The 7-year itch: non-adaptivemate change in the Eurasian beaver

Mate change in socially monogamous species can be explained by adaptive and non-adaptive hypotheses. Adaptive hypotheses describe a mate change as a strategy to correct for initial mate choice and to improve reproductive success: the 'incompatibility' hypothesis states that mate change is initiated by both mates, whereas in the 'better option' hypothesis, one partner initiates the mate change. In contrast, non-adaptive hypotheses predict mate change to be independent from previous reproduction: the 'forced divorce' hypothesis suggests that mate change is initiated by an incoming individual and the 'obligate mate change' hypothesis states that the mate change occurs after the accidental death of a partner. We investigated these hypotheses in the socially monogamous Eurasian beaver (Castor fiber), using data from a long-term study in southeast Norway between 1998 and 2014. Generally, the mate change occurred in the seventh year of a partnership and the staying individual re-paired with a younger, incoming individual. The fate of the replaced individual was mostly unknown. Resident individuals had a decreased reproductive success with increasing age but gained no benefits from a mate change in terms of reproduction. Thus, we reject the adaptive hypotheses as cause of mate change. Our results support non-adaptive mate change hypotheses, most likely the 'forced divorce' hypothesis and to a lower degree the 'obligate mate change' hypothesis. Significance statement We investigated the causes of mate change in the Eurasian beaver, a long-lived, monogamous mammal living in family groups. We found that mate change was not initiated by a member of the mated couple but rather by the intrusion of an incoming individual as suggested by the 'forced divorce' hypothesis. Additionally, mate change was partly caused by the accidental loss of a partner. Mean reproductive success did not change after a mate change but with increasing age of the resident individual suggesting senescence. Together with a study on Alpine marmots, this is one of the first studies investigating mate change in mammals.