Organism
Newborn Rabbit
TL;DR
Newborns can't shiver (lack muscle mass) but have 5% body weight as brown adipose tissue.
Exemplifies brown fat thermogenesis. Newborns can't shiver (lack muscle mass) but have 5% body weight as brown adipose tissue. Exposed to 40°F cold: brown fat activates immediately, heat production increases 5×, core temperature maintained at 98°F despite 58°F gradient for 4 hours. Without brown fat: death in 30 minutes.