Are faces special for sheep? Evidence from facial and object discrimination learning tests showing effects of inversion and social familiarity.

We have previously shown that sheep, like monkeys, have neural circuits within the temporal lobe that respond preferentially to faces. They can also discriminate between sheep, humans and other animals on the basis of facial cues using an enclosed Y-maze. In the present study we investigated the speed with which Clun Forest sheep learn to discriminate between familiar and unfamiliar faces, as opposed to symbols, in order to gain a food reward using the same Y-maze apparatus. Animals (n = 10) received 1 day of training where projected images of the pairs of faces or symbols were paired for 20 trials with a picture of either an empty or full bucket of food (which indicated which choice of face or symbol would result in the animal receiving a food reward) and on the next 4 days they were given a further 20 trials a day with the faces or symbols alone. Results showed that sheep learned significantly faster (by day 1 or 2 post training) to recognise sheep faces of a familiar breed compared to geometrical symbols (3-4 days post training). Learning using faces of animals of another unfamiliar breed was also significantly better than for symbols but was significantly worse than that seen using faces of a familiar breed. Inverting the faces significantly reduced learning speed for faces of a familiar breed but not for that of an unfamiliar one. Inverting familiar objects, food buckets, also did not impair discriminatory performance. In a further set of trials where discrimination learning was made more difficult by excluding cued trials and reducing the number of daily trials to eight, social familiarity was found to further improve the animal's ability to learn to discriminate between the faces of a familiar breed. Finally, while discriminatory performance for adult sheep faces was very good, that for young lamb faces was poor, with only one animal learning to choose the face associated with food. It was confirmed in maternal ewes that they were also slow to learn to recognise the faces of their lambs (2-3 weeks). Overall these results show that sheep can learn to distinguish between individual adult sheep faces but that breed and social familiarity influence the level of performance. Further, discrimination learning of familiar and unfamiliar facial stimuli is better than between simple geometrical symbols, indicating that faces may be preferentially processed by the brain compared to other objects suggesting that faces are indeed special in this species as has been claimed for human and non-human primates.