Azurocytes in wild field voles: factors associated with their occurrence.

The occurrence of azurocytes (AZ), a type of leukocyte unique to voles and previously described for three Microtus species, is now reported in Microtus agrestis. The goal of this study was to shed new light on the possible function and significance of these cells and on how they play a role in the natural history of rodent species. Individuals from three vole populations were sampled monthly for 2 years. A hemogram was produced for each individual, and AZ counts estimated. The counts of AZ were much higher in pregnant females, and these levels were higher the higher the past vole density. Males had low prevalences and counts, both for breeding and nonbreeding individuals, but they showed a seasonality that varied with age, body condition, and current and past vole density. Also, the occurrence of AZ in males was more likely after they had had low levels of indicators of condition, suggesting that azurocytes may result from a response to infection. Hence, overall our results suggest that, in females, these cells may be important for reproduction and may have a role in inducing abortion when conditions are not favorable, while in males they might be a response to infection.