Absence of adaptive nonshivering thermogenesis in a marsupial, the fat-tailed dunnart (Sminthopsis crassicaudata)

The presence of nonshivering thermogenesis in marsupials is controversially debated. Survival of small eutherian species in cold environments is crucially depen- dent on uncoupling protein 1 (UCP1)-mediated, adaptive nonshivering thermogenesis that is executed in brown adipose tissue. In a small dasyurid marsupial species, the fat-tailed dunnart (Sminthopsis crassicaudata), an ortho- logue of UCP1 has been recently identified which is upregulated during cold exposure resembling adaptive molecular adjustments of eutherian brown adipose tissue. Here, we tested for a thermogenic function of marsupial brown adipose tissue and UCP1 by evaluating the capacity of nonshivering thermogenesis in cold-acclimated dunn- arts. In response to an optimal dosage of noradrenaline, cold-acclimated dunnarts (12°C) showed no additional recruitment of noradrenaline-induced maximal thermo- genic capacity in comparison to warm-acclimated dunnarts (24°C). While no differences in body temperature were observed between the acclimation groups, basal metabolic rate was significantly elevated after cold acclimation. Therefore, we suggest that adaptive nonshivering thermo- genesis does not occur in this marsupial species despite the cold recruitment of oxidative capacity and UCP1 in the interscapular fat deposit. In conclusion, the ancient UCP orthologue in marsupials does not contribute to the classical nonshivering thermogenesis, and may exhibit a different physiological role.