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: Cerebral lateralization was once thought to be unique to humans, but is now known to be widespread among the vertebrates. Lateralization appears to confer cognitive advantages upon those that possess it. Despite the taxonomic ubiquity and described advantages of lateralization, substantial individual variation exists in all species. Individual variation in cerebral lateralization may be tied to individual variation in behaviour and the selective forces that act to maintain variation in behaviour may also act to maintain variation in lateralization. The mechanisms linking individual variation in the strength of cerebral lateralization to individual variation in behaviour remain obscure. We propose here a general hypothesis which may help to explain this link. We suggest that individuals with strong and weak lateralizations behave differently because of differences in the ability of one hemisphere to inhibit the functions of the other in each type of brain organization. We also suggest a specific mechanism involving the asymmetric epithalamic nucleus, the habenula. We conclude by discussing some predictions and potential tests of our hypothesis.

Abstract: Cerebral lateralization is the portioning of the cognitive functions between the two cerebral hemispheres. Several factors, like embryological manipulations, light exposure, health conditions, sex and age can influence the left-right brain asymmetries and contribute to increasing the variability in the strength and direction of laterality within most species. We investigated the influence of an environmental constraint, namely space availability, as a new source of variation on laterality in an adult vertebrate model, the donkey. In a baseline condition we tested whether donkeys show a motor lateralization bias at population level, while in an experimental condition we manipulated space availability to verify if a reduction in this parameter could represent a new source of variation in laterality. Results show that donkeys are lateralized at population level with a strong bias to standing with the right forelimb advanced over the left and that a reduction of space availability is an important source of variation in the laterality strength and direction within this species. The comparative analysis of the environmental and developmental factors that give origin to neural and behavioural laterality in animal models will be very important for a better understanding of the evolutionary origin of such multifaceted phenomenon.