Immunocompetence of animals fluctuates seasonally; however, there is little consensus on the cause for these fluctuations, as some studies have suggested that these fluctuations are influenced by changes in reproductive condition, whereas others have suggested that differences result from seasonal variations in energy expenditures. The objective of our study was to examine these contrasting views of immunity by evaluating seasonal patterns of immune response and reproduction in wild populations of deer mice (Peromyscus maniculatus) exposed to Sin Nombre virus (SNV).
Over 3 consecutive fall (September, October, November) and 3 consecutive spring sampling periods (March, April, May), we used titration ELISAs to quantify virus specific antibody production in 48 deer mice infected with SNV. Levels of reproductive hormones were quantified using ELISA. Antibody titers reached their lowest level during November (GMT = 364) and their highest levels during September (GMT = 5545) and May (GMT = 1422), suggesting that the immune response of deer mice to SNV has seasonal patterns. The seeming decrease in antibody titer over winter coupled with the consistency in body masses suggests that during winter, immunocompetence may be compromised to offset the energetic costs of maintenance functions, including those associated with maintaining body mass. Deer mice showed distinct sex-based differences in SNV antibody production, with males producing higher antibody titers (GMT = 2571) than females (GMT = 555). Reproductive hormones appear to differentially affect antibody production in males and females, as female deer mice showed a significant positive correlation between estradiol concentrations and SNV antibody titer (r2 = 0.46), whereas there was no significant relationship between levels of testosterone and SNV antibody titers in males (r2 = 0.20). Collectively, this study demonstrates that immunocompetence of wild deer mice is seasonally variable; however, reproduction is not the primary stressor responsible for this variation. Rather, the data suggest that deer mice may compromise immunocompetence during winter to offset other maintenance costs during this period.