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.
Endotherms of different species maintain body temperatures that can differ by a few degrees.
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.
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?!
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.
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.
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.
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.
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.
This is done through the hypothalamus. The hypothalamus acts as the body’s thermostat able to control increase and decrease in body temperature.
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…
…and the ‘visceral’ organs.
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…..
…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.
If 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.
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.
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.
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.
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.
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.
4. Countercurrent heat exchange
Many endotherms use ‘countercurrent heat exchange’ to prevent their core temperature falling.
Take 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 exchange provides two significant advantages:
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.
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.
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.
6.Changing body shape
Another effective way of reducing the surface area to volume ratio is to curl up in a ball!
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
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.
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.