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The purpose of this study was to examine the relationship
between the 5-HTT transporter gene polymorphism and personality
traits in chimpanzees. Personality data was collected by questionnaires
given to care givers. The genotyping was done from buccal swabs.
Significant differences between subjects that are homozygous for
the long allele and those with the short (whether homozygous or
heterozygous) were found on two factor loadings (Surgency and Openness)
and five personality items (Dominant, Sociable, Timid, Depressed,
and Intelligent). The factor loading of Dominance and the items
Solitary and Affectionate were borderline significant. These preliminary
results suggest that specific genetic factors may play a role in
the determination of personality attributes in chimpanzees.
Recent research as shown that genetic factors can
account for individual differences in personality as well as clinical
dimensions of personality disorder and psychopathology in humans
(Lesch et. al, 1996). There are some limitations to the use of human
subjects in behavioral-genetic research because humans often seek
out situations and social contexts that accommodate their personalities.
As a result, it is difficult to isolate the independent effects
of genetic and social determinants on behavior. Behavioral-genetic
studies in animals are not subject to these limitations and therefore
offer an alternative and potentially stronger approach to assessing
the influence of genetics on behavior. Chimpanzees are ideal subjects
for behavioral-genetics studies because they share 98.6% of the
same genetic proteins with humans. However, they lack the same social
determinants of personality. One area of genetic research in personality
that has employed only humans is on the 5-HTT serotonin transporter
gene (Melke et al, 2001; Ebstein et. al, 1997). The gene contains
a polymorphism consisting of two alleles, long and short, which
affect the number of serotonin receptor sites on post-synaptic neurons.
The short allele codes for less sites than its longer counterpart,
and in human subjects with short alleles, both homozygous and heterozygous
pairs are associated with higher rates of anxiety and psychopathology
(Kim et. al, 2000; Bellivier et. al, 2000; Mann et. al, 2000). This
preliminary study examined the relationship between personality
traits and allelic variation of the 5-HTT serotonin transporter
gene in chimpanzees.
Subjects consisted of 23 chimpanzees (Pan troglodytes)
housed at the Yerkes National Primate Research Center (YNPRC). Fourteen
of the subjects were females and nine were males. Personality Measures:
Personality data was collected on the chimpanzees by administering
questionnaires to the caretakers and researchers at the YNPRC. The
questionnaire used was the 49-item Personality Trait Assessment
designed for use with chimpanzees by the Jane Goodall Institute.
The questionnaire was designed around the Five Factor personality
model used in humans and previously validated on chimpanzees in
zoos and other research institutions. For each item on the questionnaire
there was a 1 to 7 point Likert-scale with indicating no or negligible
display of the item related behaviors and seven representing a large
display of the trait. Genotyping: Buccal samples were collected
from the subjects using sterile swabs. The DNA was extracted using
the buccal cell protocol provided by the PuregeneTM DNA purification
kit. The primers used in the PCR amplification were as follows:
stpr5 (5’-GGCGTTGCCGCTCTGAATGC-3’) Intl (5’-CAGGGGAGATCCTGGGAGGG-3’)
PCR products were evaluated by electrophoresis on an ethidium bromide
stained agarose gel and examined under a UV light.
The genotyping showed that out of the 23 individuals,
8 had two long alleles, 4 had two short alleles, and 11 were heterozygous,
a distribution comparable to some human populations. A factor analysis
with varimax rotation was performed on the 49 questionnaire items
and 6 basic personality dimensions emerged from this analysis. The
item structure loadings were similar to previous analyses in chimpanzees
(King et al, 1997). The item loadings for 6 factors: Dominance,
Surgency, Dependability, Agreeableness, Emotionality, and Openness.
Two nonparametric statistics were performed comparing the subjects
with the presence of one or two short alleles to those with homozygous
long alleles. In the first analysis, the two groups were compared
on the factor scores. In the second analysis, the groups were compared
on the 49 item scores from the questionnaire. The first analysis
revealed a significant difference on the factor loadings of Surgency
and Openness at p < .05. A borderline significant difference
was found for the factor loading of Dominance (p < .10). The
second analysis revealed significant differences on 5 of the personality
items including Dominant, Sociable, Timid, Depressed, and Intelligent
(all p < .05). Borderline significant differences were found
for the personality items Solitary and Affectionate (p < .10).
Subjects with homozygous long alleles had higher ratings for the
factor loadings of Surgency, Openness, and Dominance, as well as
the personality items Dominant, Sociable, Intelligent, and Affectionate.
Dominance and Affectionate were borderline. Subjects with the short
allele (both heterozygous and homozygous) had higher ratings for
the personality items Timid, Depressed, and Solitary. Solitary was
borderline. The results indicate that chimpanzees with the short
allele on the 5-HTT transporter gene have personality traits characteristic
of higher levels of anxiety and other maladaptive behavior. Limitations
of this study include the fact that a small sample size (23 subjects)
was used. Moreover, only questionnaire data was used and ideally
observational measures of chimpanzee behavior would be better.
This research was made possible in part by funding from the University
Research Committee grant 2-80308, NIH grant NS-36605, and the SURE
program of Emory University.
For this study, we attempted to find a relationship
between genetics and personality traits. Knowing that levels of
a bodily chemical called serotonin effects anxiety level and that
the activity of this chemical is determined by a specific gene (5-HTT
transporter gene), we tried to see if there was a statistical relationship
between the type of gene and the personality traits of the subjects.
We used chimpanzees as subjects instead of humans because with humans
it is difficult to determine whether personality traits are determined
by genetics or by other social factors. Since chimpanzees don't
have the same social determinants as humans, the are ideal subjects
to exmaine a purely genetic contribution to personality traits.
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