Mammals, Endotherms and Warm Blood

Endotherms are ‘warm blooded’ organisms which derive most of their body heat from their internal metabolism.  In contrast ectotherms (‘cold blooded’ organisms) rely on environmental factors to create internal heat.

Endotherms include all mammals and birds whilst ectotherms include all reptiles, amphibians and invertebrates. Fish and insects can be either endotherms or ectotherms with the vast majority being ectotherms.

Endotherms are mostly homeothermic; homeothermic endotherms are able to maintain a constant body temperature within a narrow temperature range no matter what the environmental conditions. For example within the same river eco system an otter’s core temperature fluctuates very little whereas a large mouth bass’s core temperature changes considerably.

enothermic river otter and ectothermic sea bass graph of body temperature against ambient environmental temperature

Endotherms of different species maintain body temperatures that can differ by a few degrees.

Body temperatures of different endotherms

There is a large metabolic cost to maintaining internal heat. To maintain a constant body temperature an endotherm needs to expend more energy than an ectotherm of a similar mass.

The rate at which an endothermic body uses energy while at rest to maintain vital functions such as breathing and keeping warm is defined as the basal metabolic rate (BMR). The ‘mass specific’ Basal Metabolic Rate represents the resting energy demand adjusted to take account of the mass of the animal.

This graph demonstrates the relationship between ‘mass specific BMR’ and body mass. The smaller the animal, the greater the energy requirement per kilogram of body weight. The larger the animal, the smaller the energy requirement per kilogram of body weight.

mass specific metabolic rate for mammals of different sizes

  • So why do animals of a small mass have a greater energy requirement per kilogram of body weight than animals of a larger mass? To answer this question we need to investigate surface area to volume ratios.

Thermoregulation- surface area to volume ratio

A big problem for smaller animals is that they have a larger surface area (ie surface in contact with the environment through which heat can be lost) relative to their volume of body tissue.

A large surface area with a relatively small bodily volume means that there is less tissue inside the body of the animal available with which to generate heat. The greater the surface area to volume ratio, the faster the body transfers heat to the environment.

An impossible body shape for an endotherm?!

squirrel snake with high surface area to volume ratio is an impossible shape for an endotherm

The metabolic rate rises so fast with decreasing body size that animals less than 5 grams make energy demands very difficult to meet.

Shrews are the smallest of terrestrial endotherms. With a high surface area to volume ratio, they lose metabolic heat more quickly than larger-bodied mammals. Since shrews endure a life of endothermy, their metabolisms need to work overtime to keep their tiny bodies warm.


To stoke their metabolic fire, many species of shrew need to eat every 1-2 hours or risk death by starvation.

You might think that shrews can only survive in warmer climates where they don’t have to expend so much energy staying warm.


This is not the case! Shrews survive perfectly well in colder climates. They do this by shrinking their body mass and vital organs during the winter; by so doing they reduce their resting metabolic rates by 18% and lessening their need for food. This ability to shrink vital organs during very cold weather is known as the Dehnel Phenomenon.

Body shape is also a significant consideration when it comes to reducing the rate of heat loss. Small endothermic mammals, like this kangaroo rat, have evolved round body shapes which reduce their surface area to volume ratio.  Contrast the round body shape of the kangaroo rat ….


…with the long, slender shape of an even smaller ectothermic gecko.


In larger endotherms, with low surface area to volume ratios, heat is gained and lost more slowly. This means that the larger endotherms do not have to replace lost heat as quickly; as a consequence they not have to eat as much in proportion to their body weight as the smaller endotherms.

Two-African endothermic lions

Thermoregulation – variation in size and appendage

Within any given species, animals tend to be larger in colder climates and smaller in warmer climates. This observation is known as Bergmann’s Rule. For example whitetail deer from Central America tend to have smaller body sizes and less overall mass than whitetail deer in the northern United States.

Bergmann's rule in deer

There are exceptions but, overall, this rule holds true. Smaller enotherms of the same species lose heat relatively quickly and cool down faster and so are more likely to be found in warmer climates. Larger endotherms of the same species, with their lower surface area-to-volume ratios, lose heat more slowly and are more likely to be found in colder climates.

body size of birds on scale

Endothermic vertebrates from colder climates also have shorter appendages, including ears and noses, than closely related species from warmer climates. With less surface area shorter appendages lose less heat than longer appendages.

In this diagram the species of fox and hare with the shortest ears and noses both inhabit Arctic regions.

Allen's rule showing hares and foxes

Longer appendages, such as the oversized ears of this Fennec fox (Vulpes zerda), are more effective at dissipating heat and are therefore more effective in hot climates.


Thermoregulation of  body temperature- the hypothalamus

  • So how do endotherms monitor their internal temperature?

This is done through the hypothalamus. The hypothalamus acts as the body’s thermostat able to control increase and decrease in body temperature.

Hypothalamus acts as the primary thermostat for thermoregulation

The hypothalamus monitors body temperature by processing information it receives from peripheral thermoreceptors located around the body. These peripheral thermoreceptors are located both in the skin…

heat and cold receptors

…and the ‘visceral’ organs.

visceral organs of a human from the front

Cold receptors respond to reduction in temperature of the body surface and warm receptors are activated when the skin temperature rises. The human body has more cold receptors than warm receptors. Central thermoreceptors in the hypothalamus itself detect changes in blood temperature.

After receiving reading sent as nerve impulses about body temperature…..

Hypothalamus receives signals from skin and viscera

…the hypothalamus initiates coordinated adjustments in heat gain or heat loss to correct any deviations in core temperature from the acceptable range. The acceptable temperature range in humans is between 36.5 °C and 37.5 °C.

Correcting deviations in core temperature from an acceptable range is known as homeostasis. Homeostasis is the tendency of the body to seek and maintain a condition of  temperature equilibrium when faced with external increases or decreases of temperature.

These external increases of decreases of temperature can be caused by several factors including environmental radiation.

rabbit thermoregulation

  • So how does the endothermic body initiate heat loss or heat gain to ensure that homeostasis is maintained?

Homeostasis- returning to temperature equilibrium when the body is too hot

homeostasis and temperature control if body temperature rises

Normal blood vessel and vasodilationIf the body temperature rises above normal the hypothalamus signals the blood vessels in the dermis to dilate (‘vasodilation’). The more warm blood that flows through the skin in dliated blood vessels, the more heat is lost from the body.

When body temperature rises above normal the hypothalamus also signals the sweat glands to secrete moisture.

epidermis dermis and sweat glands

As the dilute, salty sweat solution builds up on the skin it accumulates heat (water has a high heat capacity) until it evaporates and carries the heat away from the body. On hot and humid days less heat can be lost by sweating because the atmosphere already contains relatively high concentrations of water.

  • What about animals covered in fur; how do they sweat?

Take dogs as an example; dogs have some sweat glands located on the pads of its feet and on its nose. However the primary cooling mechanism for dogs is ‘panting’ which involves rapid, shallow breathing. When air moves quickly across the wet surfaces of the inner mouth, the process of evaporative cooling is accelerated.

Of course one of best strategies to prevent overheating altogether is avoidance behavior by moving into shade.


Homeostasis- returning to temperature equilibrium when body is too cold

  1. Vasoconstriction

If  body temperature drops below normal the hypothalamus sends signals through the nervous system that the dermal blood vessels should contract (‘vasoconstriction’) to reduce heat lossVasoconstriction of superficial blood vessels in Galapagos iguana

homeostasis and temperature control if body temperature falls

2.Involuntary contraction of muscles

When the muscles contract involuntarily an animal will ‘shiver’ producing extra heat to warm the animal up.  Animals are not the only organisms that ‘shiver’.

Endothermic hawkmoths uses a pre-flight ‘shivering’ routine to warm up their powerful flight muscles. As a result they are able to fly during cold weather and even at night. A hawkmoth typically shivers for up to 2 minutes before it takes off.

hawkmoth abdomen and thorax shivering reflex plotted on a graph showing temperature set against time from onset of warm up

The powerful flight muscles of the hawkmoth are located in the thorax; the insect can maintain a constant temperature of about 30 °C in the core of the body at times when the outside temperature is much colder.

hawkmoth thorax heating up by heat exchange

3. Voluntary contraction of muscles

When a human makes a conscious decision to take exercise, the locomotory muscles provide extra heat to maintain the core temperature.

Lewis Pugh record breaking attempt in Antarctica Photo:

4. Countercurrent heat exchange

Many endotherms use ‘countercurrent heat exchange’ to prevent their core temperature falling.

Take ducks standing on ice.

ducks standing on ice

A network of arteries pumps warm blood into a duck’s feet. Cold blood flows in the opposite direction along the veins. With ‘heat exchange’ warm arterial blood gives up some of its heat to the vein transporting cold blood back to the heart.

countercurrent heat exchanger in ducks feet explained

Countercurrent heat exchange provides two significant advantages:

  • Recently cooled blood from the duck’s feet is warmed before it reaches the bird’s core
  • Blood from the duck’s core is cooled significantly before it reaches the feet. This ensures that a sustainable temperature is maintained in the feet, allowing its feet to endure freezing temperatures with no significant heat loss.

duck infrared image showing cool feet

Another creature that uses a countercurrent heat exchange system is the gray whale. gray whales take in vast amounts of cold water into their mounts as they feed.

Gray-whale-snout-covered-in-barnacles-passes-by-kelp has Countercurrent heat exchange system in tongue

If the whale were to lose significant amounts of heat to the water, it wouldn’t be able to eat enough food to produce the energy required to heat its body.

As warm blood flows to the tongue through arteries, the arteries give heat to the blood returning in the opposite direction to the whale’s core through its veins.

whale heat exchange on tongue

5. Piloerection

The arrector pili muscles are small muscles attached to hair follicles.  Contraction of these muscles cause the hairs to stand on end. Air becomes trapped between the erect hairs, helping the animal retain heat.

Arrector pili muscles contracted and relaxed

6.Changing body shape

Another effective way of reducing the surface area to volume ratio is to curl up in a ball!


Abandoning endothermy-hibernation

When it becomes a bit too challenging to keep the core warm some mammals temporarily abandon endothermy by hibernating.

Hibernation is a state of inactivity and metabolic depression characterized by low body temperature, slow breathing, slow heart rate and low metabolic rate.

To get through the cold British winters, dormice need to hibernate.


From October onwards a dormouse will build itself a winter hibernation nest at or below ground level. When it hibernates it allows its body temperature to fall to match that of its surroundings. Its heart rate and breathing slow by 90% or more and it will remain in this state of hibernation until warmer conditions return in the spring, when it ‘wakes up’.

If a frost causes the ground temperature to fall below zero the dormouse can increase its metabolism just enough to keep it from freezing, all the while remaining in a state of hibernation. Dormice can hibernate six months out of the year, or even longer if the weather does not become warm enough. During the summer they accumulate fat in their bodies to nourish them through the hibernation period

Abandoning endothermy-torpor

A humming bird’s small body size and lack of insulation means that it rapidly lose body heat to its surroundings. Even sleeping hummingbirds have huge metabolic demands that must be met simply to survive the night when they cannot forage.

humming bird in a state of torpor

In order to survive cold nights, a hummingbird can lower its core temperature to the point of becoming hypothermic. This reduced physiological state is an evolutionary adaptation that is commonly called torpor.

By drastically reducing its metabolic rate a  torpid hummingbird will consumes up to 50 times less energy than when awake.

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