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De Boyer Des Roches, A., Richard-Yris, M. - A., Henry, S., Ezzaouia, M., & Hausberger, M. (2008). Laterality and emotions: visual laterality in the domestic horse (Equus caballus) differs with objects' emotional value. Physiol. Behav., 94(3), 487–490.
Abstract: Lateralization of emotions has received great attention in the last decades, both in humans and animals, but little interest has been given to side bias in perceptual processing. Here, we investigated the influence of the emotional valence of stimuli on visual and olfactory explorations by horses, a large mammalian species with two large monocular visual fields and almost complete decussation of optic fibres. We confronted 38 Arab mares to three objects with either a positive, negative or neutral emotional valence (novel object). The results revealed a gradient of exploration of the 3 objects according to their emotional value and a clear asymmetry in visual exploration. When exploring the novel object, mares used preferentially their right eyes, while they showed a slight tendency to use their left eyes for the negative object. No asymmetry was evidenced for the object with the positive valence. A trend for an asymmetry in olfactory investigation was also observed. Our data confirm the role of the left hemisphere in assessing novelty in horses like in many vertebrate species and the possible role of the right hemisphere in processing negative emotional responses. Our findings also suggest the importance of both hemispheres in the processing positive emotions. This study is, to our knowledge, the first to demonstrate clearly that the emotional valence of a stimulus induces a specific visual lateralization pattern.
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Sankey, C., Henry, S., Clouard, C., Richard-Yris, M. - A., & Hausberger, M. (2011). Asymmetry of behavioral responses to a human approach in young naive vs. trained horses. Physiology & Behavior, 104(3), 464–468.
Abstract: The aim of this study was to investigate the impact of training experience on young horses (Equus caballus)’ lateralized responses to an approaching human. The results show that the one year old untrained horses display asymmetrical responses to an approaching human, with more negative reactions (escapes, threats) when approached from the left side, while approaches towards the right shoulder elicited more positive behaviors. On the contrary, two years old trained horses reacted equally positively to approaches and contact on both sides. Our findings support those of previous studies investigating a link between emotionality and laterality and confirm the role of the left hemisphere in the processing of novel or negative stimuli. Moreover, the data underline the impact work and training can have on this laterality in horses.
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Henry, S., Zanella, A. J., Sankey, C., Richard-Yris, M. - A., Marko, A., & Hausberger, M. (2012). Adults may be used to alleviate weaning stress in domestic foals (Equus caballus). Physiology & Behavior, 106(4), 428–438.
Abstract: The present study aims to investigate whether the presence of unrelated adult horses at weaning would reduce the social stress of weaning and the emergence of undesirable behaviours. We tested this hypothesis in 32 domestic foals by comparing short and medium term behavioural and physiological responses to weaning in foals maintained in homogeneous groups of peers (PW) to those of foals grouped with both peers and unrelated adults (AW). In total, three trials were conducted, which each trial consisting of one AW group and one PW group. In all foals, weaning was followed by increased vocalization, increased locomotion and increased salivary cortisol concentration. However, signs of stress were less pronounced and shorter in duration in weanlings housed with unrelated adults (e.g. whinnies: p < 0.05; salivary cortisol: p < 0.05). Only foals without adults exhibited increased aggressiveness towards peers (p < 0.05) and abnormal behaviours (p < 0.05) such as excessive wood-chewing and redirected sucking towards peers. In conclusion, introducing adults to minimize weaning stress in foals and later on aggressiveness and abnormal behaviours appears as the most promising approach to date.
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Hausberger, M., & Richard-Yris, M. - A. (2005). Individual differences in the domestic horse, origins, development and stability. In D. S. Mills, & McDonnell (Eds.), The domestic horse : the origins, development, and management of its behaviour (pp. 33–52). Cambridge: Cambridge University Press 2005.
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