Lateralization of agonistic and vigilance responses in Przewalski horses ('Equus przewalskii')

Abstract

Eye and limb preferences were scored in the closest undomesticated relative of ('Equus caballus') using the same methods as used previously to study laterality in feral horses. Observations were made of 33 Przewalski horses ('Equus ferus przewalskii') (male N = 20, female N = 13) living under natural social conditions on a large reserve in France. Significant left-eye/side biases were found in agonistic interactions within harem bands (M ± SE bias to left 58% ± 0.01 for threats, P < 0.001; 68% ± 0.05 for attacks; P < 0.001) and in stallion fights (threats, 52% ± 0.01 left, P < 0.001; attacks, 63% ± 0.02 left, P < 0.001): as many as 80% of the horses were significantly lateralized in attack responses within harem bands. Laterality of vigilance was measured as lifting up the head from grazing and turning it to the left or right side: a directional bias to the left was found (M ± SE 53% ± 0.02 left, P < 0.001). Side bias in reactivity was calculated as the percent of head lifts above the level of the withers on the left or right side and this was also left side biased (M ± SE 73% ± 0.03 left, P < 0.001). These results indicate right-hemisphere specialization for control of aggression and responses to novelty. The left bias in attack scores within harem bands was stronger in males than females (P = 0.024) and in immature than adult horses (P = 0.032). Immature horses were also more strongly lateralized than adults in vigilance scores (P = 0.022), which may suggest that experience reduces these side biases. Our results show that Przewalski horses exhibit left eye preferences, as do feral horses, and do so even more strongly than feral horses. Considering feral and Przewalski horses together, we deduce that ancestral horses had similar lateral biases. Also similar to feral horses, the Przewalski horses showed no significant forelimb preference at the group level or in the majority of horses at the individual level, confirming the hypothesis that previously reported limb preferences in domestic breeds are entrained or generated by breed-specific selection.