Three main hypotheses have been proposed to explain mate switching in monogamous species: the "better option" hypothesis, the incompatibility hypothesis, and the "forced divorce" hypothesis. We tested the predictions of these hypotheses for the first time in a monogamous mammal using long-term data from a natural population of Alpine marmots ( Marmota marmota). Generally, pair disruption resulted in one of the pair members staying on the territory and re-pairing with a younger incomer, whereas the other disappeared from the territory. Replaced individuals were rarely found as dominant in a territory but were often injured or found dead. Individuals gained no benefit from mate switching: new mates were neither heavier, larger, or more heterozygote nor more genetically compatible than previous mates. Moreover, no increase in reproductive success was observed after re-pairing. The relationship between reproductive failure and occurrence of mate change was mainly due to infanticide by the incomer. Our results support the "forced divorce" hypothesis in the Alpine marmot and suggest that mate switching has strong consequences on breeding success. We discuss the importance of taking into account the cases of forced divorce while studying mate switching process and its evolutionary consequences in monogamous species.