Mammals generate heat mainly by keeping their metabolic rate high.
And resting metabolic rate is a big contributor to calories burning.
Mammals generate heat mainly by burning calories.
Mammals generate heat mainly by what?

[1] hugging mammals
[2] calorie counting
[3] with their fur
[4] energy of moving objects
[5] burning calories
[6] sweating
[7] selling fur coats
[8] flow of electrons