Behavior Genetics of the Horse (Equus caballus) - Genetics and the Behavior of Domestic Animals - page 268

right or left side, and to gallop on the right or left lead ( McGreevy and
Thompson, 2006 ; McGreevy and Rogers, 2005; Murphy et al., 2005; Wells
and Blache, 2008; Williams and Norris, 2007 ). Training horses to work
equally well in either direction is important in all equitation disciplines
( Manroe, 1992; Maringer, 1983; Sivewright, 1986 ). Under saddle, horses
tend to bend easily to one side and with more difficulty to the other side
( Clayton, 1990; Murphy et al., 2005 ). Horses have long been considered
asymmetrical ( Steinbrecht, 1886 ), and many training exercises are aimed at
straightening the horse.

Motor Asymmetry
Whether side preference is a result of genetic predisposition or if it results
from training and use was unknown until recently. Grizmek (1968) looked into
motor asymmetry and found that most horses begin to gallop with the left fore-
limb first, and that they also turn left more easily. McGreevy and Thompson
(2006) found differences in motor laterality between breeds of performance
horses. The more temperamental Thoroughbreds showed a higher left foreleg
preference during grazing when compared to Standardbred and Quarter
Horses. Little or no side preference was shown in Quarter Horses. McGreevy
and Rogers (2005) noted the relative position of the horses' forelegs every 60 s
for 2 hours in Thoroughbreds grazing on pasture. Results show a population
bias of standing with the left foreleg advanced. Forty-three out of 106 horses
demonstrated a significant left leg preference, 10 horses demonstrated a signif-
icant right leg preference, and 53 horses were ambidextrous in their forelimb
preference. In addition, motor laterality bias increased with age, suggesting
maturation or influence of training. In contrast to the left foreleg bias observed
by McGreevy and Rogers (2005) , Murphy et al. (2005) , found that 52% of
horses preferred to start walking or trotting with the right foreleg, while 40%
preferred the left foreleg, and 7.5% showed no preference. The horses is this
study were of mixed breeds (predominantly Thoroughbred

Irish Draught).
The findings show strong sex-related directions of laterality, but not degrees of
laterality. Male horses exhibited significantly more left-lateralized responses
and female horses exhibited significantly more right-lateralized responses.
Williams and Norris (2007) studied stride lead preferences in racing Arabian,
Thoroughbred, and Quarter horse and found that 90% of ridden racehorses
show a right lead stride bias. Wells and Blache (2008) studied both laterality
in grazing stance and laterality in lunging circle tests in 15 ridden horses
(5

3

3 years of age) that had not been rid-
den. The breeds consisted of 60% Thoroughbred, 2% Arabian, 28%
Warmblood, and 10% Riding Ponies.
The results showed that un-ridden horses did not have a grazing stance
bias, but the ridden horses showed a right forelimb bias. The lunging test mea-
sured three indicators of bias; time spent in canter, correct lead in canter, and

20 years of age), and 15 horses (1.5