Behavior Genetics of the Horse (Equus caballus) - Genetics and the Behavior of Domestic Animals - page 245

is simple, takes little time, and can easily be applied to any dam foal pair,

as it is not intrusive and presents no risks of disrupting mare foal bonds.” In

a subsequent study, Henry et al. (2007) re-tested the same foals at six
months old and found that they remain sensitive to their mares' influences.
The study also revealed high individual variability between the foals, sug-
gesting a stronger effect of the foals' own behaviors at this age.

Serotonin Genes and Maternal Behavior
The “protective” behavior of mares in the Henry et al. (2005) study repre-
sents a complex phenomenon. Naturally occurring variations in maternal
care and protection are widespread in mammals, although researchers con-
tinue to differ in how much emphasis they place on genetic or environmental
factors ( McGue, 2010 ). In recent years, however, the pattern of care given
an offspring by its mother has been linked to variation in the serotonin trans-
porter (5-HTT) gene (reviewed by Heiming and Sasher, 2010 ). Serotonin is
an inhibitory neurotransmitter in the central nervous system that is linked to
the regulation of mood and emotions. The 5-HTT polymorphism has two
common forms: short and long. In human studies, the short 5-HTT gene
form is related to increased stress sensitivity in young human females
( Gotlieb et al., 2008 ), and in rats, maternal anxiety associated with the short
form has also been found to influence infant temperament in the early post-
natal period ( Champagne and Curley, 2005 ). When mothers carrying a short
form of the gene live in an environment that they perceive as dangerous or
threatening, the effects of the short form are expressed, and their behavior
can influence the behavior of their offspring. In a safe environment, the gene
is not expressed. Consequently, Sachser et al. (2010) argues that the behav-
ioral effects of maternal stress during the postnatal period in mice are not
necessarily “pathological,” but that the mothers may be adjusting the off-
spring to their environment in an adaptive way. Mothers may be preparing
their offspring to live in a dangerous environment. Whether a mother carries
a short form of the 5-HTT gene or not, pre- and postal effects of stress influ-
ence sensitivity to stress in offspring.
The effects of maternal stress or other postnatal environmental stressors
are also influenced by genetically based individual differences in nervous
system reactivity and temperament in foals. The research of Henry et al.
(2009) on social behavioral development in “imprint trained” foals empha-
sizes this fact. Individual differences in behavior were observed in the inten-
sively handled foals. Three out of nine foals approached and investigated an
unfamiliar human, whereas almost all (nine out of 10) of the control foals
left their mother and approached the human. One-third of the intensively
handled foals approached the unfamiliar human, suggesting an effect of
genetics on this response.