Intraspecific aggression in beavers

Blog written by Martin Mayer. Read the full paper here. Picture of Eurasian beaver from study population in Norway by Martin Mayer.

Animals fight for different reasons, e.g., for access to food, territories or mating partners, to reinforce their social status within a group, or to fight off predators. Thus, by looking at what affects aggression between individuals, we can learn a lot about the underlying mechanisms driving population dynamics such as dispersal and mate change patterns. Often population density affects aggression, because crowded places lead to more competition between individuals. These effects of population density might be mediated by landscape structure, because landscape features can facilitate or inhibit connectivity between habitat patches. Thus, understanding the combined effects of landscape structure and population density is valuable to improve our understanding of intraspecific aggression and its consequences for population dynamics.

Beavers can tell us a lot about how landscape structure and population density affect intraspecific aggression, because they live in territories that they will defend by all means, especially in saturated populations. Moreover, they are social animals that live in single family groups that comprise the parents and their offspring. Young beavers leave their parents to establish a territory of their own, typically when they are about two years old, but they sometimes delay this dispersal process until as much as seven years old if population densities are high. This is because high beaver densities make it tough for young individuals to successfully obtain a territory for themselves. In order to detect available territories, individuals regularly make exploratory movements outside their parents’ territory, before leaving for good. Aggressive encounters are not rare and usually occur when a dispersing individual tries to take over the territory of an established beaver.

Here, we used long-term data from Eurasian beavers in Norway and North American beavers in the US to investigate the effects of population density and landscape structure on the number of tail scars, our measure for intraspecific aggression. These beavers live either in large lakes and rivers or in ponds. For a beaver, the main difference between these two landscape types is that in a large lake or river, you can freely move between territories, whereas if you live in a pond it’s tough to get to other areas, because it’s much harder to walk over land than swim. The Norwegian beavers, inhabiting well-connected rivers, had more tail scars compared to the beavers in the US. Within the North American beavers, individuals inhabiting large inter-connected lakes had more scars compared to individuals in ponds, where beavers had the fewest scars. This emphasizes that intraspecific aggression depends to a large degree on landscape composition. Lower levels of intraspecific aggression in ponds were likely caused by lower connectivity to other areas, in turn reducing the number of dispersing individuals reaching these places.

The different landscape types in the North American beaver population in Voyageurs National Park, showing a large river and lake (a) versus an isolated pond (b). Pictures: Thomas Gable (a) and Steve Windels (b).

Moreover, in the Norwegian population, where all territories were occupied, the number of tail scars decreased with increasing population density, but then slightly increased at the highest population densities. This pattern was likely related to inverse density-dependent dispersal, i.e., individuals in saturated populations delay dispersal to await lower population densities that increase their chances of obtaining a territory. In turn, this leads to high intruder pressure and thus intraspecific aggression at lower densities, and emphasizes that aggressive encounters might mediate natal dispersal decisions by individuals. The slight increase in intraspecific aggression at the highest population densities probably indicates that the sheer number of subordinates in the population led to increased intrusions. Conversely, in North America, aggression increased with increasing population density in well-connected large lakes, but population density had no influence on the number of tail scars in beavers that lived in ponds.

The tail of a North American beaver with a fresh bite mark from a conspecific. Picture: Steve Windels.

Interestingly, male territory owners had more tail scars than females in Eurasian beavers, but not in North American beavers. We speculate that these population differences were also caused by physical landscape differences during winter, the mating season of beavers. The study area in Norway remains ice-free for most of the year and never completely freezes over, allowing individuals to move freely throughout the year (though movement can be constrained by cold water). Conversely, the North American water bodies (both lakes and ponds) in Voyageurs National Park are covered by ice from December to April, preventing inter-territorial movement. Thus, in the ice-free Eurasian population, males will try to mate with females from neighboring territories, leading to fights between males (that either try to achieve extra-pair copulations or to defend their female) and consequently to the observed sex differences, whereas this is not possible for the beavers in the US where territories are separated by the icy conditions.

Our results show that landscape structure in interaction with population density is an important driver of intraspecific aggression in beavers, which is likely mediated by dispersal patterns. In well-connected landscapes increased levels of intraspecific aggression, caused by density-dependent dispersal, can lead to increased mate change and population turnover, in turn potentially regulating population densities. In less-connected landscapes, aggression appears to be density-independent, suggesting that other factors than agonistic encounters via dispersers drive population dynamics there, such as resource availability. The question arises how landscape structure and connectivity generally affect patterns of dispersal, mate change and territory occupancy, factors that will affect settlement patterns and reproductive success of individuals, and ultimately population dynamics. A better understanding of these processes could aid the management and conservation of wildlife species.

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