Shivering, muscle tone, and uncoupling proteins in a developing marsupial, the Tasmanian bettong (Bettongia gaimardi)

This study examined the development of several physiological parameters that enable the development of endothermy during pouch life of the Tasmanian bettong (Bettongia gaimardi). By using several key age groups, we investigated the electromyography response of pouch young under different thermal conditions (35 or 20 °C), with and without injection of the β-agonist norepinephrine. We also used molecular techniques to examine the possible expression of uncoupling proteins 1, 2, and 3 (UCP1, 2, and 3), and if they were expressed, any association these may have with the timing of endothermic development. AT 6 weeks of age, pouch young were unable to thermoregulate via shivering or non-shivering mean, maintain a constant metabolic rate, or show any response to the β-agonist injection. When the animals were exposed to a cold stress (20 °C), 8-week-old pouch young initially shivered for 2–3 min before tapering off completely, causing body temperature to sharply decline. During the 10th week, cold-exposed pouch young began shivering, with the bout lasting approximately 10 min before ceasing and body temperature beginning to decline. It was also at this age that the expression of UCP2 was initially expressed. By the 12th week, cold exposed pouch young initially employed classical shivering; however, after approximately 12 min, this was replaced with an increase in muscular tone. This increase in muscular tone was also recorded in response to β-agonist injection at thermoneutral this age and was associated with an increase in metabolic rate. Also by this age, total body fat increased by approximately 300% from the levels expressed at 6 weeks of age and UCP2 was significantly upregulated. Although pouch young B. gaimardi did not show UCP1 expression at any time, UCP3 was expressed at every age investigated from 6 weeks of age. This study proposes that pouch young B. gaimardi use a mechanism of increased muscle tone as a source of heat production, and shows that UCP2 has an association with the onset of thermogenesis.