FACTORS AFFECTING LIFE EXPECTANCY OF THE PRAIRIE VOLE, MICROTUS-OCHROGASTER

We determined life expectancy (to within approximately 3 d) of 1457 pr airie voles, Microtus ochrogaster, in a free-living population over a period of 6 yr. During this lime the population underwent four cycles of abundance, with peak densities of 160, 250, 350, and 624 individual s per ha. Life expectancy was calculated based on the interactions of season, phase of the population cycle, population density, type of nat al social group, and weather conditions 21 and 30 d before and after b irth. Life expectancy was greatest for animals born (1) in autumn, (2) during the increase phase oi the population cycle, (3) at population densities > 100/ha, and (4) into communal groups. The combination of t hese factors appears to result in a surge in population density in lat e autumn. The short life expectancy of animals born during the peak an d decline phases of the population cycle may accelerate the rate of th e decline phase. During periods of high temperatures before: birth in summer, life expectancy increased with increased precipitation. There aas no such interaction of precipitation and temperature after birth. In both summer and autumn, high precipitation after birth resulted in lower life expectancy, possibly reflecting increased mortality of youn g forced to leave flooded underground nests.