Ectotherms

They don't use internal energy sources to maintain body temperature when cold, but once they're active their muscle contractions will generate some heat from increased respiration

Temperature regulation relies upon increasing exchange of heat with environment:

• When it's cold, it will change its behaviour or physiology to increase absorption of heat from its environment
• When it's hot,  it will change its behaviour or physiology to decrease absorption of heat and increase loss of heat to its environment

To warm up: they will bask in sun or lie on warm surface.

Too hot: they will stay underground or in shade.

Some ectotherms possess physiological or anatomical adaptations to help exchange heat with environment.