Behavior Genetics of the Horse (Equus caballus) - Genetics and the Behavior of Domestic Animals - page 270

showed greater reactivity and flight responses for left side approach,
whereas horses tested first on the right side displayed less reactivity and
shorter flight distance when subsequently approached on the left side. The
evidence suggests that stimuli first presented to the right side allowed the
horses to learn that the stimulus posed no threat. This information then
transferredtotherighthemisphereof the horses' brains, leading to a less-
ened reaction when approached with the umbrella from the left side. Larose
et al. (2006) studied visual asymmetry during exposure to novel objects in
Trotters and French Saddlebreds. The temperaments of the two breeds were
similar, but expressions of visual laterality differed between individual
horses. Horses with higher emotional scores assessed during behavior tests
glanced at a novel object with the left eye (right hemisphere) more than the
right eye (left hemisphere). This evidence shows that laterality is stronger
in more reactive (fearful) animals. In addition, stimuli that animals fear the
most, such as sudden rapid moving objects, or objects that animals have
been conditioned to fear influence laterality. De Boyer des Roches et al.
(2008) found that emotionally laden objects influence lateral responses.
Horses showed a slight tendency to use the left eye (right hemisphere) to
explore objects with negative emotional value (a veterinarian's white shirt)
more than objects with neutral emotional value (a plastic cone) or positive
value (a feed bucket). The age of a horse can also influence laterality.
Sankey et al. (2011) found that yearling horses approached on the left side
showed more escape and threat (avoidance) behaviors, as opposed to more
positive (approach) behaviors when approached from the right side. Two-
year-old horses, on the other hand, reacted positively to approach and con-
tact from both sides. These results show the influence of work and training
on laterality in horses. It has been argued by some that asymmetries in
horses are caused by humans traditionallyhandlinghorsesontheleftside.
In Trotter horses that are, to an extent, handled on both sides, Larouse
et al. (2006) found no association between eye preference and reactivity.
However, field studies of feral horses by Austin and Rogers (2007) shows
that lateralization is a characteristic of horses as a species that is not caused
by human handling. Observations revealed a left-side bias of reactivity
(heads held high and looks to the left or right caused by the presence of
humans), a left-side bias in vigilance (raising the head from a grazing posi-
tion and turning the head to attend to stimuli), and a left-eye bias during
aggressive exchanges (eye used by stallion to look at its opponent). The
left-eye bias was stronger during attacks than during threats. There was no
observed population bias of foreleg preference in grazing stance in feral
horses,comparedtopopulationbiases in domestic horses. The absence of
population-level forelimb preference in feral horses suggests that training
and work influence forelimb preference in domestic horses ( Austin and
Rogers, 2007 ), and that limb preference does not indicate brain lateraliza-
tion, but may better reflect a horse's attention to its environment.