ruptors such as dioxin, PCBs, and DDT can interfere with
the development of gonads, external genitalia, and other
sexually dimorphic structures, impede gamete production,
and alter behaviors such as parenting, aggression, and ter-
ritoriality (Palanza et al. 1999). Although rodents are cur-
rently used as models for how endocrine disruptors influ-
ence human development, the effects of synthetic chemicals
on development and reproduction in free-living small mam-
mals are likely to be the focus of future investigations, per-
haps involving collaborations among behavioral ecologists,
conservation biologists, and environmental toxicologists.
Integration of levels of analysis
“Ecological developmental biology” (also known as “eco-
devo”) examines how gene expression is mediated by envi-
ronmental factors such as climate, day length, population
density, and predators to influence developing phenotypes.
Although this approach has been around for decades, we
are now seeing it integrated into our studies on behavioral
plasticity in natural contexts (see Gilbert 2001 for a de-
tailed discussion). A separate, yet related, approach to be-
havioral development focuses on individual differences in
the outcomes of developmental processes. “Behavioral syn-
dromes” are consistent between-individual differences in
behaviors across a variety of situations, such as levels of ag-
gression in foraging, predator, and mating contexts (Sih
et al. 2004). This suite of correlated traits is of interest from
a proximate perspective (e.g., the developmental and phys-
iological mechanisms underlying expression of the traits) as
well as an ultimate perspective, because the syndrome prob-
ably represents life-history parameters that were positively
or negatively selected in temporally or spatially variable en-
vironments, rather than traits that evolved independently.
Studies of behavioral syndromes necessarily bridge areas of
psychology and biology, with a focus on the expression of
individual differences in context. “Evolutionary traps” re-
fer to situations in which animals encounter drastically dif-
ferent environments (due to human disturbance) and ex-
hibit behaviors based on previously reliable environmental
cues, behaviors that may now be maladaptive. Phenotypic
plasticity, particularly of behavioral traits, may allow these
individuals to “escape” the traps, thereby adjusting their
behaviors according to the new environments in which they
find themselves. Emergence from hibernation by yellow-
bellied marmots (Marmota flaviventris) in central North
America occurs earlier than it did decades ago due to a
warming climate. As a result, marmots become active well
before snowmelt and risk starvation until food becomes
available, unless they can change their seasonal activity pat-
terns and emerge later in the spring (Inouye et al. 2000). As
an example, leatherback turtles (Dermochelys coriacea) can
adjust their food search strategy to avoid consuming plastic
bags that resemble jellyfish, the turtles’ typical prey (Schlaep-
fer et al. 2002). Studies of ecological development, behav-
ioral syndromes, and ecological traps allow connections be-
tween population-based ecological approaches to behaviors
(e.g., foraging, predator-prey interactions) and developmen-
tal studies of individual variability, learning, and plasticity.
Such integrated studies could then fruitfully link early so-
cial and environmental experiences and behavioral plas-
ticity with reproductive outcomes, life-history parameters,
and fitness, and contribute to the growing interest in inter-
disciplinary research spanning multiple levels of analysis.SummaryOne of the hallmarks of rodent behavioral development is
its sensitivity to the social and physical contexts in which
ontogeny occurs. From a functional perspective, such sen-
sitivity makes adaptive sense if one views juvenile behav-
ioral patterns as ontogenetic or age-specific adaptations and
recognizes that the factors that influence survival and repro-
duction in one environment will often differ from those in
another environment. Further, behavioral ecologists should
not think of young animals as small adults, but as organ-
isms with age-specific needs and adaptations. Indeed, young
have very different selective pressures than do adults, and
thus what is functional for one age group may not be for
another.
Development influences almost all aspects of rodent bi-
ology, particularly social behaviors. This chapter presents a
necessarily broad survey of how anatomical, hormonal, and
behavioral development are affected by an animal’s physi-
cal and social environments through the life span, from the
prenatal period to death. For example, maternal stress, ges-
tational photoperiod, and androgen exposure from adjoin-
ing fetuses can cause variation in aggression, parenting be-
haviors, and learning abilities well into adulthood. Social
and physical environments after birth continue to influence
the expression of behaviors, as an animal’s needs and vul-
nerabilities adjust with increasing age. Natural selection
will favor such plasticity when an animal experiences spa-
tial or temporal changes in microhabitat, food availabil-
ity, predation pressure, social dynamics, and reproductive
status.
When an emerging trait is studied descriptively, research-
ers tend to focus on particular aspects of the experiences
of the developing individual in isolation from other experi-
ences, deflecting attention to the interactive nature of epi-
genesis. When traits are examined in context, however, re-Ontogeny of Adaptive Behaviors 205