There’s the classic two liner: A horse walks into a bar. The landlord asks ‘why the long face?’
This one joke perfectly sums up the difficulties of interpreting facial expressions across species. Human beings can learn to read social cues in animals (I’m reasonably sure I can tell when my cat is angry with me), but are animals any good at reading human behaviour?
Amy Smith and her colleagues at the University of Sussex wanted to find out if horses can discriminate between human emotional states. Their findings have huge implications for our understanding of how animal domestication affects evolution (and vice versa), and how the brain decodes human facial expressions. Horses are a great species in which to study this question, as they have a 5000 year history of domestication and co-evolution with human beings.
Smith set out to test the ability of horses to discriminate between a happy and an angry human face. They recruited a group of horses, with each horse shown two photographs of a man expressing an emotion. In one photograph the man was smiling, and in the other he was aggressively snarling. During the encounters, the researchers recorded the horses’ behavioural responses and monitored their heart rate.
The horses showed that they were able to distinguish between the smiley and aggressive pictures. When looking at the snarling man, the horses’ heart rate jumped up quicker than when they viewed the smiling man; this might be a signal of a heightened fight or flight response. But the most intriguing finding was to do with their behaviour – in particular, which eye the horses used to glance at the photograph. Unlike people, horses have eyes which are found on either side of the skull. Many prey animals have this in common, as this helps them to spot predators from different directions. In order to focus on a particular object, horses have to turn their head to get a good look.
The horses showed a tendency to look at the snarling picture with one particular eye – the left one. The choice of which side to use may seem trivial at first glance, but there is more to it. Because optic nerves cross over to the other side of the brain, the image from the left eye is processed by the right side of the brain. Similar studies in dogs and in humans have found that the right side of the brain has a role in interpreting emotionally negative images. With this in mind, Amy Smith’s findings hint that horses are conscious of the emotional state of the humans around them.
The existence of emotional perception across a species barrier opens up all sorts of avenues of thought. Is this ability pre-programmed or learned? Is this a result of domestication, or the reason for it? Perhaps the horse’s reply to the barman will be: ‘I’m fine, but you don’t look so good’.
Smith AV, Proops L, Grounds K, Wathan J, McComb K. 2016 Functionally relevant responses to human facial expressions of emotion in the domestic horse (Equus caballus). Lett. 12: 20150907. http://dx.doi.org/10.1098/rsbl.2015.0907