Many biological variables related to energy turnover including torpor, the most efficient energy-saving mechanism available to mammals, scale with body size, but the implications for animals living in their natural environment remain largely unknown. We used radio-telemetry to obtain the first data on the activity patterns and torpor use of two sympatric, free-ranging dasyurid marsupials, the stripe-faced dunnart Sminthopsis macroura (16.6±1.5 g) and the more than six-times larger kowari Dasyuroides byrnei (109.4±16.4 g), during winter in arid Queensland, Australia. Eight dunnarts and six kowaries were surgically implanted with temperature-sensitive radio-transmitters and monitored for 14–59 days. Both species commenced activity shortly after sunset, but while kowaries remained active through most of the night, dunnarts usually returned to their burrows before midnight. In dunnarts, short activity was associated with the frequent use of daily torpor (99.1% of observation days). Torpor often commenced at night, with body temperature (Tb) decreasing to a minimum of 11.3 °C, and torpor lasted up to 26 h. In contrast, only 50% of the kowaries entered torpor and torpor was brief (maximum 4 h), shallow (minimum Tb 25.3 °C) and restricted to the daytime rest-phase. Our study suggests that in winter, the smaller dunnarts can remain active only during the warmer first half of the night and energy-saving torpor becomes part of their daily routine. In contrast, it appears that the larger kowaries are less affected by cold winter nights and can maintain high night-time activity levels and commence reproduction already in winter. Hence, they enter torpor only occasionally and only during the rest phase.