<p>Daily torpor and hibernation in adult mammals and birds have been extensively studied, but there is still much to learn about these fascinating physiological states. Because torpor in marsupials is widespread, partially because many marsupial species are small in size and heterothermy is common in small species, a growing number of studies have successfully investigated the use of heterothermy in this group. </p> <p>However, most available data on heterothermy in marsupials exist for adults, and very little for growing young, mostly due to the challenges of measuring body temperature (T_b) in very small animals. There is currently much well-placed interest in the relationship between heterothermy and climate change, and how increasing air temperatures (T_as) may influence the use of heterothermy in these species. However, an animal may be most at risk of being negatively affected by increasing or variable T_as when it is still developing, especially in the case of marsupials which have potentially vulnerable altricial young. While some studies have investigated the development of thermoregulation in these tiny animals, continuous temperature measurements, that do not disturb the animal, have not been obtained due to the lack of appropriate technology. </p> <p>The species chosen for my study were fat-tailed dunnarts (<i>Sminthopsis crassicaudata</i>) and stripefaced dunnarts (<i>S. macroura</i>), which are small marsupials from the family Dasyuridae (carnivorous marsupials). These two species are common in the wild and are ideal for laboratory work as they are easy to maintain and breed in captivity. The central theme of my study was the development of thermoregulation and torpor in very small, developing dunnarts. My first aim was to find a method of taking continuous measurements of T_b in very small animals that would not interrupt torpor use, as conventional transmitters are too large to be used in such small animals. By testing and confirming the reliability and accuracy of small temperature-sensitive transponders, I was able to use these to obtain continuous T_b readings in animals only 60 days (d) old, at approximately 8 g, and still in the nest.</p> <p>The second aim of my study was to measure the development of endothermy and torpor use in the fat-tailed dunnart (<i>S. crassicaudata</i>). When animals were placed at T_a of 18 °C at 40 d they were poikilothermic, rapidly cooling to T_a, at 48 d animals cooled more slowly and could maintain T_b at approximately 25 °C, and at 56 d animals were endothermic, maintaining a high, normothermic T_b at the low T_a. Animals at ~60 d entered an apparent state of torpor, but being unable to rewarm, became hypothermic. However, these animals could rewarm when given access to radiant heat. Basking in this instance was not an optional method of reducing the cost of rewarming, but was instead necessary to rewarm and avoid hypothermia, and therefore essential to be able to use torpor. The incidence of hypothermia decreased until ~120 d when all animals could actively rewarm, without the aid of radiant heat. This is the first time, to my knowledge, that torpor use in an animal that is not yet fully endothermic has been observed, and indicates that the development of thermoregulation in this, and likely other species occurs in three stages: poikilothermy, partial endothermy and heterothermic endothermy. This discovery may also have some evolutionary implications that need to be considered, as heterothermy was possibly an intermediate stage between poikilothermy and homeothermy in the evolution of endothermy in mammals. The observation of basking in juveniles could reveal the step that explains how pre-endothermic animals were able to move beyond the heterothermic stage in the evolution of endothermy.</p>