Seasonal changes of pelage in the vole (Microtus agrestis). I. Correlation with changes in the endocrine glands

The vole (Microtus agrestis) has seasonal changes of pelage. Both summer and winter coats are composed of two types of hair: long, coarse, guard hairs and fine underhairs. A spring hair replacement produces a sparse coat with coarse guard hairs. An autumn replacement produces a dense coat with a greater number of underhairs and with finer guard hairs. Thus, there is a change both in the number of hairs (fewer in summer) and in the amount of hair secreted by the follicles (greater in summer). Accompanying these changes in pelage there are seasonal differences in activity of the gonads, and there is also evidence that the pituitary, thyroid, and adrenal glands are more active at the time of the spring molt than at the autumn molt.