When access to resources is limited, organisms must shift energy investment among
physiological processes to survive, reproduce, and respond to unpredictable events.
The shifting of these limited resources among processes may result in physiological
tradeoffs, often mediated by glucocorticoids. We assessed relationships among the
physiological processes of immunity, reproduction, and the stress response in wild
adult red-eared slider turtles (Trachemys scripta elegans). Red-eared sliders exhibit
a multi-clutching reproductive strategy that requires high energetic investment in
reproduction at the beginning of the nesting season in females. Males mate in spring
and undergo spermatogenesis and mating in late summer/early fall. We expected to observe
tradeoffs when investment toward reproductive processes was particularly demanding.
To test this, we subjected 123 individuals to a standardized acute stressor and collected
blood to measure innate immunocompetence and circulating steroid hormone concentrations.
Tradeoffs between female reproduction and immunocompetence occurred early in the nesting
season. This high reproductive investment was evident by heightened circulating progesterone
and reduced baseline innate immunity. Corticosterone (CORT) was also high during this
period, indicating a role in facilitating allocation of energy. Tradeoffs were not
as evident in males, though males upregulated innate immune function, baseline CORT,
and testosterone prior to fall spermatogenesis and mating. Throughout the entire sampling
period, both males and females increased CORT and immunocompetence following the acute
standardized stressor. Taken together, we concluded that reproduction requires shifts
in energy allocation in during the highest reproductive period for females but all
individuals in this population remain able to respond to the standardized stressor
even during increased reproductive investment. These findings reinforce the continuing
evidence that physiological relationships are context-dependent and resource demands
are dynamic across the reproductive season.
