The Reproductive Cycle of the Field Mouse, Microtus californicus

The cricetid rodent Microtus has been studied extensively in recent years due to its widespread geographic distribution and to the cyclic abundance shown by most of its species. Reports are available on certain aspects of the reproduction of Microtus californicus (Hatfield, 1935), M. agrestis (Brambell and Hall, 1939), M. pennsylvanicus (Hamilton, 1941), and M. guentheri (Bodenheimer, 1949). With these exceptions, the remaining work is primarily concerned with the population dynamics of Microtus. The present report, based on work in the field and in the laboratory, describes some of the anatomical and physiological features of the reproductive cycle of females of Microtus californiens and emphasizes certain aspects in which M. californicus differs from other microtines or from other rodents. The present study is based on dissection of 581 female Microtus californicus collected over a 21- month period from 1951 to 1953. Most of these animals were trapped in grassland adjacent to San Francisco Bay, near San Rafael, Marin County, California. Approximately 50 were kept in captivity. Appropriate reproductive tracts were preserved in Bouin's fluid, paraffin embedded by the dioxane method, sectioned at 10 micra, and stained with Delafield's hematoxylin and eosin.