Behavior Genetics of the Horse (Equus caballus) - Genetics and the Behavior of Domestic Animals - page 276

associated with obvious fear responses, in others the fear responses may
become attenuated over time ( Cayado et al., 2006 ), and can easily be mistaken
for motor asymmetry or disobedience. For example, young horses trained in a
round pen on a lunge line often travel better in either a clockwise, or counter-
clockwise direction. If horses have a left-side hair whorl, the preferred direc-
tion is clockwise. In the counter-clockwise direction, the horse makes smaller
circles and tries to turn its head to the outside. The head is often held high, the
speed of travel is increased, and the tail is held high. These are all signs of
emotional arousal ( McGreevy, 2004; Visser et al., 2001; Waring, 1983; Wolff
et al., 1997 ). Older horses show the same signs of arousal but to a lesser
degree. As we discussed earlier, horses with left-side whorls resist horseshoe-
ing on the left side. The resistance may involve attempts to flee, or refusal to
allow a leg to be touched. In other instances, the resistance may be subtle and
horses stand and tolerate the leg being lifted, but lean on the farrier or try
repeatedly to pull the foot and leg away. These are not clear and obvious signs
of fear, but may in fact be attenuated fear responses. Leaning on a farrier may
be a defense reaction motivated by fear, and more evident on one side. The
“assumed” motor asymmetries shown by horses in ridden work ( Murphy and
Arkin, 2008 ) may in fact be attenuated lateralized fear responses ( Cayado
et al., 2006 ). In early stages of training for riding, fear may motivate horses to
prefer to work in a rightward or leftward direction. Horses with left-eye (right
hemisphere) control of fearful stimulus may be less fearful when working in a
rightward direction. This direction allows the horse to keep the left eye to the
outside of a circle where it feels more confident to defend itself against poten-
tial threats. Strong one-sidedness motivated by fear in young horses during the
early stages of training may be attenuated with experience and mistaken for
motor asymmetry.
Research in humans and animals show that trauma experienced early in
life, or at any time in life can cause shifts in cognitive bias toward the right
hemisphere leading to prolonged anxiety-like behavior ( Chamberlain and
Sahakian, 2007; Mathews and McLeod, 2005 ). Adamec et al., (2005)
exposed rats to a cat and found a shift to the right hemisphere, and subse-
quent changes in neural activity and neurotransmitter levels lasting up to
12 days after this stressful event.
Fear stress is highly aversive and subjecting animals to extreme fear
stress is very detrimental to welfare ( Grandin, 1997 ).
Experience-Dependent Lateralized Learning
In a previous section, the first author discussed his approach to horses that
are difficult to shoe on one side or the other. He found that switching sides
and shoeing the “less difficult” side first was helpful. When he finished the
less difficult side, horses were often more cooperative when he returned
to the difficult side. This may be an example of experience-dependent