 7. Personality in Animals  179
behavioral ecologists now show a greater 
appreciation of the need to examine the 
construct validity of behavioral measures 
(Araya-Ajoy & Dingemanse, 2014; Carter, 
Feeney, Marshall, Cowlishaw, & Heinsohn, 
2012).
Finally, it is important to show that per -
sonality measures are sensitive to expected 
species differences—  an issue that has not yet 
been discussed much (see, however, Uher et 
al., 2013, p. 429). For example, among hy -
enas, females are physically stronger than 
males and dominate them. Given this sex 
role reversal in power relations among hy -
enas compared to humans, it makes sense 
that female hyenas are higher on assertive -
ness than males (Gosling, 1998), and lower 
on neuroticism (Gosling & John, 1999). 
From a measurement perspective, these 
findings provide evidence for what might be 
called the “comparative validity” of the per -
sonality measures.
Evolutionary Explanations 
for Personality  Differences
Biologists and psychologists have advanced 
several evolutionary explanations to explain 
the presence of persistent, heritable varia -
tion in behavior (for reviews, see, e.g., Ding -
emanse & Wolf, 2010; Penke, Denissen, & 
Miller, 2007). Although there is not enough 
space in this chapter to cover all of these 
proposed explanations, it is worth discuss -
ing those that have received the most atten -
tion or support.
The default (or null) hypothesis is that per -
sonality variation reflects the accumulation 
of mutations that do not influence survival 
or reproduction (i.e., they are “neutral” in 
terms of selection; Kimura, 1968, 1983, 
1986). This hypothesis was advanced by evo -
lutionary psychologists whose main focus 
was on describing cognitive modules that, 
they argued, were human universals (Tooby 
& Cosmides, 1990). However, considerable 
evidence has come to light that personality 
is associated with health (Strickhouser, Zell, 
& Krizan, 2017) and reproductive success 
(Alvergne, Jokela, & Lummaa, 2010; Eaves, 
Martin, Heath, Hewitt, & Neale, 1990; 
Jokela, Alvergne, Pollet, & Lummaa, 2011) 
in humans. Likewise, evidence in nonhuman animals shows that personality traits are as -
sociated with survival and reproductive suc -
cess (see, e.g., Smith & Blumstein, 2008, for 
a review). As such, personality variation in 
both humans and animals can probably not 
be attributed to neutral selection.
A second possibility is that balancing se -
lection maintains variation in personality. 
This possibility is supported by studies of 
birds, lizards, insects, small mammals, and 
ungulates, which find that whether and in 
what direction a personality trait is associ -
ated with survival and reproduction varies 
across environments, time, and/or develop -
mental stages (for reviews, see Dingemanse 
& Réale, 2013; Dingemanse & Wolf, 2010). 
For example, Dingemanse, Both, Drent, and 
Tinbergen (2004) captured wild great tits 
and measured how often and how rapidly 
they explored a novel laboratory environ -
ment. They then released the birds into the 
wild and followed them over three winters: 
Availability of the birds’ preferred food dur -
ing the first winter was poor, availability of 
the birds’ preferred food during the second 
winter was good, and availability of the 
birds’ preferred food during the third winter 
was poor. For male birds, exploration in the 
laboratory was associated with lower surviv -
al rates during winters when food was scarce 
but higher survival rates during the winter 
when food was abundant; for female birds, 
the pattern was in the opposite direction.
In another study, Nicolaus, Tinbergen, 
Ubels, Both, and Dingemanse (2016) found 
that the association between exploration 
and fitness in great tits varied as a function 
of population density: Faster explorers were 
favored when the population density was 
low; slower explorers were favored when 
the population density was high. Critically, 
although birds changed their behavior as a 
function of population density, it was in the 
opposite direction as one would expect if 
doing so was to maximize fitness: Birds be -
came faster explorers when the population 
density was high and slower explorers when 
the population density was low.
A third possibility is that personality 
variation is maintained by trade-offs related 
to differences in life-  history strategy, a con -
tinuum that was originally used to charac -
terize species or populations (MacArthur & 
Wilson, 1963). At one end of this continuum 