Table 1.
Description of the tested horses clustered in two groups according to their bonds (Pair 1: Gina/Calippo; Pair 2: Julia/Betsie).
In addition to the information regarding sex, age, breed, test timing, we provided the color of the fur and the relative color of the mark (yellow or blue) that we selected in order to be clearly visible by both experimenters and tested horses.
Fig 1.
Detailed description of the testing area.
A: the area of the L-shaped enclosure in which mirror was positioned; B: the other part of the L-shaped enclosure; f: the short fence delimiting the A and B areas; xf: the thick fence added to prevent the fellow horse looked at the mirror by leaning over the fence.
Fig 2.
Shape, color and position of the sham and colored marks during the Sham and Mark test.
Day 4 (SHAM control): both cheeks of the horse were painted with the sham mark (basal control for tactile cues) (A and C). Day 5 (MARK test 1): the left cheek was painted with colored mark (B and D, yellow for Julia and Calippo, blue for Betsie and Gina) while right cheek was painted with sham mark (C). Day 6 (MARK test 2): the position of sham and colored marks was inverted and, therefore, colored mark was positioned on the right side while sham mark was on the left side. The presence of the sham mark on the opposite cheek of the colored mark (during MARK test 1 and 2), was necessary to avoid the bias due to a tactile stimulation.
Table 2.
Description of the behaviors performed by the horses once entered in the “mirror area”.
Attention to and exploration of the mirror, looking behind the mirror and mouth movements were collected under Open (OM) and Covered Mirror (CM) conditions; Scraping events were collected under the four experimental conditions (OM, CM, SHAM and MARK). Videos are provided in Supporting Materials showing the horses performing such behaviors.
Table 3.
The table shows the amount of time (seconds) that each horse spent in front of and exploring the mirror in Open Mirror (OM) and Covered Mirror (CM) conditions.
In the table, the number of the times the horse looked behind the mirror and all the mouth patterns the animal performed in front of the mirror are also reported for both conditions, OM and CM. Under OM condition all the horses spent a higher amount of time in front of the mirror and three horses out of four engaged in a significantly longer time exploring it, compared to CM condition. All of the tested horses looked behind the mirror in OM condition and three out of four horses performed mouth movements when they were in front of the reflecting surface.
Fig 3.
Frequency of scraping events directed towards both cheeks.
Bar-graphs showing the frequency of the scraping events (SCRA) that each subject directed towards both sides of their face (see S1 Fig) across the four different conditions when the colored mark was positioned on the left cheek (a) (CM, OM, SHAM, MARK 1; df = 3; Julia: χ2 = 137.05, p < 0.0001; Calippo: χ2 = 5.27, p = 0.1531; Gina: χ2 = 58.61, p < 0.0001; Betsie: χ2 = 34.68, p < 0.0001) and when it was placed on the right cheek (b) (CM, OM, SHAM, MARK 2; df = 3; Julia: χ2 = 27, p < 0.0001; Calippo: χ2 = 10.62, p = 0.014; Gina: χ2 = 28.85, p < 0.0001; Betsie: χ2 = 42, p < 0.0001).
Table 4.
Comparison (Chi-square and Binomial tests) between the amount of scraping (SCRA) behaviors in MARK and SHAM conditions: The frequency of SCRA behavior towards both left and right cheeks is reported in the two conditions (SHAM and MARK 1, 2).
There was a significant increase of SCRA towards both sides of the face under MARK 1 condition compared to SHAM condition in three out of four horses. One out of four horses performed SCRA behavior at higher frequency in MARK 2 compared to SHAM condition. The table also shows the SCRA activity performed only towards the left cheek which corresponded to the mark side (MARK 1, left), with the same three horses showing a significant increase of SCRA in MARK 1 compared to the SHAM condition (left). Regarding the SCRA events towards the right cheek (MARK 2, right), there was no difference in SCRA directed to the right cheek between MARK 2 (right) and SHAM condition (right).
Fig 4.
Frequency of scraping events directed towards the left or the right cheek.
Bar-graphs showing the frequency of the scraping events (SCRA) that each subject directed specifically towards the left side of their face (see S1 Fig) across the four different conditions when the colored mark was placed on the left cheek (a) (CM, OM, SHAM, MARK 1; df = 3; Julia: χ2 = 86.02, p < 0.0001; Calippo: nd—expected values less than 5; Gina: χ2 = 32.52, p < 0.0001; Betsie: nd—expected values less than 5) and when it was placed on the right cheek (b) (CM, OM, SHAM, MARK 2; df = 3; Julia, Calippo, Gina, Betsie: nd—expected values less than 5).