Abstract
Faces are an essential source of information for social species. Sheep are one of the most studied farm species in terms of their ability to process information from faces, but little is known about their face-based emotion recognition abilities. We investigated whether sheep could use images of faces displaying different emotional states as cues in a simultaneous discrimination task. To that end, we took photos of faces of four sheep in three social situations: two with a negative valence (social isolation or aggressive interaction) and a neutral situation (ruminating in the home pen). In a two-armed maze, sheep (n=16) were then presented with pairs of images of the same familiar individual taken in the neutral situation and one of the negative situations. Sheep had to learn to associate one type of image from a pair with a food reward. Once they had reached the learning criterion, sheep then had to generalise the task to new pairs of images of different conspecifics displaying the same emotions as in the previous phase. For every run in the maze, the latency to choose an arm and the outcome of the choice (success or error) were recorded, as well as the total number of runs needed to learn the task (learning speed). Influence of the type of image rewarded and of the side of presentation of the rewarded image were analysed by linear mixed models and learning speed was analysed by Mood’s median test. All sheep learned the task with images of faces. Sheep that had to associate a negative image with a reward learned faster that sheep that had to learn the neutral-reward combination (medians: 40vs 75 runs, χ2=4.00, df=1, P=0.046). With the exception of sheep from the Aggression-rewarded group, sheep could generalise the discrimination task to images of new faces (F3,26.6=3.37,P=0.033). Sheep chose an arm correctly more often (F1,288.1=5.02, P=0.026) and more quickly(F1,289.3=23.92, P<0.001; right: 8.4±0.6 sec, left: 9.6±0.8 sec) when the rewarded image was displayed on the right side, suggesting the influence of a right hemisphere/left visual field bias in face-based perception of emotions. Our results strongly suggest that sheep can perceive the emotional valence displayed on faces and that this valence affects learning processes.