“Ground squirrels, which replenish their fat supplies regularly during hibernation, can awaken from their deep sleep in less than three hours. In this time, large amounts of fat are burned as fuel to raise the body temperature. This is accompanied by intense shivering and muscle contraction, which also generate heat”. Read more about brown fat at www.asknature.org.
Bats use brown fat as well to wake up from hibernation :
As do hedgehogs: