Diel activity patterns of northern Australian small mammals: variation, fixity, and plasticity

The temporal niche has received less attention than the spatial niche in ecological research on free-ranging animals. Most studies that have examined the effect of season on the diel activity patterns of small mammals have been conducted in temperate climates where daily temperatures and day length are important predictors of activity. Extremely seasonal rainfall in northern Australia possibly exerts a strong influence on mammalian activity due to the influx of food resources. Using camera traps set over a 3-year period, we documented the diel activity patterns of 5 species of small mammals co-occurring on Groote Eylandt, in the wet-dry tropics of northern Australia. All species were strictly nocturnal but some responded differently to the effect of season. The northern quoll (Dasyurus hallucatus) displayed a bimodal activity pattern that did not differ between the seasons. The northern brown bandicoot (Isoodon macrourus) displayed bimodal activity in the wet season and unimodal activity in the dry. The more sustained activity of I. macrourus in the dry season may be the result of this species utilizing more cellulose-rich food in times of lower insect abundance, whereas D. hallucatus possibly exhibits lower dietary plasticity. The northern hopping-mouse (Notomys aquilo) was consistently active throughout the night in both seasons. Conversely, the delicate mouse (Pseudomys delicatulus) showed great plasticity in its nocturnal activity which altered significantly depending on both season and habitat. The disparity in activity pattern between these 2 rodents possibly reflects differences in predation risks. The grassland melomys (Melomys burtoni) was recorded only during the dry season in coastal grassland habitat, when its activity peaked sharply after nightfall. Our study highlights the interspecific variation in small mammal activity between the wet and dry seasons in northern Australia, which may be explained by differences in diet, habitat use, and predation risk in these species.