Social interactions serve as an evolutionarily important source of stress, and one
that is virtually ubiquitous among mammalian species. Animal models of social stress
are varied, ranging from a focus on acute, intermittent, or chronic exposure involving
agonistic behavior, to social isolation. The relative stressfulness of these experiences
may depend on the species, sex, and age of the subjects, and subject sex also appears
to influence the value of hypothalamic--pituitary--adrenal (HPA) axis activity as
a general criterion for stress response: higher glucocorticoid levels are typically
found in dominant females in some species. Social stress models often produce victorious
and defeated, or dominant and subordinate, animals that may be compared to each other
or to controls, but the appropriateness of specific types of comparisons and the interpretations
of their differences may vary for the different models. Social stress strongly impacts
behavior, generally reducing aggression and enhancing defensiveness, both inside and
outside the stress situation. Social and sexual behaviors may be reduced in subordinate
animals, as is activity and responsivity to normally rewarding events. However, some
components of these changes may be dependent on the presence of a dominant, rather
than representing a longer-term and general alteration in behavior. Social stress
effects on brain neurotransmitter systems have been most extensively investigated,
and most often found in serotonin and noradrenergic systems, with changes also reported
for other monoamine and for peptidergic systems. Morphological changes and alterations
of neogenesis and of cell survival particularly involving the hippocampus and dentate
gyrus have been reported with severe social stress, as have longer-term changes in
HPA axis functioning. These findings indicate that social stress models can provide
high magnitude and appropriate stressors for research, but additionally suggest a
need for caution in interpretation of the findings of these models and care in analysis
of their underlying mechanisms.