Extra-territorial movements differ between territory holders and subordinates in a large, monogamous rodent

Abstract

Territorial animals carry out extra-territorial movements (forays) to obtain pre-dispersal information or to increase reproductive success via extra-pair copulation. However, little is known about other purposes and spatial movement patterns of forays. In this study, we GPS-tagged 54 Eurasian beavers (Castor fiber), a year-round territorial, monogamous mammal, during the non-mating season. We investigated forays in territory-holding breeders (dominants) and non-breeding (subordinate) family members. Twenty of 46 dominant individuals (44%), and 6 of 10 subordinates (60%) conducted forays. Generally, beavers spent between 0 and 11% of their active time on forays, travelled faster and spend more time in water when on forays compared to intra-territorial movements, suggesting that forays are energetically costly. Further, beavers in smaller territories conducted more forays. Possibly, smaller territories might not have sufficient resources and thus dominant individuals might conduct forays to assess possibilities for territory expansion, and potentially for foraging. Generally, besides territory advertisement (e.g. via scent-marking), forays might serve as an additional mechanism for territory owners to assess neighbours. Subordinates spent more time on forays, moved greater distances and intruded into more territories than dominant individuals did, suggesting that they prospected to gain information on the population density and available mates before dispersal.

Figure 1

The location of our study area in southeast Norway (top right), and our study species, the Eurasian beaver, with a GPS on its back (top middle). The main map shows exemplary GPS data from a subordinate individual who conducted extra-territorial movements (ETM, red triangles) through five different territories (shown with grey shadings). The territory is shown in red hatching and intra-territorial GPS positions (ITM) are shown as red dots. The figure was created using ArcMap 10.1 (Esri, Redlands, CA, USA, http://www.esri.com/arcgis/about-arcgis). Source of small map (top right): National Geographic, Esrif DeLorme.

Figure 2

Predicted relationship (solid line) between the territory size and the number of forays of 54 GPS-tagged Eurasian beavers in southeast Norway (2009–2016). Dashed lines represent upper and lower confidence intervals.

Figure 3

Box plots showing (a) the proportion of time spent in water and (b) the travel speed (both averaged per individual) for extra-territorial and intra-territorial movements of 25 GPS-tagged Eurasian beavers in southeast Norway (2009–2016). The box plots show the median, 25th and 75th percentile, the range of the data and outliers (dots).

Figure 4

Box plot showing the distance moved (in m) on individual extra-territorial movements separately for dominant (N = 20 individuals, 43 forays) and subordinate (N = 6 individuals, 8 forays) GPS-tagged Eurasian beavers in southeast Norway (2009–2016). Box plots show median values, 25th and 75th percentile, the range of the data and outliers (dot).