Temporal dynamics of dehydration differentially regulate components of innate immunity

Hydration state is an important regulator of vertebrate physiology, yet the temporal dynamics of hydration-associated immune responses remain poorly understood. We investigated how progressive dehydration and repeated dehydration-rehydration cycles influence innate immunity in Antaresia childreni (Children's pythons). Snakes were assigned to either a hydrated control group or a dehydration-treatment group, and plasma osmolality, agglutination, lytic ability, and bacterial killing ability were measured throughout two complementary experiments. Progressive dehydration produced stepwise increases in plasma osmolality, whereas immune responses changed later and differed among immune components. Agglutination and lytic ability increased during later stages of dehydration, whereas bacterial killing ability increased only at peak dehydration; following rehydration, no immune measure differed significantly between treatment and control animals. During repeated dehydration-rehydration cycles, plasma osmolality increased during both dehydration events and returned following rehydration, whereas immune responses differed in their persistence and repeatability. Agglutination remained higher than controls throughout both dehydration-rehydration cycles, lytic ability increased during both dehydration events but declined following rehydration, and bacterial killing ability increased during the first but not the second dehydration event. These findings demonstrate that innate immune function is influenced not only by dehydration, but also by the temporal dynamics and recent history of water limitation. More broadly, our results emphasize that physiological responses to environmental change cannot always be inferred from endpoint comparisons alone.