Condition and diet of cycling populations of the California vole, Microtus californicus.

-Populations of the California vole, Microtus californicus, were followed at two sites near San Francisco Bay in order to investigate changes in diet and condition of these cycling microtine rodents. Mark and recapture analyses provided information on condition during seasonal (season to season) and annual (year to year) fluctuations, and snap trapping provided stomach samples for dietary analyses and carcasses for determinations of reproductive condition and fat reserves. Seasonal changes in condition were found to be correlated with dietary changes. The end of the breeding season in late spring was associated with low growth rates, low survival rates, and low fat reserves. At the same time the vegetation was drying, and the diet changed from one dominated by grass stems and leaves to one dominated by grass seeds. Breeding and adult growth sometimes began before the autumn rains while grass seeds were still the major dietary item, but occasional meals of green forbs and perennial grass were also taken at this time. Three annual grasses, Lolium multiflorum, Avena fatua, and Bromus rigidus, were preferred foods and formed the bulk of the winter diet at both low and high populations. The standing crop and seed production of these grasses were severely reduced by high vole populations. Reduction in food availability and quality might have caused the delay in the start of the breeding season, the low fat reserves and the continued population decline that were observed after the peak population. These results indicated that nutritive factors could be involved in microtine population cycles at low latitudes and point to the need for further research on diet quality and nutritional physiology of Microtus. California voles (Microtus californicus) undergo 2to 4-year cycles of abundance as do many other microtine rodents. There is no general agreement on the causes of population declines or the causes of the delay in recovery of the populations after a decline. Christian and Davis (1964) attributed the decline of high populations to endocrine exhaustion owing to increased behavioral interactions. Chitty (1955, 1967) and Krebs (1964) also thought that mutual interference is important, but they postulated selection for aggressive animals with low viability at high populations as the cause of declines. Predation was supported by Pearson (1966) as a major influence causing the delay in population recovery but he thought it insufficient to cause the decline. Lack (1954) suggested that microtine populations crash when food supplies deteriorate and that recovery of the population is delayed until there are sufficient nutrients. Supporting evidence for this nutrient-recovery hypothesis was reported by Pitelka (1957a, 1957b) and Schultz (1964). Which factors are important for production of microtine cycles will remain in doubt until suitable field experiments are completed, but a start in that