Biology Reference
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training, shared the same training area, and had the same caretakers. Blood
samples were collected for DNA analysis, and a questionnaire survey was
used to rate the temperament traits of each horse. Interestingly, an associa-
tion was found between polymorphisms (genes that come in two or more
forms) in the dopamine D4 (DRD4) receptor gene and two temperament
traits in the horses: curiosity (novelty seeking) and vigilance. The survey had
the following descriptions of curiosity and vigilance: “The horse tends to be
interested in novel objects and approach them,” and “The horse tends to be
vigilant about surroundings.” Horses with one form of the gene had higher
curiosity (novelty seeking). Horses with the other form displayed lower vigi-
lance. The DRD4 gene is also associated with novelty seeking in humans
( Benjamin et al., 1996 ), in primates ( Livik et al., 1995 ), and in dogs ( Ito
et al., 2004 ). Momozawa et al. (2005) speculates that diversity in the
sequence of the DRD4 gene might influence differences in novelty seeking
in various species.
In more recent molecular genetics research, Anderson et al. (2012) dis-
covered mutations in the DMRT3 genes associated with locomotion in gaited
horses. A high frequency of DMRT3 mutations were found in horse breeds
such as Tennessee walking horse, Missouri fox trotters, Standardbred trotter,
and French trotters. The mutations were not present in non-gaited horses
such as Thoroughbreds, Arabians, and Przewalski's horses. In the laboratory,
Anderson et al. (2012) blocked the normal function of the DMRT3 genes in
mice (null mutants) and observed changes in locomotion and gaits similar
to those in gaited horses. This research shows that the expression of certain
specialized genes in vertebrates as different as mice and horses has remained
remarkably conserved over evolution.
GENETIC DIFFERENCES INTERACT WITH ENVIRONMENT
Hausberger et al. (2004) highlights how individual horse personalities and
temperaments interact with genetic, social, and nonsocial environmental
factors. A battery of standardized behavioral tests were performed on 702
horses and ponies stabled at 103 different riding centers, training centers,
and national stud and breeding farms in France. Only registered horses were
used to enable accurate breed identification. Sex and age were well distrib-
uted in the horses (185 mares, 305 geldings, and 212 stallions). Behavioral
tests were used to rate different facets of temperament, emotionality, and
learning ability. An arena test was used to estimate emotionality. Each horse
was released into a familiar arena and its behavior was recorded for 10 min-
utes. Emotional reactions (whinnying, locomotion, vigilance) were used to
evaluate the effects of social separation in the arena. This test differed from
a standard open-field test used to measure fear because the arena was famil-
iar to the horse. A novel object test was used to evaluate nervousness.An
object resembling a cage with a ribbon attached to it was placed in the
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