180 II. BIOLOGICAL  FOuNDATIONS
are species or populations with a “fast” life- 
history strategy. Individuals belonging to 
these species or populations breed early and 
often, but at the expense of longevity and 
offspring survival. At the other end of this 
continuum are species or populations with a 
“slow” life-  history strategy. Individuals be -
longing to these species or populations breed 
later and less frequently but have longer lives 
and more surviving offspring. Life-  history 
strategy has been extended to describe dif -
ferences between individuals within species 
and populations (Réale et al., 2010). Be -
cause different life-  history strategies have 
similar fitness payoffs (MacArthur & Wil -
son, 1963), the variation of any personality 
trait associated with life history should be 
maintained (Biro & Stamps, 2008; Wolf, 
van Doorn, Leimar, & Weissing, 2007). 
Consistent with the life-  history strategy per -
spective, Réale, Martin, Coltman, Poissant, 
and Festa-  Bianchet (2009) tested whether 
two personality traits (boldness and docil -
ity) in bighorn sheep were genetically cor -
related, and whether genetic tendencies for 
these traits were associated with survival 
and reproductive success. Boldness  was 
defined as the tendency to enter a baited 
trap, and docility  was defined as reactions 
to being handled by humans. Boldness and 
docility were associated with life-  history 
trade-offs, heritable, and had a negative ge -
netic correlation with each other. Moreover, 
the predicted genetic values of boldness and 
docility were related to a longer lifespan and 
late-life reproductive success.
A fourth possibility is that personality 
variation is maintained because of geneti -
cally or environmentally mediated corre -
lations between traits (Sih, Bell, Johnson, 
& Ziemba, 2004). These correlations can 
occur within contexts, such as the case of 
brown capuchin monkeys: Individuals high 
on a personality dimension labeled “as -
sertiveness” tend to be more aggressive, 
dominant, and independent, but less fearful 
and cautious (see Table 6 in Morton, Lee, 
Buchanan-  Smith, et al., 2013, for all of the 
loadings). These correlations can also occur 
across contexts, for example, in stickleback 
fish in which more aggressive individuals 
were more likely to explore a new environ -
ment (Huntingford, 1976). In both cases, 
because the traits are correlated, being at the optimum level of one trait (assertiveness 
in monkeys and aggressiveness in fish) is in -
compatible with being at the optimum level 
of the other trait (cautiousness in monkeys 
and the tendency to explore new environ -
ments in fish).
It is unlikely that any one of these possible 
evolutionary mechanisms, or others that are 
not discussed here, is solely responsible for 
maintaining personality variation. How -
ever, the identification of these mechanisms 
in several species testifies to how animal 
personality research can solve problems re -
lated to personality evolution. There is more 
to say about the maintenance of personality 
variation, but that involves comparing spe -
cies, a topic that I address later in this chap -
ter. Until then, I describe the search for the 
physiological bases of animal personality.
Physiological Underpinnings of Animal 
Personality  Differences
With respect to identifying the physiologi -
cal bases of animal personality, some of the 
most convincing work comes from selec -
tion studies and studies of animal coping 
styles. Animal breeders have long known 
that breeding (selecting) for physical  traits 
can lead to changes in behavioral  traits, 
and vice versa. In a classic study, Dmitry 
Belyaev bred foxes for tameness for over 40 
years (Trut, Oskina, & Kharlamova, 2009). 
In several respects, the tame foxes behaved 
more like domesticated dogs than their 
wild counterparts; they also differed from 
wild foxes in their rate of development and 
neuroendocrine profiles (Trut et al., 2009). 
Specifically, compared to wild foxes, tame 
foxes retained youthful features but reached 
sexual maturity earlier and showed higher 
levels of reproductive hormones (Trut et al., 
2009). Moreover, during development, foxes 
display a reduction in their exploratory be -
havior and an increase in glucocorticoids, 
both of which mark the end of their sensitive 
socialization period; these changes occurred 
later in tame foxes than in wild foxes (Trut 
et al., 2009). It thus seems foxes bred to 
thrive in captivity had an extended adoles -
cent period, perhaps because they were not 
selected to survive in a more competitive and 
stressful environment (the wild).