Life-history traits of voles in a fluctuating population respond to the immediate environment

Life-history traits relating to growth and reproduction vary greatly among species and populations(1,2) and among individuals within populations(3). I n vole populations, body size and age at maturation may vary considerably a mong locations and among years within the same location(4-8). Individuals i n increasing populations are typically larger and start reproduction earlie r in the spring than those in declining populations(6-8). The cause of such life-history variation within populations has been subject of much discuss ion(7,9,10). Much of the controversy concerns whether the memory of past co nditions, leading to delayed effects on life-history traits, resides in the environment (for example, predators(11,12), pathogens(13) or food(14,15)) or intrinsically within populations or individuals (age distribution(16,17) , physiological state(3), genetic(18) or maternal effects(19,20)). Here we report from an extensive field transplant experiment in which voles were mo ved before the breeding season between sites that differed in average overw intering body mass. Transplanted voles did not retain the characteristics o f their source population, and we demonstrate an over-riding role of the im mediate environment in shaping life-history traits of small rodents.