Endothermic animals rely on:
Endotherms generate most of the heat used to regulate body temperature through metabolism. Oxidation of carbohydrates and fats releases energy, much of which appears as heat rather than mechanical work. Birds and mammals can increase heat production through shivering muscles, non-shivering thermogenesis in brown adipose tissue, hormonal effects, and ordinary organ activity. Insulation and controlled blood flow then reduce or redistribute heat loss, while sweating or panting removes excess heat. Environmental heat can contribute, and many endotherms bask or seek shelter to reduce energetic cost
Ref: Evolutionary Analysis, Herron & Freeman, 5th Ed., Ch. 10