Wether ewe know me or not: the discrimination of individual humans by sheep.

A growing literature suggests that animals of various species can discriminate between individual humans. In the present study, 15 experimentally naive sheep were rewarded for making a nosepress response in the presence of one handler (S+) and non-reinforced for this behavior in the presence of a second person (S-). All animals responded significantly more to the S+ handler (P<0.001) during non-reinforced test trials. Furthermore, sheep engaged in clearly different behavior during a 10-s pre-trial period, depending on which handler was present. Control conditions preclude discrimination based on order effects, temporal cues, or a win-stay/lose-shift learning set. The ability to differentiate between individual humans, regardless of its sensory basis, suggests that animals can use humans to predict the delivery of hedonic events that routinely occur in research settings. The resultant human-based operant and Pavlovian conditioning has implications for the design of research protocols and the interpretation of results.