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 and among individuals within populations. In vole populations, body size and age at maturation may vary considerably among locations and among years within the same location. Individuals in increasing populations are typically larger and start reproduction earlier in the spring than those in declining populations. The cause of such life-history variation within populations has been subject of much discussion. Much of the controversy concerns whether the memory of past conditions, leading to delayed effects on life-history traits, resides in the environment (for example, predators, pathogens or food) or intrinsically within populations or individuals (age distribution, physiological state, genetic or maternal effects). Here we report from an extensive field transplant experiment in which voles were moved before the breeding season between sites that differed in average overwintering body mass. Transplanted voles did not retain the characteristics of their source population, and we demonstrate an over-riding role of the immediate environment in shaping life-history traits of small rodents.