Reptiles, Ectotherms and Cold Blood

‘Ectotherm’ is a word of Greek origin derived from the words ‘ektós’ meaning ‘outside’ and ‘thermós’ meaninghot’. ‘Cold blooded’ is another way of describing ‘ectotherms’.

The term ‘cold blooded’ is a less accurate way to describe ectotherms since some ‘cold blooded’ creatures often have hotter blood than ‘warm blooded’ creatures. Take the desert iguana; its maximum daytime temperature rises to a maximum of 42 °C….

Desert Iguana's maximum temperature rises to 42 °C. Wilson44691

…whereas the maximum daily temperature of a bobcat ( Lynx rufus) never rises above 39 °C.

Desert bobcats Lynx rufus maximum temperature never rises above

Ectotherms incorporate many different ‘classes’ of organism including reptiles…

Ectothermic Red-eared-slider turtles basking


Red-Eyed Tree Frog (Agalychnis callidryas)


-North-Pacific-giant-octopuses are all ectotherms


tropical fish on Great barrier reef are all ectotherms

and some insects.

American burying beetle ( Nicrophorus americanus) is ectothermic

Ectotherms depend on the environment to generate their body heat…

sources of heat for ectothermic lizard fromm the environment

whereas ‘Endotherms’ (‘warm blooded’ creatures), including birds and mammals, generate their own internal sources of heat.

Male lion shown under infrared light

Endotherms have evolved ways to reduce heat loss by growing feathers…

bird family


sleeping cat and dog

or layers of blubber.

Harbour Seal (Phoca vitulina)Denver Zoo

However ectotherms have not evolved ways to insulate their bodies against heat loss.


The images below, captured in the infrared spectrum of light, show how the body temperatures of a lizard and scorpion are identical to the surrounding environmental temperature.

Notice the difference in temperature between these ‘cold blooded’ creatures and the ‘warm blooded’ humans holding them.

ectothermic cold blooded gecko and scorpion under infrared showing how their core temperatures are the same as the environmental temperature

Being reliant on surrounding environmental temperatures to heat your body means that you do not have to produce your own internally generated heat.

As a resting ectotherm you only need to produce enough energy to fulfilll requirements such as breathing, blood circulation, cell growth and brain function.

cold blood ectothermic bearded-dragon-sleeping

In contrast a resting endotherm has to provide enough energy to maintain the function of vital internal organs (so called ‘basal functions’) and enough energy to heat the endothermic body to a constant temperature.

Sleeping lion is an endotherm

This graph shows how the body temperature of a ‘warm blooded’ jaguar stays constant irrespective of the ambient temperature.

endothermic jaguar and ectothermic snake temperature plotted against ambient temperature

Contrast the body temperature of the jaguar with that of the ‘cold blooded’ snake; the core body temperature of the snake changes as the ambient environmental temperature changes.

  • So what are the different energy requirements of endotherms and ectotherms?

Take the example of a human being and a similar sized alligator; in order to maintain a constant body temperature of 37 °C the average sized endothermic human male at rest has to expend 1600-1800 kcal/day.

In contrast a similar sized ectothermic alligator, at an ambient temperature of 28 °C, has to expend only 72 kcal/day.

cold blood Crocodile basking in Sun

Hibernation (‘brumation’) and estivation

Another way in which ectotherms reduce their energy requirement is by ‘hibernating’ (sometimes referred to as ‘brumating’) in Summer and ‘estivating’ in Winter.

The desert tortoise is one such ectotherm; it ‘hibernates’ underground in its burrow in Winter and ‘estivates’ underground in its burrow during the Summer.

cold blooded desert tortoise hibernates and estivates in its burrow

The desert tortoise remains ‘dormant’ and inactive (in a state of ‘torpor’) for more than 3 months of the year, saving a great deal of energy and removing the necessity to forage for food.

Ectothermic desert tortoise throught the year as it hibernates and estivates

Below are several examples of ectothermic animal ‘families’ which ‘estivate’ during the hot summer months.

examples of estivation habitats

Estivating helps ectotherms lower their body temperatures in the hot summer months while hibernating in the cold winter months reduces their exposure to life threatening winter temperatures.

Every year common garter snakes (Thamnophis sirtalis) living in colder regions of North America hibernate underground in ‘hibernaculums’. It is not unknown for some ‘hibernaculums’ to contain more than 8,000 snakes!

snake hibernaculum

Garter snakes leave the sanctuary of their ‘hibernaculums’ in early Spring.

cold blooded ectothermic Red-sided-garter-snakes-emerging-from-den

Change in behavior -migration

Ectotherms sometimes migrate as a behavioral response to avoiding life threatening temperatures. Monarch butterflies migrate from their summer habitat in the north-eastern US and southern Canada and fly to Mexico where they take refuge in Autumn.

fall migration of ectothermic monarch butterflies

Change in behavior- moving between ‘microclimates’

If migration is a more unusual response to regulating ectothermic body temperature, a more common response is daily ‘thermoregulation’. Ectotherms ‘thermoregulate’ by seeking out the right ‘microclimate’ depending on the time of day or night.

At night a desert iguana (Dipsosaurus dorsalis) sleeps underground in its burrow. By doing so it insulates itself against the cold and escapes nocturnal predators.

In the early morning it pokes it head out of its burrow. It stays in this position for several minutes, alert to the threat of predators, while its head absorbs solar radiation.

Desert Iguana pokes head out of burrow

After it has warmed up it climbs out of its burrow to bask in the sun. Like other lizards, the desert iguana is slow moving and vulnerable to predators until it has fully warmed up.

desert iguana basking

When its body has reached the right temperature it is alert, active and ready to catch prey.

Desert iguana with sphinx moth Yuma Desert

By noon its body temperature is too high and it retreats to the shade. When the temperature drops in the late afternoon it leaves the shade to bask in the sun once again.

Desert Iguanas in shade

Desert iguanas adopt a predicable, daily routine of thermoregulation.

Temperature regulation of lizards follows a daily cycle

like fats in cell walls of ectotherms become solid in cold weather

  • So why are desert iguanas sluggish and lethargic when their body temperatures are low?

The answer lies in the cell walls in the tissues of its body. Much like cold butter, the fats that comprise the cell walls become more solid in cold temperatures, slowing down the transfer of molecules between cells.

four types of tissue

In addition enzymes, those molecules that drive chemical reactions and control cellular metabolism, operate most effectively at higher temperatures. In ectotherms lower temperatures result in slower metabolism whilst higher temperatures increase metabolism.

Ectothermic metabolism plotted on graph showing how metabolism decreases with decreasing air temperature

Ectotherms as ‘ambush predators’

The fact that ectotherms have low metabolic rates and low energy requirements compared to endotherms means they need to eat less often than endotherms. This makes ectotherms perfect ‘sit and wait predators’ which can wait for prey to come to them rather than expending energy chasing after prey.

The Californian blunt nosed leopard lizard (Gambelia sila) is the perfect ambush predator.


It often hides camouflaged behind a bush or small tree waiting for its prey, mostly small invertebrates, to come within striking distance. When prey is sufficiently close, it will pounce.

cold blood ectothermic Blunt-nosed-leopard-lizard waiting to ambush prey

Leopard lizards lack endurance and stamina and are physically unable to run long distances. They rely on ‘anaerobic respiration’, rather than aerobic respiration, to fuel fast muscular movement. It is fast muscular movement which enables them to sprint short distances to catch prey.

Anaerobic respiration and lung function of ectotherms

Anaerobic respiration burns glucose instead of oxygen. Anaerobic energy stores, in the form of glucose, are immediately available within the lizards’ muscles to facilitate short bursts of activity.

Glucose supplies the chemical ‘ATP’ to the cells of its limb muscles; it is ‘ATP’ which provides the energy for fast muscular movement.

anaerobic respiration converts glucose to lactic acid and energy

Lactic acid is a ‘waste’ product of burning glucose. Lactic acid, which builds up in the muscles, exhausting the reptiles very quickly.

Lizards, such as this Sand lizard (Lacerta agilis) are completely exhausted after 3-5 minutes of continuous activity and require several hours to regenerate their energy stores.

Male-sand-lizard (Lacerta agilis) -feeding-on-tiger-beetle

  • So why do lizards have to rely on ‘anaerobic’ respiration to provide them with energy? Why not ‘aerobic’ respiration?

The lungs of lizards cannot produce enough oxygen. As a consequence their hearts cannot pump enough oxygenated blood to the muscles where it is needed during times of strenuous activity.

Leopard Lizards, like nearly all reptiles and amphibians, have three chambered hearts; each three chambered heart consists of two atria and a single ventricle. Deoxygenated blood enters the heart through the atria and is pumped through the lungs. The blood becomes oxygenated in the lungs before being pumped out to the body through the single ventricle.

reptile and amphibian heart consisting of two atria and one ventricle

In a three chambered heart a single ventricle pumps out both oxygenated and deoxygenated blood. Having only one ventricle means that there is some mixing of both types of blood.

In contrast the two ventricle, four chambered hearts of endothermic mammals and birds prevents blood mixing and keeps oxygenated blood separate from deoxygenated blood. There is one ventricle for deoxygenated blood and a different ventricle for oxygenated blood.

Four chambered mammalian heart with two artia and two ventricles

A four chambered heart means that there is more oxygen in the blood of mammals and birds than in comparably sized reptiles and amphibians which leads to greater energy levels.

aerobic respiration glucose and oxygen produces carbon dioxide water and energy

Having powerful lungs which produce copious amounts of oxygen means that endotherms have more stamina and can sustain prolonged periods of exertion.

Arctic wolf running in snow

One advantage of producing less oxygenated blood is that ectotherms need to inhale less oxygen into their lungs. Shovel Snouted Lizards (Meroles anchietae) from Namibia….

Shovel snouted lizard

….escape the searing heat of the desert by burrowing underneath the sand…..

Shovel snouted lizard diving under the sand

…where they descend to depths at which they can cool down to relieve the symptoms of heat stress. Air makes a third of the volume of the sand allowing the lizards to breathe. They control how much they cool off by how deeply they burrow under the sand.

Shovel snouted lizard descends into sand

Similar sized endotherms could never survive the shortage of oxygen found in desert sands.

Physiological modification- change in skin color

Ectotherms have many physiological modifications which help them survive, hunt and reproduce.

The Carolina anole (Anolis carolinensis) can change its skin color. When in a dark, cool environment or under stress from a predator it skin appears brown; when it is feels relaxed and is in a light warm environment it appears green.

Anolis carolinensis
In the wild changing color plays an important role in thermoregulation; darker colored reptiles are able to warm faster and maintain higher body temperatures than reptiles that are lighter colored.

Physiological modification- constriction of blood vessels

Whenever a Galapagos iguana (Amblyrhynchus cristatus) swims in cold water…


…the superficial blood vessels next to the skin constrict, through a process known as ‘vasoconstriction’ ….

Vasoconstriction of superficial blood vessels in Galapagos iguana

‘Vasoconstriction’ reduces heat loss and keeps internal organs warm. Controlling blood flow to the skin is an effective adaption to survival in cold water.

In the infrared image below this Galapagos iguana can be seen just after emerging from the sea. Its temperature is far colder than the temperature of the surrounding rocks.

marine iguana in infrared spectrum newly emerged from water

After swimming in the sea Galapagos iguanas raise their core temperatures by spend considerable amounts of time ‘basking’ in the sun.


Physiological modification- freeze avoidance by ‘supercooling’

Ectothermic vertebrates have colonised regions in which the environmental temperature is often below  0°C.

One way in which ectotherms have evolved to survive in cold climates is by ‘freeze avoidance’ through a process of ‘supercooling’. Supercooling is the state in which body fluids remain liquid in temperatures below freezing.

Supercooled organisms have ‘cyroprotectants’  which prevents the build up of ice crystals in the blood. One example is the spadefoot toad (Scaphiopus bombifrons) of Canada and the northern United States….

plains spadefoot toad Scaphiopus bombifrons has supercooled

….which burrows below the frostline in winter to help it avoid the worst of the freezing temperatures. From the safety of its burrow the ‘antifreeze’ in its blood can help it withstand underground temperatures of -4.3°C.

Notothenioids, a group of more than 120 marine fish species, including this Antarctic toothfish (Dissostichus mawsoni)….

Antarctic toothfish (Dissostichus mawsoni) possesses antifreeze proteins

….possess antifreeze proteins which prevent the build up of ice crystals in their blood and keep them ‘supercooled’.

Antifreeze proteins absorbed on ice surface inhibit ice crystal growth

Physiological modification- freeze tolerance

While the blood of some ectotherms contain ‘cyroprotectants’ which prevent the freezing of the blood, there are other ectotherms which can tolerate their blood freezing. One such species that is ‘freeze tolerant’ is a tiny species of frog – the Wood frog (Rana sylvatica).


When really cold weather arrives the Wood frog freezes over and turns into a ‘frogsicle’.

wood frogsicle

Evelyn Dietz

It can survive for weeks with an incredible 65% of water in its body completely frozen!

65% of wood frog freezes

Special ‘nucleating proteins’ freeze any water present in its blood. This iced water in the blood then sucks the water out from the cells of the frog’s body.

While this is happening the frog’s liver produces large amounts of glucose which enters the frog’s cells to prevent them collapsing. The inside of the cells become very dehydrated, but become full of massive amounts of thick sugary syrup.

Brain activity ceases and the heart stops beating. The frog can stay in this state of suspended animation until warmer weather arrives.

freezing-wood frog

As temperatures warm up and the ice melts, water re enters the cells. The heart starts pumping blood once again, the frog starts breathing and hops away.

The Wood frog can endure multiple freeze and thaw episodes over the course of a single winter.

Ectotherms can often endure prolonged periods of food and water shortages

Ectotherms can often endure prolonged periods of water and food deprivation. As a result they can exploit environmental niches in which food and water are often hard to come by.

The vast majority of species which inhabit the White Sands National Monument of New Mexico, USA…..

White Sands National Monument

…. are ectotherms. Species which have been recorded inhabiting the margins of the dune field include 100 families of insects (with numerous genera and species in each family), 26 species of reptiles, 6 species of amphibian and 44 species of mammals.

As a result of their low metabolism, with as little as three heart beats per minute when at rest, ectotherms will eat far fewer meals than comparably sized endotherms. Nile crocodiles typically eat as few as 50 meals a year. In times of famine crocodiles competely ‘shut down’ and can go for up to a whole year without eating!

Nile Crocodile resting on river bank has very low metabolism

Pythons can get by eating only 6 meals a year.


Ectotherms different shapes and sizes

The bodies of endotherms generate negligible amounts of  their own internal heat, so having large surface areas relative to their body weights is no problem. Ectotherms acquire a whole range of body shapes and sizes not available to endotherms;

tiny in size like some salamanders….

Desmognathus ochrophaeus Allegheny Mountain Dusky Salamander

…elongated in shape like this common long tailed seps…

Tetradactylus tetradactylus

…or having a laterally compressed shape like this Butterfly fish.

Rainford's Butterflyfish Chaetodon rainfordi

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