Heterothermic animals are those that can switch between poikilothermic and homeothermic strategies. The naked mole-rat and sloth are some of the rare mammals which are poikilothermic. This makes them homeothermic when active, and poikilothermic when at rest. The pathogenesis is, however, temperature dependent and iridoviruses are thus confined to poikilothermic hosts. REM sleep is also considered a poikilothermic state in humans. Being poikilothermic, they depend heavily upon external heat for survival. However, almost all fish are poikilothermic. Octopuses are poikilothermic or cool-blooded, and have three hearts and blue, copper-based blood. Insects are poikilothermic, but maintaining an adequate temperature range remains important. However, animals with facultative endothermy are often poikilothermic, meaning their temperature can vary considerably.