Facial and vocal discrimination in sheep

Abstract

The ability of sheep, Ovis aries, to discriminate between sheep, humans and other animals on the basis of facial and vocal cues was tested in an enclosed Y-maze. Pairs of faces or voices were presented which had a clear differential significance for the sheep (i.e. sheep versus human, dog or an unfamiliar breed or species). Some Clun Forest and Dalesbred sheep could actively distinguish (>75% choice) between different faces, with the best performances being seen for sheep versus human faces (the sheep face being preferred to that of the human). Other animals showed pronounced position preferences in the maze whatever faces were shown although they approached sheep faster than other faces, suggesting that they could discriminate between them. Dalesbred, but not Clun Forest, sheep could also discriminate between sheep and human vocalizations alone in the maze. For both breeds, combining appropriate sight and sound stimuli did not significantly enhance performance although mismatching them reduced it. In a further experiment on animals that performed at more than 75% choice criterion in the maze, inverting the faces, turning them to profile or masking the eyes all significantly reduced performance in discriminating between sheep and human faces. In another experiment, Clun Forest ewes could also distinguish between the faces of male and female breed members. During anoestrus they preferred the faces of females and when they were in oestrus they preferred those of rams. Overall, these results suggest that sheep can use facial cues to discriminate between different species, breeds and male and female members of the same breed. They also show that discriminatory performance is influenced by orientation, the presence of eyes and, to some extent, vocal cues.