Data set showing the number of tail scars of North American and Eurasian beavers

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.