Horses in an Italian study showed an ability to identify life-sized head shots of their own species, and were later willing to pick out the images of a sheep instead of a horse to obtain the food reward on offer.
However, they were not able to identify the images of individual horses or sheep within each species.
The experiments conducted by Giulia Ragonese and her fellow researchers from several Italian universities are described in the open-access journal PLOS ONE.
The study team hypothesized, based on findings with other ungulates, that horses would be able to discriminate between the faces of horses and the faces of other domestic species.
The research involved 10 adult Franches-Montagnes horses, comprising six females and four males, who were stabled in Palermo.
A wooden testing frame was built comprising two by side-by-side trapdoors which could be pushed back by a horse’s nose, revealing a shelf on which a food reward could be retrieved.
Each of the two trapdoors had a transparent front cover in which life-sized pictures of the animals could be inserted.
The trapdoors, which were built at the height of the horse’s nose, had a wooden panel between them, which required the horse to choose between one of the two panels at a minimum distance of 40cm.
A total of 20 digitized life-sized color pictures were used in the experiments. Ten pictures featured frontal views of the faces of different horses, while the other 10 pictures were frontal views of faces of different domestic animals — two cows, three sheep, four donkeys and one pig.
All of the faces were unknown to the horses used in study.
The horses were trained in the use of the apparatus before the first experiment, in which they were shown the face of a horse on one side, and a different species on the other, with the side randomly assigned.
The food reward — 30 grams of oat flakes — was placed behind each trapdoor to eliminate any influence from smell, but only the trapdoor with the horse image was unlocked.
The experiment was carried out with different images 10 times to complete one session.
Eight out of the 10 horses were able to distinguish between the pictures of the horses and the other animals.
When the experiment was reversed, and the horses got the food reward for identifying the sheep instead of the horse, they showed a similar ability.
Each horse was deemed to have succeeded if it picked the correct image eight out of 10 times in two consecutive sessions (there were three sessions for each phase of the experiment).
The performance of the horses generally got better with each session.
Interestingly, the time required by the horses to make the choice in each trial increased as the sessions proceeded.
“This may indicate that the decision-making process requires some sort of ‘greater concentration’ to become more accurate, which results in a longer time needed to make the choice.
“However, there was no evidence of a speed-accuracy trade-off at the individual level, as already reported in horses performing a spatial task.”
The study team concluded that the horses were able to discriminate between two-dimensional images of other horses and those of other domestic animals, similar to previous findings in other ungulates.
“In addition, despite some difficulties … the horses tested also demonstrated their learning ability in the reversal task, in contrast to cattle.”
However, they did not appear to be able to categorize individual faces among the images.
“It is therefore probable that only the shape of the face drives their discrimination and reversal abilities.”
The authors said specific investigations are required to understand whether two-dimensional social stimuli are treated differently from real animals.
The study team comprised Ragonese, Adriana Ferlazzo, Esterina Fazio, and Cristina Cravana, all with the University of Messina; Paolo Baragli, Chiara Mariti, and Angelo Gazzano, all with the University of Pisa; and Antonio Lanatà, with the University of Florence.
Ragonese G, Baragli P, Mariti C, Gazzano A, Lanatà A, Ferlazzo A, et al. (2021) Interspecific two-dimensional visual discrimination of faces in horses (Equus caballus). PLoS ONE 16(2): e0247310. https://doi.org/10.1371/journal.pone.0247310