 7. Personality in Animals  181
Another selection study compared birds 
(great tits) selected to be fast or slow explor -
ers of novel environments (van Oers, Bu -
chanan, Thomas, & Drent, 2011). Fast ex -
plorers had better immune functioning and 
lower testosterone levels than slow explor -
ers. The latter findings surprised van Oers 
and his colleagues, because fast explorers 
are more aggressive. However, this differ -
ence may mean that, like Belyaev’s domesti -
cated foxes, birds selected for lower levels of 
a trait related to aggression reached sexual 
maturity more rapidly.
In the animal personality literature, cop -
ing style1 refers to a suite of behavioral and 
physiological tendencies related to how in -
dividuals react to stressors (Koolhaas et al., 
1999). At one end of this spectrum are pro -
active  individuals. At the other end of this 
spectrum are reactive  individuals. In behav -
ioral tests, proactive individuals were found 
to attack more quickly and were more likely 
to engage in defensive behavior (Koolhaas et 
al., 1999, Table 2). Animals with a proac -
tive coping style also differ with respect to 
their hypothalamic–  pituitary–  adrenal axis 
activity and reactivity, and their parasympa -
thetic reactivity, both of which are low rela -
tive to animals with a reactive coping style 
(Koolhaas et al., 1999, Table 3). Finally, the 
sympathetic and testosterone reactivity of 
individuals with a proactive coping style are 
high compared to individuals who have a 
reactive coping style (Koolhaas et al., 1999, 
Table 3).
Differences in neurophysiology have also 
been identified as proximate explanations 
for personality variation. A magnetic reso -
nance imaging study of 74 chimpanzees 
revealed that the percentage of grey matter 
in the brain, but not the asymmetry of the 
brain’s subgenual cingulate cortex region, 
was associated with lower dominance and 
higher conscientiousness (Blatchley & Hop -
kins, 2010). Personality measures in that 
study were derived by questionnaire ratings: 
Dominance was made up of traits related to 
boldness, fearlessness, assertiveness, and ag -
gressiveness; conscientiousness was made up 
of traits related to self-  control, predictabil -
ity, and emotional stability (Weiss, King, & 
Hopkins, 2007).
A later magnetic resonance imaging study 
of 107 chimpanzees by Latzman, Hecht, Freeman, Schapiro, and Hopkins (2015) 
revealed associations between personal -
ity and other brain regions: The volume 
and asymmetry of the frontal region of the 
brain’s grey matter were associated with 
higher dominance and extraversion, the lat -
ter being related to sociability, activity, and 
other traits that make up its human coun -
terpart (Freeman et al., 2013). Latzman and 
his colleagues (2015) also found associations 
between frontal grey matter volume and 
higher openness, a personality dimension re -
lated to exploratory behavior and curiosity 
(Freeman et al., 2013), and between frontal 
grey matter asymmetry and higher reactiv -
ity/unpredictability, a personality dimension 
related to low conscientiousness (Freeman et 
al., 2013).
Evolutionary Explanations for Human 
Personality  Variation
The findings reviewed earlier suggest that 
there is promise in the use of animals to 
understand the evolutionary and physi -
ological underpinnings of personality varia -
tion. Turning again to questions about the 
evolution of personality, researchers have 
conducted many studies on the association 
between human personality and health, sur -
vival, and reproductive success (see Lewis 
& Buss, Chapter 1, this volume). However, 
researchers have been considerably less suc -
cessful in answering questions related to 
human personality evolution than they have 
been in answering questions about animal 
personality evolution. Fortunately, studies 
that compare species provide some of the 
best tests of some of these hypotheses about 
the evolution of human personality, and so 
can help move the study of human personal -
ity evolution forward.
The comparative method involves con -
ducting natural experiments by comparing 
species with different evolutionary histo -
ries (see Gosling & Graybeal, 2007, for an 
overview). To see how one tests evolution -
ary hypotheses in comparative studies, con -
sider the case of studying two species. These 
species can either be closely related to one 
another (e.g., humans and chimpanzees) or 
distantly related to one another (e.g., chim -
panzees and brown capuchin monkeys). In 