poikilotherm – an organism that cannot regulate its own body temperature at a constant temperature i.e. cold blooded

(Pronounced poy-kih-luh-therm)

Note: The best example of a poikilotherm is a snake. 

To remember the meaning of poikilotherm, use the following mnemonic:

To make poison and put a kilogram of it in a thermos (poikilotherm) is a cold-blooded thing to do.

A poikilotherm is an animal whose internal body temperature varies with its surrounding environment, also known as being 'cold-blooded'. This includes reptiles, amphibians, fish and all invertebrates, which cannot generate sufficient internal heat to maintain constant body temperature like mammals or birds can. 

Poikilotherms rely on external heat sources such as sunlight, warm surfaces, or ambient air temperature to regulate their body temperature, and they use behavioural adaptations to control their temperature, such as basking in the sun to warm up, seeking shade to cool down, or moving to different depths in water where temperatures vary. 

The advantage of poikilothermy is that these animals require much less food and energy than homeotherms because they don't need to fuel constant internal heat production, allowing them to survive where food is scarce. However, the disadvantage is that their activity levels and metabolic rates are dependant on environmental temperature - they become sluggish or inactive when it's cold and may need to hibernate or become dormant during winter months.