Fact 1: Mammals can also generate little bursts of heat by shivering.

Fact 2: Shivering costs the bird, however, by depleting energy stores.

Given the two facts above, answer the question "How can mammals generate heat?" with the following options: 
- layers of fat
 - Ice cubes
 - wishing
 - Depleting energy stores
 - death
 - with their fur
 - raising their temperature
 - by keeping them warm


 

    Depleting energy stores     