TEMPORAL VARIATION IN HUMORAL AND CELL-MEDIATED IMMUNE-RESPONSE IN A SIGMODON HISPIDUS POPULATION

We examined the hypothesis that environmentally induced alteration of immunocompetence is a primary physiological mechanism by which populat ion survival is regulated. The involvement of disease and immunocompet ence in the regulation of small mammal populations is not well underst ood but a strong relationship between physiological stress and immunit y has been demonstrated. Its involvement is further supported by sever al studies, which have provided evidence that survival, incidence of d isease, and selected gross measures of immunity change temporally with density and season. We examined this hypothesis by documenting the ex istence of temporal alterations in selected measures of both humoral a nd cell-mediated immune function in a population of cotton rats (Sigmo don hispidus), a species known to fluctuate widely in density, inhabit ing a tallgrass prairie in central Oklahoma. A total of 310 cotton rat s was collected from August 1989 to May 1991 (10 collections) using co nventional live-trap methods. In addition to morphological characteris tics of primary and secondary lymphoid organs, the ability of splenocy tes to produce specific antibody in response to a single injection of sheep erythrocytes and to proliferate in response to in vitro exposure to mitogens was measured to assess immunocompetence. Whole blood was collected for the determination of total peripheral blood leukocyte co unts, packed cell volume, and total serum proteins. Statistically sign ificant differences in all variables except total serum protein existe d among the 10 collections and most of this variability was independen t of specific seasonal time periods. Bimodal rhythmicity was character istic of temporal changes for several measures of immunocompetence. Ch anges over time in the average immunocompetence of this population cou ld have reflected a shift in genotypic polymorphism; several other pla usible explanations are offered to explain the observed temporal patte rns.