Minding your own business: low pair cohesion in a territorial, monogamous mammal

Abstract

Social interactions among individuals play a central role in behavioural ecology and studying cohesion of monogamous pairs is important to understand their social relationships. Here, we tested five hypotheses to investigate patterns of pair cohesion in an obligate monogamous mammal, the Eurasian beaver, Castor fiber. We hypothesized that pair cohesion is related to the spatial structure of the territory, territory defence, resource competition, offspring provisioning and the age difference between pair members. We analysed GPS data of 14 beaver pairs and calculated the distance between pair members, and the proportion of territory and core area overlap. While overall territory size was similar between partners and overlap was high, the distance between pair members was large, averaging 500 m, and increased with increasing territory size. Moreover, the proportion of core area overlap was comparatively low and decreased with territory size, also indicating low pair cohesion. Pair members were closest at the beginning and end of their nightly activity periods, suggesting that variation in pair cohesion was caused by the spatial structure of territories with the lodge serving as a central place. We found no support for the other hypotheses, but the overall low pair cohesion might nevertheless serve to increase territory defensibility and reduce intrapair competition for resources. Ultimately, low pair cohesion might be caused by the obligate monogamous mating system of beavers, leading to the defence of spatially structured territories rather than the defence of a partner.