Evidence of heterospecific referential communication from domestic horses (Equus caballus) to humans
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Referential communication occurs when a sender elaborates its gestures to direct the attention of a recipient to its role in pursuit of the desired goal, e.g. by pointing or showing an object, thereby informing the recipient what it wants. If the gesture is successful, the sender and the recipient focus their attention simultaneously on a third entity, the target. Here we investigated the ability of domestic horses (Equus caballus) to communicate referentially with a human observer about the location of a desired target, a bucket of food out of reach. In order to test six operational criteria of referential communication, we manipulated the recipient’s (experimenter) attentional state in four experimental conditions: frontally oriented, backward oriented, walking away from the arena and frontally oriented with other helpers present in the arena. The rate of gaze alternation was higher in the frontally oriented condition than in all the others. The horses appeared to use both indicative (pointing) and non-indicative (nods and shakes) head gestures in the relevant test conditions. Horses also elaborated their communication by switching from a visual to a tactile signal and demonstrated perseverance in their communication. The results of the tests revealed that horses used referential gestures to manipulate the attention of a human recipient so to obtain an unreachable resource. These are the first such findings in an ungulate species.
KeywordsDomestic horse Referential communication Human–animal communication Intentional communication Referential gesture
We wish to thank EquiLuna A.S.D. for granting us the permission to use their facilities and involve in this research the horses they host. In particular, we thank Laura Ascione, Claudio Saba, Andrea Montagnani and Lesley Moore for the help provided, and all those volunteered in this research. Our warmest thanks go to Christian Postiglione, our camera operator and technical assistant. We are grateful to Christian Postiglione, Elisabetta Visalberghi, Ian Couzin and Corsin Müller for helpful discussions and statistical consultation, to Debbie Kelly and Amelia Wein for improving the English and to Alan McElligott for his invaluable help at the very beginning of this project. This research was supported exclusively by private funding.
Authors declare not to have any source of funding.
RM conceived of the study, designed the study, collected field data, participated in data analyses and in statistical analyses and contributed to draft the manuscript; LH participated in data analyses and in statistical analyses and contributed to draft the manuscript. Both authors gave final approval for publication.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest. All applicable international, national and/or institutional guidelines for the care and use of animals were followed.
Online Resource 2. The video shows samples of the experimental conditions and coded behaviours. The target (bucket of food) is on the other side of the visible fence. The first two samples show gaze alternation between the horse and the experimenter during the condition ‘Forward’ (the experimenter was frontally oriented towards the center of the arena). In both samples, the experimenter stayed about in the direction of the video camera. The third sample shows a horse pointing to the bucket while at the same time performing a head gesture (very quick movement of the head along the sagittal plane). The fourth sample shows a horse elaborating its communication from visual to tactile during the condition ‘Forward’: while close to the target, she first pointed at it and used some head gestures, then walked back to the experimenter, touched her and again to the target. When she stopped close to it, she alternated her gaze between the walking experimenter and the target. The fifth sample shows another case of elaboration of communication, this time during the condition ‘Backward’ (the experimenter faced away from the gates, i.e. her back was oriented to the center of the arena). The horse walked back to the experimenter and touched her. (MP4 70,313 kb)
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