Mammals can also generate little bursts of heat by shivering.
Polar bears are mammals.
Polar bears can generate little bursts of heat by shivering.
What can generate little bursts of heat by shivering?

[1] alpacas
[2] bacteria
[3] animals
[4] polar bears
[5] phytoplankton
[6] rabbit
[7] viruses
[8] humans