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.

Methods
We sampled adult P. dorsalis lizards of both sexes across three seasons (pre-breeding, early breeding, and late breeding) and multiple sites. We extracted 30-min handling stress-induced blood samples to quantify circulating glucose and corticosterone levels. Lizards were also gut flushed in the field to identifiy prey eaten in the wild to the level of Order. Identified prey Orders were later captured from the wild and subjected to a CHNS analysis to determine their carbon and nitrogen content. We used the absolute prey numbers identified in the diet, prey elemental content and prey mean weights to determine the total dietary elemental composition in terms of Carbon and Nitrogen. We also collected fecal samples from a separate group of lizards to determine the elemental composition (C and N) of feces. Additionally, we used pit-fall traps and sticky traps at the various sampling sites to determine the natural prey abundance.