Contrasting patterns of female house mouse spatial organisation among outbreaking and stable populations

The size and distribution of home ranges reflects how individuals within a population use, defend, and share space and resources, and may thus be an important predictor of population-level dynamics. In eruptive species like the house mouse in Australian grain growing regions, contrasting space use between stable and outbreaking populations allows us to test predictions regarding social or life history strategies that may contribute to an outbreak. In this study we use spatially explicit capture-recapture models to compare home range overlap (as a proxy for territoriality) in female mice from populations showing different outbreak trajectories. We found that female space use in spring varied between outbreaking and stable populations. Our analysis indicated greater home range overlap in populations with stable trajectories compared to those that would later experience an outbreak, suggesting females in these stable populations may have had greater potential for cooperative group formation as indicated by shared space use. We discuss the results with respect to intrinsic factors, such as kin structure, with potential implications for better predicting mouse outbreaks at a microgeographic scale.

Methods
The data used for this study were collected from grain paddocks located near Mallala on the Adelaide Plains, South Australia (SA) (-34° 26' 59.99" S, 138° 29' 59.99" E) (paddocks A and B,) and near Parkes, Central West, New South Wales (NSW) (33° 8' 12.56'' S, 148° 10' 22.93'' E) (paddocks C and D). In each paddock (n= 4), two independent live-capture trapping grids, each separated by a minimum of 100 m were established (n= 8 trapping grids). On each live trapping grid, 64 single capture Longworth traps (25 × 6.5 × 8.5 cm, Longworth Scientific, Abingdon, UK) were set at 10 m intervals on an 8 x 8 grid. Traps contained polyester fibre bedding and wheat grains for food. They were checked and closed each morning starting at approximately 0630 hours (h) and opened in the evening at 1700 h. Traps are designed for single capture, however during this study some traps had multiple captures on a single trap night. 

Near Mallala SA (paddocks A and B), trapping data were collected between October 2019 and February 2020, across four trapping sessions. Near Parkes NSW (paddocks C and D), trapping data used here is from a subset of data collected between November 2020 and February 2021, across four trapping sessions. Sessions are defined by trap grid and the corresponding trapping session (e.g., A1.1 or A1.2).

Each trapped mouse had a radio frequency identification passive integrated transponder (PIT) tag (MiniHPT8 8 mm x 1.4 mm; Biomark, Inc.) inserted subcutaneously between the scapulae allowing individuals to be uniquely identified through time. For each trapped individual, we recorded data on trap location, PIT tag identification number, weight, length, sex. All data were collected under the approval of the CSIRO Wildlife, Livestock, and Laboratory Animal Ethics Committee under the approved application AEC 2019-18.