Abstract: Growing evidence shows a variety of sensorial and motor asymmetries in social and non-social interactions in various species, indicating a lateralized processing of information by the brain. Using digital video cameras on tripods and drones, this study investigated lateralization in frequency and duration of social behavior patterns, in affiliative, agonistic, and resting contexts, in a feral population of horses (Equus ferus caballus) in Northern Portugal, consisting of 37 individuals organized in eight harem groups. Affiliative interactions (including grooming) were more often performed, and lasted longer, when recipients were positioned to the right side. In recumbent resting (animals lying down) episodes on the left side lasted longer. Our results of an affiliative behavior having a right side tendency, provide partial support to the valence-specific hypothesis of Ahern and Schwartz (1979) – left hemisphere dominance for positive affect, affiliative behaviors. Longer recumbent resting episodes on the left side may be due to synchronization. However, in both instances it is discussed how lateralization may be context dependent. Investigating the position asymmetries of social behaviors in feral equids will contribute to a better understanding of differential lateralization and hemispheric specialization from the ecological and evolutionary perspectives.

Abstract: Despite the abundant empirical evidence on lateralized social behaviours, a clear understanding of the relative roles of two brain hemispheres in social processing is still lacking. This study investigated visual lateralization in social interactions of free-ranging European bison (Bison bonasus). The bison were more likely to display aggressive responses (such as fight and side hit), when they viewed the conspecific with the right visual field, implicating the left brain hemisphere. In contrast, the responses associated with positive social interactions (female-to-calf bonding, calf-to-female approach, suckling) or aggression inhibition (fight termination) occurred more likely when the left visual field was in use, indicating the right hemisphere advantage. The results do not support either assumptions of right-hemisphere dominance for control of various social functions or hypotheses about simple positive (approach) versus negative (withdrawal) distinction between the hemispheric roles. The discrepancy between the studies suggests that in animals, the relative roles of the hemispheres in social processing may be determined by a fine balance of emotions and motivations associated with the particular social reaction difficult to categorize for a human investigator. Our findings highlight the involvement of both brain hemispheres in the control of social behaviour.

Abstract: The left and right sides of the brain are specialised to process information in different ways and to control different categories of behaviour. Research on a range of species has shown that the left hemisphere controls well-established patterns of behaviour performed in non-stressful situations, whereas the right hemisphere responds to unexpected stimuli and controls escape and other emergency responses. The known functions of each hemisphere are summarised in this paper. Then it is hypothesised that stressed animals rely on predominant use of the right hemisphere, and that a bias to use the right or left hemisphere, respectively, may explain the behavioural differences between animals with a negative cognitive bias and those with a positive cognitive bias. In some species of primates it has been shown that the preferred limb used to pick up food when the animal is in a relaxed state reflects the dominant hemisphere and may be an accessible measure indicating susceptibility to stress and tendency towards positive versus negative cognitive bias. Hence, limb preference might be a useful measure of such tendencies in domesticated species. Some difficulties in determining a relevant measure of limb preference in non-primate species are mentioned, followed by the suggestion that eye preferences for viewing certain stimuli may be a useful measure in species with laterally placed eyes. Finally, effects of experience on the development of hemispheric dominance are discussed, leading to a suggestion that the welfare of domestic animals may be enhanced by ensuring development of left hemisphere dominance (e.g. by exposing chick embryos to light) and by shifting right to left hemisphere dominance in animals with negative cognitive bias.