Lateralization in Feral, Przewalski and Domestic Horses

Abstract

Lateralization, referring to differential processing of information by the left and right sides of the brain and to side biases in behaviour, has been reported in domestic horses. To determine whether lateralization is characteristic of the horse ('Equus caballus') as a species and not simply a result of training or domestication, field studies were carried out observing Przewalski horses ('Equus ferus przewalski') and two groups of feral horses that differed in number of generations removed from domestication. Przewalski horses are the closest wild relative of domestic horses. Left-side biases of approximately 57–68% were found in agonistic interactions, 63–73% in high alert and 53–54% in vigilance. As in other vertebrates, the right hemisphere of the horse is specialised to control agonistic interactions and responses to potential threats. Also consistent with previous research, showing right-hemisphere control of the expression of strong emotion, leftwards bias was stronger in measures of behaviour involving higher aggression and reactivity. Domestic riding horses were also studied using the same methodology and found to display left-side biases for the above measures, with the exception of vigilance, but the strength was weaker than in feral and Przewalski horses. This suggests (a) ancestral horses were more strongly lateralized for attack, vigilance and reactivity than are present-day domestic horses, and/or (b) lateralization is stronger in horses living in natural habitats. Limb preference, measured as the forelimb placed in front of the other during grazing, was investigated because previous research has shown limb preference in some breeds of domestic horse.