-cocking responses while looking at the real-time screen, respectively, and none while looking at the delayed-image screen. Heiji produced three “whistle series” vocalizations while looking at the real-time image, and 11 while looking at his delayed image. Finally, Pigmon scratched himself twice while looking at his real-time image, and 7 times while looking at his delayed image.
Phase 3: Previous self vs. other
There were no significant group-level or individual-level differences in measures of looking at the two screens in Phase 3. Only one facial expression occurred, shown by Zinnia toward his previously recorded image. The same individual was the only one to show lateral movements/head-cocking, which he did three times when looking at the image of the other monkey.
Although Phase 3 consisted of only 4 sessions, frequency and total duration measures of looking exceeded those in Phase 2 (see ), reversing a clear reduction in visual interest shown towards the video images between Phases 1 and 2.
Group means for measures of looking at the two screens in the three phases.