Carrion use by a reptile is influenced by season, habitat, and competition with an apex mammalian scavenger

Scavenging on carrion is critical and often fiercely competitive for a range of vertebrate species, from native apex predators to invasive species and even reptiles. In Australia, a notable reptilian scavenger is the lace monitor (Varanus varius). In this study, we quantified lace monitor activity at carcasses and compared their use of the resource to common co-occurring predators that also scavenge; the invasive red fox (Vulpes vulpes), and native apex predator, the dingo (Canis dingo). To do so, we deployed 80 macropod carcasses equally across seasons (summer and winter) and habitats (open and closed canopy), in a temperate bioregion and monitored vertebrate scavenging with camera traps. Lace monitor activity was 1.67-times higher in summer than winter, but it did not differ across closed and open habitats. Monitor activity occurred earlier after carcass deployment at sites deployed in summer than winter (1.47-fold earlier), and at carcasses in open than closed habitats (0.22-fold earlier). Lace monitors initially discovered carcass sites faster in summer than winter and before both red foxes and dingoes in summer. Lace monitors were active diurnally in both summer and winter, differing from the red fox, which was strictly a nocturnal scavenger, and the dingo, which was significantly more active at night across both seasons. Finally, we found that lace monitor activity at carcass sites decreased slightly with higher rates of activity for dingoes (0.04-fold decrease as dingo activity increased), but not with red fox activity. Our results have implications for understanding lace monitor foraging and scavenging and highlight the value of monitoring carcasses to provide important insights into the behaviour of varanid lizards that scavenge.

Methods
This dataset was collated by placing 80 eastern grey kangaroos (Macropus giganteus) carcasses within open and closed habitats and winter and summer seasons over four independent sampling periods (n = 20 in each deployment period, n = 10 in open and closed habitats in each deployment period). The study area was the Wolgan Valley in the west of the Sydney Basin Bioregion, NSW, Australia. Carcasses were monitored with camera traps and all scavengers recorded were identified using the program Digikam. We then used the package 'camtrapR' within R to extract the tagged images and created independent 'events' for each species occurrence. These events were classified as when an animal entered the frame of the camera and ended when the animal left the frame. Unique events were considered to be when more than ten minutes elapsed between events of the same species or when a different species entered the camera frame within 10 minutes of another. We then filtered this dataset to only include lace monitors, red foxes, and dingoes. Carcass sites were monitored for 30 days continuously.