Modified open field tests
We employed modified open field tests (also referred to as emergence tests: see Brown & Braithwaite 2004; López et al. 2005; Carteret al. 2013; Jolly et al. 2019) to assess boldness in grassland melomys and whether the arrival of a novel predator resulted in behavioural shifts in invaded populations. All open field tests were conducted on the night after the last trap night (night 5) and in opaque-walled experimental arenas (540mm x 340mm x 370mm). Experimental arenas were modified plastic boxes that had an inverted Elliott trap sized hole cut in one end and were illuminated by strings of red LED lights (Jolly et al. 2019). Each experimental arena had natural sand as substrate, and a rolled ball of universal bait (peanut butter, oats and honey) located both in the centre and along one wall of the arena (Jolly et al. 2019). After dark, Elliott traps containing a melomys were inserted into the hole in the side of each experimental arena and melomys were allowed to habituate for 10 min. At the start of each trial, Elliott trap doors were locked open—the inverted orientation of the trap prevented them from being triggered closed. Melomys were given 10 min to explore the open field arena. After 10 min, individuals were rounded back into their retreat (the Elliott trap) and a novel object (standard red, plastic disposable bowl) was placed at the end of the arena opposite the Elliott trap (Jolly et al. 2019). Melomys were then given a further 10 min to explore the arena and interact with the novel object. Elliott traps remained open during the open field tests and melomys could shelter and emerge from them under their own volition. All trials were recorded using a GoPro HERO 3. A previous study in this system determined that melomys showed repeatable behaviour between trials (boldness: R [± 95%CI] = 0.67 [0.47, 0.80], P < 0.001; emergence time: R[± 95%CI] = 0.73 [0.53, 0.83], P < 0.001; novel object: R [± 95%CI] = 0.61 [0.209, 0.974],P < 0.001; Jolly et al. 2019), therefore the data presented in this study were from a single behavioural trial of each animal (n = 146). Once trials were complete, each melomys was released at its point of capture.
To measure the boldness of individual melomys, we scored three behaviours typically associated with boldness and neophobia in rodents (Dielenberg & McGregor 2001; McGregor et al. 2002; Réaleet al. 2007; Cremona et al. 2015): whether melomys fully emerged from their Elliott trap hide and entered the open arena during the 0–10 min period (scored 0 or 1, respectively); whether they fully emerged and entered the trial arena during the 10–20 min period (scored 0 or 1); and whether they interacted (touched) with the novel object that was placed in the arena during the 10–20 min period (scored 0 or 1). Videos were scored by a single observer who was blind to each melomys’ origin and identity. Because interacting with the novel object was predicated on a melomys’ willingness to emerge from their hide during the 10–20 min period, for analysis we combined their emergence during this period and interaction with the novel object into a single binary score: 0 (neophobic) = did not emerge or emerged but did not interact with novel object; or 1 (not neophobic): emerged and interacted with novel object.