SUPPRESSED BREEDING IN THE FIELD VOLE (MICROTUS-AGRESTIS) - AN ADAPTATION TO CYCLICALLY FLUCTUATING PREDATION RISK

densities of microtine rodents and their main predators, small musteli ds, fluctuate synchronously in 3-5-year cycles in central and northern Fennoscandia. Predation by small mustelids has been suggested as one of the driving forces in microtine cyclicity, causing deep synchronous declines of several vole species. We studied experimentally the effec ts of small mustelids on mating behavior, foraging, and breeding in no nwintered field voles (Microtus agrestis) originating from a cyclic po pulation. By using mustelid odors, we simulated a crash phase environm ent with high predation risk for breeding pairs of voles. In our exper iments, 87% of the female field voles suppressed breeding when exposed to mustelid odors. Both female and male behavior changed, and no mati ng behaviors were observed under the simulated predation risk. Weights of both sexes decreased when exposed to mustelid odor, probably due t o decreased foraging; weights of the control females increased due to pregnancy; and no weight changes occurred in control males. Decreased breeding and foraging possibilities under high predation pressure may form the basis for the ultimate explanation for breeding suppression. There are at least two different mechanisms for breeding suppression: either mating does not take place or malnutrition in females does not allow breeding to occur. Delayed breeding under high risk of predation , for whatever reason, could increase the probability of individuals, especially that of the females, to survive over the crash to the next, safer breeding season when their young would have better possibilitie s to survive.