The winter immunoenhancement hypothesis: associations among immunity, density, and survival in prairie vole (Microtus ochrogaster) populations

Seasonal variations in photoperiod, temperature, and population density hav e been shown to modulate immune responsiveness of animals in laboratory stu dies. To examine these associations under natural conditions, we monitored 3 populations of prairie voles (Microtus ochrogaster) for temporal variatio ns in selected immunological parameters, population density, and survival r ate from winter 1996 to spring 1997. Spontaneous and cytokine-stimulated T- cell proliferative responsiveness of prairie voles peaked in winter and dec lined in spring. Relative organ mass, hemolytic-complement activity, and in vivo hypersensitivity responses varied temporally but showed no clear seas onal trend. The population density and survival rate of all 3 prairie vole populations varied temporally and correlated with measures of immunity. Mul tiple regression analysis indicated that the model containing relative sple en mass, cytokine-stimulated T-cell proliferation, and in vivo hypersensiti vity explained a significant amount of variability in population density, w hile cytokine-stimulated T-cell proliferation and relative thymus mass expl ained a significant amount of variability in survival rate. The results sug gest that seasonal environmental changes can enhance immune responsiveness of a host and may counteract the immunoenhancing effects of photoperiod in wild populations of prairie voles. Our results also suggest that there is a n association between immune function and demography in wild populations.