Physiological state is an emergent property of the interactions among physiological
systems within an intricate network. Understanding the connections within this network
is one of the goals in physiological ecology. Here, we studied the relationship between
body condition, two neuroendocrine hormones (corticosterone and insulin-like growth
factor 1 [IGF-1]) as physiological regulators, and two physiological systems related
to resource metabolism (glucose) and oxidative balance (malondialdehyde). We measured
these traits under baseline and stress-induced conditions in free-living house sparrows
(Passer domesticus). We used path analysis to analyze different scenarios about the
structure of the physiological network. Our data were most consistent with a model
in which corticosterone was the major regulator under baseline conditions. This model
shows that individuals in better condition have lower corticosterone levels; corticosterone
and IGF-1 levels are positively associated; and oxidative damage is higher when levels
of corticosterone, IGF-1, and glucose are elevated. After exposure to acute stress,
these relationships were considerably reorganized. In response to acute stress, birds
increased their corticosterone and glucose levels and decreased their IGF-1 levels.
However, individuals in better condition increased their corticosterone levels more
and better maintained their IGF-1 levels in response to acute stress. The acute stress-induced
changes in corticosterone and IGF-1 levels were associated with an increase in glucose
levels, which in turn was associated with a decrease in oxidative damage. We urge
ecophysiologists to focus more on physiological networks, as the relationships between
physiological traits are complex and dynamic during the organismal stress response.
