T
he behavioral biology of rodents signifi-
cantly influences their life-history strategies and re-
productive success, and development creates much
of the variation in these traits on which natural selection
can act. In this chapter I present an overview of the ontog-
eny of rodent behavior, including pre- and postnatal effects
of social experiences, stress, and seasonality on the expres-
sion of developing phenotypes. I focus on several factors
modulating both the processes and outcomes of behavioral
development, such as maternal influences, observational
learning, hormonal effects, and ecological conditions con-
tributing to individual and population differences in social
behaviors. I also discuss in detail, from a comparative ap-
proach, how social systems and life-history parameters me-
diate the development, plasticity, and functional significance
of a suite of survival behaviors. Examples are drawn from
studies of both captive and free-living animals, with a focus
on comparative research with ground-dwelling squirrels.
The framework presented here integrates both proximate
and ultimate levels of analysis, to understand how processes
of development influence, and are influenced by, social and
environmental contexts (summarized in fig. 17.1).
Behavioral development, long the realm of comparative
psychology, is emerging as an area of interest among evo-
lutionary biologists and ecologists because of their interests
in phenotypic plasticity and the effects of experiences on
not only morphological and physiological traits but also
behavioral traits. Recent research on parental effects — the
ways in which parents’ phenotypes influence the develop-
ment of their offspring —has also invigorated investigations
into the processes and outcomes of behavioral development.
Compelling evidence shows that multiple phenotypes can be
favored when animals experience spatial or temporal vari-
ation in selection pressures or competition within one envi-
ronment, with alternative phenotypes differentially favored
depending on frequency distributions of current traits (re-
viewed by Stamps 2003). Thus multiple developmental pro-
cesses would be expected when individual differences are
favored, either between or within environments.
Detailed field studies of behavioral development in ro-
dents are uncommon. In many species, adults are difficult
to view and follow, and their young are often reared in in-
accessible burrows or nests. Social interactions among par-
ents and offspring or among littermates, so influential in
later adult relationships, are therefore unknown to us. Many
discoveries about early social interactions, such as paternal
behaviors in hamsters or voles, communal nesting, and so-
cial influences on timing of puberty, resulted from observa-
tions in captivity (e.g., Vandenbergh 1983; Manning et al.
1992; Jones and Wynne-Edwards 2000). In the lab or field
enclosure, researchers can observe a developing animal’s
social and physical environment from birth (or earlier)
through adulthood, revealing the potential effects of litter
size, hormones, olfactory cues, sensory stimulation, diet,
and parental behaviors on emerging behavioral phenotypes.
However, observers must be cautious in generalizing results
from the lab to development in nature, where a young ani-
mal experiences the full range of social and physical stimu-
lation and interacts with and modifies those environments
in ways that cannot be replicated in captivity (e.g., Wolff
2003c).