Cold blood vs warm blood? Neither one nor the other

When we are at school and at science class we are taught about the different groups of animals, we are taught that animals can be divided into “warm blooded” and “cold blooded”. Even though this refers to the different thermoregulation mechanisms found in the different animals, this differentiation between cold and warm blood is not completely right. In this entry we’ll explain, in a more scientific way, the different temperature-controlling mechanisms present in the animal kingdom and we’ll give you examples of different species that cross the line between cold blood and warm blood.


The first thing we must ask ourselves is where body heat comes from. This may come from two different sources:

Endothermy: Endothermy (“endo”, inside) is the mechanism of obtaining body heat by intern production. Endotherm animals have metabolic mechanisms that generate heat (thermogenesis). To generate heat is energetically costly, which means that these animals have high energetic and nutritional requirements.

seal-255522_1280The blue-footed booby (Sula nebouxii) and the Galápagos sea lion (Zalophus wollebaeki) are two good examples of endothermic animals. Photo by Peter Stuart Mill.

Ectothermy: Ectothermy (“ecto”, outside) happens in animals that get their body heat from the environment, basking in the sun (heliothermy) or staying in contact with heat sources like sun warmed rocks (tigmothermy), etc… These animals do not present any metabolic mechanisms to generate inner heat, but ectotherms have many behavioural adaptations to obtain or release heat. Therefore, as they do not spend any energy to generate heat, their energetic requirements are usually lower and they do not need to eat as often as endotherms.

16. Podarcis liolepisA Catalonian wall lizard (Podarcis liolepis) basking in the sun, is a good example of an ectothermic animal. Photo by David López Bosch.

These two concepts only refer to the source of heat, independently of the animal’s ability to regulate or not its body temperature.


The next pair of concepts refers to whether the body temperature of an animal is constant or if it varies over time:

Homeothermy: Homeotherms (“homeo”, same) control their body temperature, making it relatively constant while the environmental temperature varies. As they have constant body temperature, their activity is not conditioned by environmental conditions.

Homeothermy-poikilothermyRange of temperatures at which an homeotherm and a poikilotherm can be. while the temperature of the rat is usually around 37oC, the temperature of the lizard is much more variable. Image by Petter Bockman.

Poikilothermy: Poikilotherms (“poikilo”, varied) present a body temperature similar to the environment. Their inner temperature varies as the environment temperature does, and so their activity is pretty much conditioned by environmental conditions. For example, on the Iberian Peninsula, lizards have their optimum temperature at 33-38oC and snakes at 28-34oC. Still, as they are used to sudden changes in body temperature, they have an advantage over homeotherms in some habitats, as the latter can die if their body temperature increases or decreases a few degrees.

Wiki_stranglesnakeThermogram of a snake aroung a human arm, showing how the snake's temperature is similar to that of the environment. Photo by Arno.

Yet these are relative concepts. Homeotherms, while having pretty constant average body temperature, do not have exactly the same temperature in all their body parts (temperature in the trunk and internal organs is usually higher than in the extremities). Similarly, poikilotherms do not always have exactly the same temperature as the environment, because, as most of them are ectotherms, they can increase their body temperature using external heat sources.

Cold_nose,_warm_touch_-_Thermography_of_CatThermograms of a cat, showing that homeothermic animals do not have exactly the same temperature on different body parts. Photos by Yellowcloud.

Using these four definitions, birds and mammals are endotherms (generate metabolic heat) and homeotherms (constant body temperature) while reptiles, amphibians and the rest of animals are ectotherms (obtain heat externally) and poikilotherms (body temperature varies with the environment). But in practice, this isn’t an impassable line. Hereafter we’ll show you various examples of animals that make the line dividing these four concepts even more blurred.


Heterothermy (“hetero”, different) occurs in animals which are able to switch from endothermy to ectothermy. This usually happens in small birds and mammals with high metabolic rates (quite active and with very high energetic requirements), which decrease their body temperature during inactivity periods. These inactivity periods usually are yearly or daily.

Bear_hibernatingAmerican black bear (Ursus americanus) and cubs hibernating. Photo by National Park Service.

Yearly inactivity periods are usually known as hibernation (or estivation if it happens in summer). Usually hibernators are endothermic mammals such as squirrels, small primates, hedgehogs and many marsupials, and even though bears also hibernate and decrease their metabolism, their body temperature is not so much lowered (only one or two degrees).

Fat-tailed_Dwarf_Lemur,_Kirindy,_MadagascarThe fat-tailed dwarf lemur (Cheirogaleus medius) usually estivates during the malgasy dry season. Photo by Frank Vassen.

During hibernation, the animal’s metabolic rate is drastically lowered and so, they stop regulating their body temperature, making it similar to that of the environment (that’s why hibernator mammals seek refuge in places where the temperature is not as cold as the exterior). When typically homeotherm and endotherm animals aren’t able to find enough food, they pass to a poikilotherm and ectotherm state to save energy.

F1.largeGraphic of body temperature variation during daily torpor on the Syrian hamster (Mesocricetus auratus). Source Fatemeh Talaei et al.

Other homeotherm animals go through daily poikilotherm periods called torpor, during which their metabolism is also greatly reduced. This typically happens in many species of bats, which have a high body temperature during the night (when they are awake) but this decreases during the day (when they are sleeping). Yet as bats sleep during daytime when temperatures are higher, their temperature does not decrease as much as it would if they slept at night.

Cluster_of_hibernating_virginia_big_eared_batsCluster of hibernating Virgina big-eared bats (Corynorhinus townsendii virginianus). Bats usually congregate during hibernation, helping them to stay warm enough during cold weather. Photo by Stihler Craig, U.S. Fish and Wildlife Service.

Hummingbirds can also lower their temperature during night time, but in their case it depends on the quantity of nectar they have consumed during the day. Hummingbirds feed exclusively on nectar, which contains mainly sucrose, which cannot be stored in the organism and passes directly to blood and tissues. This means hummingbirds have an extremely high metabolism and that they need to feed very often to be able to keep up their activity every day.

Purple-throated_carib_hummingbird_feedingPurple-throated Carib hummingbird (Eulampis jugularis) feeding on nectar. Photo by Charles J. Sharp.

If during the day they have been able to consume a lot of nectar, at night they are able to maintain their typical body temperature of 39oC. However, if during the day they haven’t consumed enough nectar, at night they enter a state of torpor, during which their metabolism is greatly reduced and body temperature drops to 12-17oC. This allows them to save energy to be able to look for more nectar the following day.

Video of a hummingbird that fell into torpor during the night in an artificial feeder in Tennessee. Video by Chip Curley.

Next, we have the case of the naked mole rat (Heterocephalus lager), a rodent from east Africa which is the only known poikilotherm mammal. The naked mole rat’s body temperature is the same as the environmental temperature. Yet some scientists have studied that while its body temperature is exactly the same as the environmental temperature from 12 to 36oC, because it is not able to thermoregulate, at 28oC and over its metabolism becomes homeothermic, even though it is not known how it generates or regulates its body heat.

Angry female naked mole rat. Credit: Buffenstein/Barshop Institute/UTHSCSAFemale naked mole rat (Heterocephalus lager). Photo by Jedimentat44.


Finally there are typically ectotherm animals which are able to generate heat and increase their body temperature using various adaptive strategies. Most of these animals use muscle activity to generate body heat.

Many oceanic fish present a complex of veins and arteries called rete mirabile. This complex is found typically in mammals and birds, and is a countercurrent exchange system (artery-vein) which is used to level different parametres (temperature, pH or gas concentrations) in different body parts.

8041255171_bf47050eef_oDrawing about the functioning of the rete mirabile. The thick orange arrows indicate heat exchange that returns to the body. Image by Arne Hendriks.

In many oceanic fish like sharks, tunas and marlins, this system allows them to raise their body temperature because their inner muscles are very strong and their temperature is very high. The rete mirabile allows them to distribute through all the body and to keep the heat generated while swimming, making them practically endotherms.

Rete_Mirabile_04Drawing where we can see the high temperature on the inner muscles of a shark. Drawing by Vittorio Gabriotti.

In fact, the fish called opah (genus Lampris) are the first fish known to be completely endothermic, because thanks to their rete mirabile located in their gills and to a layer of fat covering their bodies and insulating them from the exterior, they can keep their temperature 5oC above water temperature. Yet they are not homeothermic, as their body temperature can still vary depending on the environment.

Opah_6Photo of an opah fish (Lampris guttatus). Photo by USA NOAA Fisheries Southwest Fisheries Science Center.

Muscle-driven heating is not found only in fish. Mammals also shiver when we are at risk of hypothermia, because muscular contraction generates heat. Even if it cannot be said that they shiver, many insects and some reptiles also use muscle activity to raise their temperature. When insects need to activate their metabolism, they flutter violently to increase their temperature. This, together with a counter current system (similar to the rete mirabile), makes the insect’s body temperature raise considerably compared to that of the environment.

Insect_warm-upThermograms of an insect increasing its body temperature by fluttering. Photos by Crespo J.

Similarly, some reptiles can generate heat. Many pythons incubate their eggs after laying them. To do so, they roll around their eggs and start contracting their body muscles voluntarily to raise the temperature of their clutch.

Female_Python_sebae_brooding_eggs_Tropicario,_FINAfrican rock python (Python sebae) brooding eggs. Photo by Tropicario.

We’ve seen mammals and birds reducing their body temperature, fish generating heat, non-thermoregulating rodents, and reptiles and insects that get warmer moving. As you can see, each species is a unique example of adaptation to its habitat and, even if for an elementary school class it may be useful, dividing animals into warm-blooded and cold-blooded not always the most suitable.


The following sources have been consulted during the elaboration of this entry:


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