Variability of immune response to heterologous erythrocytes during population cycles of red (Clethrionomys rutilus) and bank (C-glareolus) voles

Immunocompetence is regarded to be one of the principal links in mechanisms of population regulation in small mammals. We assessed immune responsiveness to injection of sheep red blood cells (SRBC) in sympatric populations of red and bank voles in southern West Siberia, Russia, during the cycle of density from 1991 to 1995. We used a splenic plaque-forming cells (PFC) technique to measure the immune response to the antigen. In an attempt to establish relationships of immunity with intrinsic factors involved in population regulation we monitored the following parameters: population numbers, recruitment rate, and adrenocortical activity. The analysis of covariance showed a significant effect of reproductive status of voles and independent significant effects of month and year of capture on immune response to SRBC. In general, immature young was significantly higher responders as compared to mature (breeding) adults. The seasonal and long-term variations in numbers of PFC were similar in both species. Seasonal minimum of the immune response was observed within the period of intensive breeding in June and July. Among the years the immune responsiveness was maximal in the phase of population depression. The increase in maturation rate as density increased coincided with a decrease in immune responsiveness. The correlation analysis showed little or no evidence of links between the immunity and adrenocortical activity in the populations under study. Based on the patterns of seasonal and long-term variations of surveyed variables it is likely that physiological mechanisms governing the processes of sexual maturation and reproduction of red and bank voles are involved in the regulation of immunity.