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

Fact 2: Polar bears are mammals.

Given the two facts above, answer the question "What can generate little bursts of heat by shivering?" with the following options: 
- alpacas
 - bacteria
 - animals
 - polar bears
 - phytoplankton
 - rabbit
 - viruses
 - humans


 

    polar bears     