Density-Dependent Difference in Serum Levels of Immunoglobulin G in Brandt's Voles and Its Potential Influencing Factors.

Understanding the density-dependent immune response is an important theme in ecological and evolutionary studies. Animals may elevate their immune response with an increase in population density. There is substantial evidence supporting the density-dependent immune response in some insects, reptiles, and birds, but evidence is still lacking in wild rodents. Here, we tested the density-dependent immune response on Brandt's voles by manipulating their population change under both laboratory (with none or little parasite infection) and field conditions (with strong parasite infection). In the field experiment, we found that the parasite prevalence and infection intensity as well as the IgG levels increased with population density, suggesting evidence of density-dependent immune response. In the lab experiment, Brandt's voles in the high-density group experienced high crowding stress exhibited by a high frequency of locomotion and aggression, and they had a higher IgG level than those in the low-density group, but with no significant difference in parasite infection. Brandt's voles in the field had significantly higher parasite intensity and higher IgG levels than Brandt's voles in the lab. Sheep grazing and rainfall supplmentation increased IgG level but food supplementation had no significant effect on IgG level. Our study confirms density-dependent immune response in Brandt's voles, likely driven by increasing agressive behavior of voles and parasite transmission, and provides novel insight into density-dependent population regulation in small rodents oscillations.