For humans, facial expressions are important social signals, and how we perceive specific individuals may be influenced by subtle emotional cues that they have given us in past encounters. A wide range of animal species are also capable of discriminating the emotions of others through facial expressions [], and it is clear that remembering emotional experiences with specific individuals could have clear benefits for social bonding and aggression avoidance when these individuals are encountered again. Although there is evidence that non-human animals are capable of remembering the identity of individuals who have directly harmed them [], it is not known whether animals can form lasting memories of specific individuals simply by observing subtle emotional expressions that they exhibit on their faces. Here we conducted controlled experiments in which domestic horses were presented with a photograph of an angry or happy human face and several hours later saw the person who had given the expression in a neutral state. Short-term exposure to the facial expression was enough to generate clear differences in subsequent responses to that individual (but not to a different mismatched person), consistent with the past angry expression having been perceived negatively and the happy expression positively. Both humans were blind to the photograph that the horses had seen. Our results provide clear evidence that some non-human animals can effectively eavesdrop on the emotional state cues that humans reveal on a moment-to-moment basis, using their memory of these to guide future interactions with particular individuals.

Results and Discussion

8 Proops L.

McComb K. Cross-modal individual recognition in domestic horses (Equus caballus) extends to familiar humans. 9 Proops L.

McComb K.

Reby D. Cross-modal individual recognition in domestic horses (Equus caballus). 2 Smith A.V.

Proops L.

Grounds K.

Wathan J.

McComb K. Functionally relevant responses to human facial expressions of emotion in the domestic horse (Equus caballus). 10 Wathan J.

Proops L.

Grounds K.

McComb K. Horses discriminate between facial expressions of conspecifics. 11 Stone S.M. Human facial discrimination in horses: can they tell us apart?. Figure 1 Diagram of the Experimental Design and Setup Show full caption (A) Images of the experimental setup in exposure phase (left) and test phase (right). (B) Photographic stimuli presented in the exposure phase in relation to the permutations of the test phase. In the exposure phase, each horse was presented with a photograph of model A or model B either happy or angry. In the test phase, subjects in the experimental group were presented with the live model previously depicted in the photograph, but this time adopting a neutral expression. Subjects in the mismatch control group viewed the previously unseen model adopting a neutral expression. The solid border around the images of the test phase denotes the experimental group, and the dashed border denotes the mismatch control group. Both models were blind to the original condition (happy or angry). The presentation duration, stimulus movement, and post-test period were the same in both presentation and exposure phases. See also Table S1 for details of exposure phase. As well as recognizing individual conspecifics and humans [] and discriminating between different facial expressions in both species [], horses can also learn to differentiate unknown people based on facial features alone and transfer this discrimination from photographs to live models []. In the current study, we first presented horses with a photograph of a happy or angry face belonging to one of two human models for 2 min. Half of the subjects saw the happy or angry face of model A, and the other half the happy or angry face of model B ( Figure 1 ; see Table S1 for details of the response in this exposure phase). Several hours later, the horses in the experimental group were presented with the live model previously depicted in the photograph, but this time adopting a neutral expression. Critically, the live models were blind to the emotional valance of the photograph that the horses had previously seen. In order to determine whether any memory of the past emotional encounter was specific only to the individual seen adopting this expression, we presented a control group with a different, mismatched, live person (the other model in the experimental group) who was also adopting a neutral expression. Thus, in our design, not only was each individual model presented both as a happy and angry variant in the photographs, but the models also acted as live mismatch controls for each other—matching the identity of the person in the photographs in the experimental trials and contrasting with it in the control trials. If horses remember a single brief exposure to an emotional facial expression of a particular individual, then we would expect the experimental group to react either positively or negatively to the neutral model based on which facial expression they had previously seen, but the mismatch control group not to differ in response.

12 MacNeilage P.F.

Rogers L.J.

Vallortigara G. Origins of the left & right brain. 12 MacNeilage P.F.

Rogers L.J.

Vallortigara G. Origins of the left & right brain. 13 Rogers L.J. Relevance of brain and behavioural lateralization to animal welfare. 14 Leliveld L.M.C.

Langbein J.

Puppe B. The emergence of emotional lateralization: evidence in non-human vertebrates and implications for farm animals. 2 Smith A.V.

Proops L.

Grounds K.

Wathan J.

McComb K. Functionally relevant responses to human facial expressions of emotion in the domestic horse (Equus caballus). 15 Racca A.

Guo K.

Meints K.

Mills D.S. Reading faces: differential lateral gaze bias in processing canine and human facial expressions in dogs and 4-year-old children. 1,22 = 2.67, p = 0.02, r = 0.49; see 1,22 = 2.33, p = 0.04, r = 0.44; see 1,22 = 4.06, p = 0.001, r = 0.65; see 11 = 4.12, p = 0.002, r = 0.78) and no significant bias after the presentation of the happy photograph (t 11 = −1.16, p = 0.27). In contrast to these laterality biases in the experimental group, there were no significant differences in lateralized looking behaviors in the mismatch control group as a function of whether subjects had previously seen an angry or happy photograph (see 1,22 = 0.52, p = 0.61) or the mismatch control groups (see Figure 2 Responses of the Experimental and Control Groups to the Neutral Person after Viewing the Happy versus Angry Photographs Show full caption (A) Time spent viewing the stimuli with a left gaze bias. (B) Time spent viewing the stimuli with a right gaze bias. (C) Laterality indices LI = (L − R) / (L + M + R), where L, M, and R represent time spent looking left, right, and in the middle. Positive scores indicate a left-gaze bias, and negative scores indicate a right-gaze bias. (D) Time spent engaging in displacement behaviors. ∗p < 0.05, ∗∗p < 0.001. See also Mean ± SEM are shown. n = 48.p < 0.05,p < 0.001. See also Tables S2 and S3 Looking behaviors (lateralized and binocular looking), displacement and stress behaviors, approach, avoidance, and heart rate measures were recorded to evaluate responses to the neutral person and gain information on the subjects’ emotional state. Lateralized responses provide a useful window into what animals are experiencing []. Across a wide range of species including horses, negative and potentially threatening stimuli tend to be preferentially processed in the right hemisphere, indicated by a left gaze bias, and more pro-social stimuli in the left hemisphere, indicated by a right gaze bias [], and lateralized responses to human facial expressions have been observed in dogs and horses []. In our study, there was a significant difference in the first gaze biases of horses between the positive and negative groups (n = 21, p = 0.008), with the subjects that had previously seen the negative photograph showing an initial left gaze bias and those that had seen the positive photograph no gaze bias (negative: n = 11, K = 10, p = 0.01; positive: n = 10, K = 3, p = 0.34). In addition, subjects originally shown the negative photo spent significantly more time overall looking at the live model with their left eye (mean ± SE = 15.2 ± 4.3 s) than did subjects shown the positive photo (mean ± SE = 3.6 ± 0.9 s, t= 2.67, p = 0.02, r = 0.49; see Figure 2 A). The opposite is true when we look at right gaze bias time—subjects previously shown the happy photo spent more time viewing the model with their right eye than did subjects shown the angry photo (mean ± SE = 7.5 ± 2.9 s versus 0.7 ± 0.3 s, t= 2.33, p = 0.04, r = 0.44; see Figure 2 B). These results are also reflected in significant differences between standard laterality indices after the angry versus happy presentations (mean ± SE = 0.36 ± 0.07 versus −0.07 ± 0.06, t= 4.06, p = 0.001, r = 0.65; see Figure 2 C). The response to the neutral person showed a significant right hemisphere bias after the presentation of the angry photograph (t= 4.12, p = 0.002, r = 0.78) and no significant bias after the presentation of the happy photograph (t= −1.16, p = 0.27). In contrast to these laterality biases in the experimental group, there were no significant differences in lateralized looking behaviors in the mismatch control group as a function of whether subjects had previously seen an angry or happy photograph (see Table S2 ). The time that horses spent looking directly at the models previously portrayed as happy versus angry was not significantly different in either the experimental (mean ± SE = 30.5 ± 5.7 s versus 26.2 ± 5.9 s, t= 0.52, p = 0.61) or the mismatch control groups (see Table S2 ).

16 Maestripieri D.

Schino G.

Aureli F.

Troisi A. A modest proposal: displacement activities as an indicator of emotions in primates. 1,15.5 = 2.30, p = 0.036, r = 0.42; see Displacement behaviors are actions that appear unrelated to the current situation, such as scratching, and are thought to be a coping mechanism in stressful situations. These behaviors may also provide observable insights into how animals are perceiving emotional expressions []. Horses in the experimental group that had been shown the angry photo spent more time engaging in displacement behaviors (scratching, floor sniffing, and performing a species-specific behavior termed “lick and chew”) when viewing the live neutral person than did those that had been shown the happy photo (mean ± SE = 12.5 ± 3.5 s versus 3.6 ± 1.6 s, t= 2.30, p = 0.036, r = 0.42; see Figure 2 D). Looking at additional stress-related behaviors, only one horse showed avoidance behaviors and two showed nostril dilation during the test (all in the angry condition); thus, no statistical analyses were performed on these measures. There were also no differences in the number of horses that approached the live model (Fisher’s exact test [FET] n = 23, p = 0.68). In the mismatch control group, there were no significant differences in any of the above behavioral variables as a function of whether subjects had previously seen an angry or happy person in the photograph (see Table S2 ). Furthermore, no differences in heart rate measures were found between the two conditions in the experimental group, although all values were in the predicted direction of higher heart rate and lower heart rate variability for the angry condition (see Table S3 for details).

Using the facial expressions of others to gage the correct response to those individuals in the future requires a combination of cognitive abilities including sensitivity to emotional facial expression, identification of the individual, and memory for specific emotional events. Our results demonstrate that some animals are capable of taking into account a single encounter with an individual displaying an emotional facial expression when subsequently interacting with that same individual in a neutral context 3–6 hr later. This result is particularly striking because the horses did not have a strongly positive or negative direct experience with the person—they merely viewed a photograph of them with either a happy or angry facial expression. This short-term exposure to a facial expression was enough to generate clear differences in subsequent lateralized looking and levels of displacement behavior that were consistent with the angry expression being perceived negatively and the happy expression positively.

17 Tate A.J.

Fischer H.

Leigh A.E.

Kendrick K.M. Behavioural and neurophysiological evidence for face identity and face emotion processing in animals. 18 Barber A.L.A.

Randi D.

Müller C.A.

Huber L. The processing of human emotional faces by pet and lab dogs: evidence for lateralization and experience effects. It is notable that the mismatch control groups—where the subjects saw a different live person to the one seen in the photographs—showed an overall left gaze (right hemisphere) bias. This bias may be driven by a number of factors, including the activation of right hemisphere face-processing centers or the subjects perceiving the experimental setup negatively []. Our difference between the two test groups (angry versus happy) could therefore be driven by the response to the happy one generating a reduction in left gaze, as well as by the angry one producing an increase in left gaze and/or a reduction in right gaze. The extent to which differences in response are driven primarily by a positive response to a happy expression or a negative response to an angry expression (or a combination of the two) would be an interesting area for future research. In addition, there are a number of possible (non-mutually exclusive) factors that may give rise to horses’ abilities to remember transient human facial expressions and use these expressions to guide subsequent interactions. Horses may have an innate ability to remember the facial expressions of conspecifics, and this ability automatically extends to humans. Alternatively, the ability could have specifically evolved during the process of domestication or may be learnt during a lifetime of experience with people. Further work could usefully assess the evolutionary and ontogenetic mechanisms involved by comparing the responses of domestic and wild species, as well as individuals with varying degrees of human exposure.