Staying warm whatever the weather is energetically costly and many mammals resort to torpor and abandon their stable body temperature during challenging periods in order to save energy. While entering torpor is energetically cheap, coming out is expensive as the body has to warm up, although the costs can be mitigated by basking in the sun. Even though the cost of emerging from torpor may be even more extreme for juveniles – because they are smaller than adults and have a high surface area to volume ratio – they use it frequently, which raises the question: do juveniles attempt to reduce their rewarming costs by basking in the sun?
An Australian team led by Chris Wacker from the University of New England was interested in whether juvenile marsupials – which are the size of a jellybean at birth and spend their first months of life snuggled in their mother’s warm pouch – might use the sun to aid rewarming from torpor. Dunnarts (Sminthopsis crassicaudata), small nocturnal marsupials that live in desert areas, are known to use torpor throughout the year and often bask in the early morning sun. The team looked at juvenile dunnarts between the ages of 40 and 160 days – the age at which they leave the mother's pouch and switch from depending on her warmth to maintaining their own body temperature. They measured the dunnarts’ ability to maintain a stable body temperature, whether and when juveniles used torpor and whether they would use an artificial heat source to reduce the costs of heat production as they aroused from a bout of torpor.
Interestingly, the team found that the juveniles are already capable of entering torpor even before they can regulate their body temperature fully and they were able to achieve this by basking. When the juveniles emerged from the pouch at around 60 days, they were able to maintain a more or less stable body temperature for a few hours, although they entered torpor during the later part of the night. However, they were unable to restore a warm body temperature after torpor unless they had the opportunity to crawl under the warmth of a heat lamp.
To find out whether the juveniles were just losing too much heat, causing them to cool down involuntarily, or had entered a genuine state of torpor, the team compared the juvenile dunnarts’ heat loss rate with the heat loss of dead animals that had been warmed to the same initial body temperature. This comparison showed that the live dunnarts cooled faster than the dead animals, indicating that they controlled the decrease in body temperature and entered torpor. And when the team monitored the juveniles’ use of torpor, they found that animals were still entering torpor during the night at around 90 days of age when they were already able to maintain a stable warm body temperature during the course of the entire night, although the older youngsters were able to rewarm and emerge from torpor with greater ease. Nonetheless, dunnarts of all ages still basked under a heat source whenever they had the chance.
Wacker and colleagues conclude that juvenile dunnarts are able to enter torpor even before they are capable of producing sufficient heat to maintain a stable temperature, and they need to bask in the sun to resume a warm body temperature after torpor. This observation also raises the possibility that basking may be a crucial step in the development of heterothermy – where an animal does not always maintain a single stable body temperature – in small animals that live in warm habitats. Importantly, the team has revealed that basking is not an option for juvenile marsupials wishing to capitalize on energy savings during torpor; it is essential.