Does seasonal variation in the corticosterone response affect the nutritional ecology of a free-ranging lizard?

Challenging events in the environment that are both predictable (e.g.
seasonal patterns in breeding activities) and unpredictable (e.g. predator
encounter) are known to induce a glucocorticoid response that facilitates
metabolic requirements during the challenge.   Given its role in
mobilizing energy, glucocorticoid levels can influence the nutritional
ecology of an individual by shifting dietary intake or retention patterns,
but this relationship has not been tested in free-ranging vertebrates.
Using a tropical lizard species (Psammophilus dorsalis) as a model system,
we tested whether the elemental composition of dietary intake and
excretion (faecal samples) varies with stress-induced corticosterone
levels in males and females across different seasons. From free-ranging
lizards in the wild, we measured levels of stress-induced corticosterone
and glucose in blood and determined diet composition from gut-flushing.
Elemental composition of the diet was determined by analysing the carbon
and nitrogen content of identified prey Orders caught from the wild. We
also collected faecal samples and estimated their elemental composition.
We found that stress-induced corticosterone levels varied across seasons,
with the lowest levels during the breeding season for both males and
females. Despite high variation in corticosterone responsiveness, lizards
did not shift the elemental composition of their diets and maintained an
intake Carbon : Nitrogen ratio of 4.56. We did, however, find a negative
correlation between stress-induced corticosterone levels and faecal
elemental composition, suggesting selective retention of both carbon and
nitrogen in individuals that have higher corticosterone responsiveness.
This study highlights the interplay between corticosterone responsiveness
and nutritional ecology, challenging the existing links in literature, and
illustrating how free-ranging animals, such as lizards, adjust the
elemental composition of excretion and not dietary intakes as a potential
strategy to modulate natural physiological and ecological challenges.