Causes and consequences of inverse density‐dependent territorial behaviour and aggression in a monogamous mammal

Abstract

Territoriality is an important process shaping population dynamics, and the defence of a territory is crucial for individuals to increase the duration of territory occupancy and, consequently, reproductive success. However, little is known about how the frequency of territory intrusions and subsequent territorial behaviours and aggression by territory owners are affected by external factors, such as population density. This is important because it can affect mate change (the replacement of one pair member) and dispersal, a key ecological process. The aim of this study was to investigate the behavioural and spatial response of territory owners to intruder pressure as a function of population density in a territorial, monogamous mammal, the Eurasian beaver (Castor fiber). Using a combination of GPS technology, scent experiments, camera trap data and tail scar observations from an individual‐based long‐term study, we investigated the factors influencing spatial movement patterns by territory owners in response to a simulated intruder and the factors affecting territory intrusions. We found consistent inverse density‐dependent patterns in territorial behaviours and evidence of conspecific aggression. At lower densities, territory owners detected more simulated intrusions, showed more territorial reactions and experienced increased conspecific aggression as indicated by tail scars, suggesting increased intruder pressure. Inverse density‐dependent territorial behaviour and aggression suggest a potential mechanistic link between inverse density‐dependent natal dispersal and mate change. At low population densities, increased dispersal amplifies intruder pressure, leading to the observed increases in territorial behaviours, conspecific aggression and previously observed mate turnover, which in turn might increase natal dispersal. Our study demonstrates how population density can affect the behaviour and space use of individuals, which is important for territory occupancy and fitness.

Ecological forecasts reveal limitations of common model selection methods: predicting changes in beaver colony densities

“…Territoriality between beaver colonies may therefore function as a mechanism of site‐dependent regulation (Rodenhouse et al 1997), causing average fecundity rates to decrease as population density increases (as previously found by Payne 1984, Pietrek et al 2017). However, a recent study suggests levels of conspecific aggression may actually be inversely density‐dependent as a result of delayed dispersal at high densities (Mayer et al 2020), which may result in lower rates of change at higher densities.…”

Outsized effect of predation: Wolves alter wetland creation and recolonization by killing ecosystem engineers

“…2 and 4). Because dispersing beavers are primarily solitary individuals (45), the only way for a newly created or recolonized pond to persist once a wolf kills a dispersing individual is if another dispersing beaver reaches that pond and continues to maintain the dam. Our work suggests that such a scenario is rare and that once a Fig.…”

Landscape structure and population density affect intraspecific aggression in beavers

“…We counted the family group size for every territory in our study area using a combination of live captures and observations, allowing us to calculate the local population density (hereafter population density) separately for each river and each year (Figure 3), defined as the number of individuals per km shoreline (calculated for both sides of the river) (Mayer et al, 2020).We recorded tail scars every time we (re)captured an individual.…”

“…Population density can also affect aggression in beavers. Mayer et al (2020) demonstrated that territory owners have more tail scars at lower population densities due to increased intruder pressure via dispersers, because dispersers are more likely to emigrate at lower population densities and remain in their natal family group at high densities when the chances of obtaining a territory are low (Mayer, et al, 2017;Mayer Zedrosser et al, 2017c). Moreover, landscape structure can influence intraspecific aggression, with beavers in large rivers having more injuries compared to ones in smaller streams (Crawford et al, 2015).…”