Intraspecific competition plays an important role for territory acquisition and occupancy, in turn affecting individual fitness. Thus, understanding the drivers of intraspecific aggression can increase our understanding of population dynamics. Here, we investigated intraspecific aggression in Eurasian (Castor fiber) and North American (C. canadensis) beavers that are both monogamous, territorial mammals. Combined, we examined tail scars from >1000 beavers (>2000 capture events) as part of two long-term studies in Norway and the USA. We investigated the influence of landscape structure, population density, sex, age, and (for Eurasian beavers only) social status and group size on the number of tail scars caused by conspecifics. The number of tail scars was affected by population density in well-connected landscape types (large lakes and rivers), but not in more isolated areas (ponds), where individuals generally had fewer tail scars. Further, the relationship of population density was not linear. In the North American beaver population occurring in large lakes, intraspecific aggression increased with population density. Conversely, in the saturated Eurasian beaver population, intraspecific aggression was in a negative relationship with population density (except at the highest densities), likely due to inverse density-dependent intruder pressure via dispersers. Our findings emphasize that population density can affect intraspecific aggression depending on landscape structure, which might have important consequences for local patterns of dispersal, mate change and territory occupancy, all of which can affect population dynamics.

Tail scar data based on two individual-based long-term research projects on beavers.

'Area' is the area where beavers were captured (NAM = Kabetogama Lake; RAINY = Rainy Lake; PONDS = ponds; see Fig. 2 in the publication). 'Scars' shows the number of tail scars. Population density ('pop_dens') is shown as number of individuals for Eurasian beavers ('Species' = EU_beaver) and as number of lodges per km shoreline for North American beavers (NA_beaver). 'Weight' is shown in kg.