Behavior Genetics of the Horse (Equus caballus) - Genetics and the Behavior of Domestic Animals - page 272

interacting with humans, Farmer et al. (2010) writes, “Intriguingly, the right-
eyed horses in both groups, although only a very small sample, showed the
same trend with regard to the right eye, that is to say they showed stronger
lateralization under

the stranger (response to an unfamiliar human)

condition.”
In the studies reviewed in the previous section, the few horses in each
sample showing a reverse of left-eye (right hemisphere) processing of emo-
tional information have received little attention. It is interesting to speculate
that reversed hemispheric dominance might reflect a distinct cognitive dispo-
sition, or bias. An association between cognitive bias, handedness in humans,
and limb preference in animals supports this view ( Rogers, 2010 ). Cognitive
bias refers to the particular way the brain perceives, forms memories, and
makes judgments. Denny (2009) found that left-handed people report more
depression symptoms than right-handed people. In chimpanzees, Hopkins
and Bennett (1994) found that right-handed chimps interact more with novel
objects than left-handed individuals. More evidence suggests that preferred
use of the left paw (and right hemisphere) is characteristic of more fearful
marmosets ( Cameron and Rogers 1999 ). Rogers (2009) also found that left-
handed marmosets are more likely to have higher levels of the stress hor-
mone cortisol when compared with right-handed individuals. Rogers (2010)
also suggests that right-handed marmosets are active, and left-handers are
reactive. In marmosets, handedness predicts a tendency to adopt a positive or
negative cognitive bias.
In horses, no studies exist that specifically address differences in left-eye
system versus right-eye system dominance and cognitive bias. It may be pos-
sible that right-hemisphere-dominant and left-hemisphere-dominant horses
respond differently in similar circumstances. Anecdotal evidence collected
by the first author suggest that horses with high left- or high right-side hair
whorls respond differently to either stationary or sudden novelty. Horses
with high right-side whorls display higher levels of arousal such as prancing,
vigilance, whinnying, snorting, and tail raising, compared to horses with
high left-side whorls. To illustrate this point, the first author has two
Warmblood mares; Bobbi has a high left-side hair whorl, and Sam has a
high right-side hair whorl. The mares are both 6 years old and share the
same sire, but different mothers. The first author bred, raised, and trained
both mares himself. Sam and Bobbi are both reactive to sudden novelty and
social separation; however, Sam (right-side whorl) vocalizes when separated
and makes more attempts to return to the herd. Sam also nickers and whin-
nies more than Bobbi at feeding time. Emotional responses to positive and
negative stimuli between the two are obvious. Recently, we tested both
mares on flight and escape-turning responses using the method described by
Austin and Rogers (2007) . Placing an oat bucket in the center of a large pad-
dock, the first author released the mares one at a time and allowed them to
eat from the bucket for 1 2 minutes. He positioned himself in front of each