Domestic horses use a sophisticated cognitive system to identify individuals in species other than their own, British research reveals.
Researchers at the University of Sussex, in a previous prize-winning study, showed that horses had the ability to combine auditory and visual information to recognise each other.
Now, in their paper published by the Proceedings of the Royal Society B, Dr Leanne Proops and Dr Karen McComb have been able to show that horses also use this system for distinguishing between different people they know.
They also found females were better at it than males.
Lead researcher Proops, from the university’s Mammal Vocal Communication and Cognition Research Group in the School of Psychology, said: “The domestic horse is an ideal animal model for this research because it has a complex social organisation and close relationship to humans, making individual recognition of humans a highly functional ability.”
McComb explained: “When we hear a familiar voice we form a mental picture of who spoke. We match visual and auditory cues to recognise specific individuals.
“Previously, we showed that horses also identify other horses cross-modally.
“We now demonstrate how flexible this ability is by showing that horses can also recognise humans in this way, despite people looking and sounding very different to themselves.”
The researchers carried out their study on domestic horses accustomed to several different handlers.
They first tested where the horse would look when two voices – one familiar, one unfamiliar – were played from a hidden loudspeaker, either side of which stood the familiar and unfamiliar person.
They then tested how the horses would perform in the more complex task of having to distinguish between two familiar voices played to them that were from different handlers that they knew.
They found that the horses were faster to respond and looked for longer and more often at the familiar human compared with the stranger when played their voice, and were better at making this match when the familiar person was on the right of their visual field, indicating that the left hemisphere of the brain is involved in this processing.
In the second experiment, the horses proved able to match a specific familiar voice to its human handler.
This indicated that the sight of the handler activated a multimodal memory of that specific individual, allowing each horse to match the sight of a particular person with the sound of their voice.
The results suggest the horses use this recognition strategy naturally to identify numerous individual people in their day-to-day lives.
The earlier research by Proops, McComb and Dr David Reby, showed it was not just humans who used complex memory processes to identify each other.
In that research, they studied the reaction of horses to the sight of one member of the same herd while they heard the call of either the same horse, or a different herd member.
They found that the horses showed a stronger reaction to the calls that did not match the herd member they had just seen compared with the congruent calls.
The results, published in Proceedings of the National Academy of Sciences of the United States of America on December 15, 2008, suggested that horses, like humans, used a “cross modal” system for recognising each other – one that involves a combination of sensory cues such as auditory and visual/olfactory information.
Proops said at the time: “Individual recognition within species is a complex process and is very poorly understood. We know that in humans it is cross-modal. For example, we recognise someone by how they look and how they sound. It now appears that horses, and perhaps other animals, also possess a cross-modal representation of known individuals.”
The 2008 study focused on 24 horses at Woodingdean livery yard in Sussex and at the Sussex Horse Rescue Trust.
Subjects watched a herd member being led past them before the individual went out of view and a call from that or a different associate was played from a loudspeaker positioned close to the point of disappearance.
The researchers measured how long the test horse looked in the direction of the loudspeaker. They found that horses responded more quickly and looked for longer in the direction of the incongruent calls (which didn’t match the horse they had seen) compared to the congruent calls, indicating that the mismatched combination violated their expectations.
Proops said: “Given that the stimulus horse was out of sight when the vocal cue was heard, it is likely that the test horse was accessing or activating some form of multimodal memory of that individual’s characteristics.”
The latest study, “Cross-modal individual recognition in domestic horses (Equus caballus) extends to familiar humans” was published on Wednesday in Proceedings of the Royal Society B.