Endothermy in Endotherms: Examples of Endothermic animals

Definition of Endotherms

Endotherms are organisms that maintain their bodies at a metabolically favorable temperature by using the heat released by their internal body functions. They don’t rely on ambient heat to regulate their body temperature. The internally generated heat is a result of routine metabolism in the animal. However, under excess cold conditions or low activities, they may adopt special mechanisms to generate heat. Such mechanisms involve special function muscular exertion like shivering. Uncoupled oxidative metabolism like within brown adipose tissue too can be another adopted mechanism. most of the heat endothermic animals need is generated internally.

Basically, only mammals and birds are existing endothermic animals. Although certain fishes like tuna, lamnid sharks, and billfishes are endothermic too. Endotherms are said to be warm-blooded animals. Endothermy permits birds and mammals to stabilize their internal temperature. Hence, allowing biochemical processes and nervous system functions to move at steady high levels of activity. However, in winter, endotherms can stay active and exploit habitats that ectotherms cannot. Though in colder climates, some endotherms that are small with high heat loss and limited access to food may reduce their activities and hibernate.

Examples of endotherms

  • Humans
  • Mammals
  • Birds
  • Tuna
  • Lamnid sharks
  • Billfishes
  • Honeybees and some snake species can exhibit facultative endothermy

What is Endothermy?

Endothermy is the generation and regulation of body temperature via metabolism. It can be defined as a state of being warm-blooded. The term cold-blooded and warm-blooded has been used for long to divide organisms into 2 groups. This term and grouping were done based on animals that feel cold to the touch and those that don’t.

Zoologists normally use poikilotherms to describe animals that their body temperature fluctuates with environmental temperature and homeotherms for those that have a regulated constant temperature regardless of the environmental temperature. However, physiologists prefer another way of contrasting and grouping organisms based on the thermoregulation mechanism. A grouping term that reflects that an organism’s body temperature is a balance between heat loss and heat gain. All animals generate heat from cellular metabolism. However, in some animals, the heat is lost as fast as it is generated. Such animals are called ectotherms. An ectothermic animal’s body temperature is dependent on the environmental temperature. Their source of heat comes from the environment and not from within their body. The majority of animals are ectothermic.

Contrarily to ectotherms, some animals can generate heat and maintain enough metabolic heat to improve their body temperature. The source of heat of such organisms is internal and the temperature is increased to a high and stable level. Such organisms are referred to as endotherms. Thereby endothermy is the opposite of ectothermy. There are few endotherms in the animal kingdom and are mostly mammals and birds with few nonavian reptiles and fast swimming fishes. Also, some certain insects are partially endothermic.

In endothermic animals, if the heat loss exceeds the heat production, metabolism is increased. It is increased to make up for the loss or the animal will shiver to raise its body temperature. Then if the heat generation exceeds the heat loss some certain mechanisms increase heat loss. Mechanisms such as panting or perspiring. Endothermic animals can be active and thrive at low external temperatures, unlike ectotherms. Though they need a high amount of food because they must produce heat continuously.

Thermoregulation in endotherms

Thermoregulation in endotherms is the ability of an endothermic animal to keep a constant body temperature despite the environmental temperature. Therefore, this internal thermoregulation is a part of homeostasis. Homeostasis, however, is the state of optimal functioning for an organism. It involves the body temperature and fluid balance in the body being kept within certain limits.

Internal thermoregulation plays a role in animals being able to maintain homeostasis within a certain temperature range. An organism should be able to maintain a constant temperature. Furthermore, a condition known as hyperthermia occurs once an organism is unable to maintain a normal temperature and its body temperature increases above normal levels. . For instance, when a wet-bulb temperature is kept above 35 °C for 6 hours, humans can experience lethal hyperthermia.

Also, another condition known as hypothermia occurs when an organism’s body temperature reduces below normal levels.  Hypothermia usually occurs when there is a malfunction of the homeostatic control mechanisms of heat within the body. This results in the body losing heat faster than the rate at which it generates heat. Since 37 °C is the normal body temperature, hypothermia occurs when the body temperature goes lower than 35 °C. This can be caused by prolonged exposure to cold temperatures.

However, there are various mechanisms employed by animals to regulate their internal body temperature. This is why thermoregulation could be either endothermy or ectothermy. Endothermy involves the organism producing most of its heat through metabolic processes whereas ectothermy involves organisms using external temperature sources to regulate their body temperatures.

How do endotherms regulate body temperature?

In endotherms, constant temperature is maintained by a delicate balance between heat loss and heat production. Most mammals have a body temperature that ranges between 36 degrees and 38 degrees celsius. Mammals’ temperatures are somewhat lower than that of birds. Birds’ temperature ranges between 40 degrees to 42 degrees Celcius.

Heat is generated by the animal’s metabolism. This involves basal cellular metabolism, oxidation of foods, and muscular contraction. Endothermic animals must eat more food than ectothermic animals. This is because most of an endotherm’s daily caloric intake is required to produce heat, especially in cold weather.

However, heat is lost via conduction, convection, and radiation to a cooler environment. Also, it is lost by the evaporation of water. Birds and mammals can control both processes of heat production and loss within wide limits. Once an endothermic animal gets too cold, it can generate heat by increasing muscular activity. The muscular activity could be increased via shivering or exercise. Also, by increasing their insulation, endotherms can decrease heat loss. However, if it becomes too warm, they decrease heat production and increase heat loss.

Thermoregulation in endotherms on a warm day
A diagram showing the exchange of heat between a large mammal and its environment on a warm day.
The RED ARROWS indicate sources of net heat gain by the animal (radiation)
The BLUE ARROWS are avenues of net heat loss (conduction to the ground, evaporative cooling, longwave radiation into space, and forced convection by the wind)
However, if the atmospheric temperature and ground temperatures were warmer than the animal, the BLUE arrows for conduction, radiation, and forced convection would be reversed. Then, the animal could only lose heat by evaporative cooling.
Photo Credit: Integrated Principles of Zoology Textbook. Fifteenth Edition By Hickman, Roberts, Keen, Eisenhour, Larson, and I’Anson

In hot environments

Even in extremely hot environments, many kinds of animals survive successfully. The desert, for example, with its harsh conditions still accommodates some animals. Smaller desert mammals living in the desert are usually fossorial or nocturnal. Fossorial animals live in the ground and nocturnal animals are active at night. The burrows in the deserts help fossorial animals to reduce water loss by evaporation. This is because the burrows have lower temperatures and higher humidity. Some desert animals like kangaroo and ground squirrels, without drinking water, can derive water from the metabolism of their dry food. Eventually, such animals will produce urine that is highly concentrated and their feces will be completely dry.

Large desert ungulates (large mammals with hooves) cannot escape the desert heat by hiding in burrows like other smaller mammals. Animals like gazelle, eland, oryx, and camels have some adaptive mechanisms to survive heat and dehydration. The mechanism for regulating water loss and avoiding overheating are closely related.

Let us use the elands for example. The glossy pallid color of the eland fur reflects direct sunlight. This fur as excellent insulation resists heat. By convection and conduction, heat is lost from the underside of the elands. However, the fur on the underside of elands is very thin. Also in a single hump on the back, fat tissues are concentrated as an essential food reserve. The fat tissues are concentrated in the hump instead of being uniformly distributed under the skin, inhibiting heat loss by radiation. Moreso, elands prevent evaporative water loss by allowing their body temperature to drop during the cool night and then during the day cause it to rise slowly as the body stores fat.

How endotherms regulate their body temperature in hot environments
A diagram showing the physiological and behavioral adaptation of the common eland for regulating temperature in hot environments
Photo credit: Integrated Principles of Zoology Textbook. Fifteenth Edition By Hickman, Roberts, Keen, Eisenhour, Larson, and I’Anson. Pg 684

However, elands prevent further temperature rise when the body temperature reaches 41 degrees celsius. The eland does this through evaporative cooling by panting and sweating. Then, as air is breathed out, respiratory moisture is condensed and reabsorbed in nasal passages. By producing dry feces and concentrated urine, they conserve water. Camels, however, have all these adaptations too. Theirs are developed to an even greater extent as they are the most perfectly adapted of all large desert mammals.

In cold environments

Endothermic animals use two main mechanisms in cold environments to maintain homeothermy. These mechanisms are:

  1. Decreased conductance
  2. Increased heat production
  • Decreased conductance

This involves the reduction of heat loss by increasing the effectiveness of the insulation. The fur thickness of all mammals living in cold regions of the earth usually increases in winter. This increase sometimes is as much as 50%. The thick under hair is the main insulating layer. Whereas, the longer guard hair that is more visible serves as protection against wear. It is also a protective coloration. The down feathers in birds function similarly to conserve heat.

Body extremities like the legs, tail, ears, and noses of arctic birds and mammals are thinly insulated compared to their well-insulated trunk body. Hence, these body extremities are exposed rapidly to cooling. Therefore, these parts are allowed to cool to low temperatures to prevent them from becoming the main avenues of heat loss. Often they cool, approaching freezing point. Thus, the heat in the warm arterial blood is not lost from the body. Rather, between the returning cold blood and the outgoing warm blood, there is a countercurrent heat exchange. Thereby preventing heat loss.

In the leg of an arctic bird or mammal, the arterial blood passes in close contact with a network of small veins. Heat is exchanged very efficiently from artery to veins as the arterial blood flow is opposite to the returning venous blood. Hence, arterial blood transfers nearly all of its heat to the veins when it reaches the foot. These veins return blood to the core body. As a result, little heat is lost to the surrounding cold air from poorly insulated distal regions of the leg.

However, these countercurrent heat exchangers in appendages are common also in aquatic mammals. Aquatic mammals like whales and seals have thinly insulated flippers. These flippers might be avenues of excessive heat loss. Therefore, without this heat salvaging arrangement, the flippers would be heat loss avenues. Nevertheless, the legs and feet of mammals and birds in cold environments must function at low temperatures. This is a consequence of peripheral heat exchange. Hence, to keep feet flexible and supple at low temperatures, fats in the extremities have very low melting points. Usually, 30 degrees lower than ordinary body fats.

Diagram showing countercurrent heat exchange for cold environments in endothermic animals
A diagram portraying countercurrent heat exchange in an arctic wolf’s leg.
The diagram to the LEFT shows how the extremities cool when the animal is exposed to low air temperature.
The diagram to the RIGHT shows how heat is exchanged between arterial and venous blood in a part of the front leg artery and vein.
Thus, heat is pushed back into the body and conserved.
Photo Credit: Integrated Principles of Zoology Textbook. Fifteenth Edition By Hickman, Roberts, Keen, Eisenhour, Larson, and I’Anson. Pg 684.
  • Increased heat production

In extremely cold conditions, all mammals can produce more heat by augmented muscular activity via shivering or exercise. One can increase heat production by as much as 18-fold when maximally stressed by cold by violent shivering. Another heat source is the increased oxidation of foods. Most especially from stores of brown fat. This mechanism is called nonshivering thermogenesis.

Small mammals like the size of lemmings, mice, and voles address the challenge of cold environments in another way.  These small mammals are not well insulated like large mammals. So in addition to augmented muscular activity and nonshivering thermogenesis, they exploit the excellent insulating qualities of snow. They do this by living under snow in runways on the forest floor where their food is also located. In this subnivean environment, the temperature rarely drops below -5 degrees celsius. Though the air temperature may fall to -50 degrees celsius. Snow insulation decreases thermal conductance from small mammals, an equivalent way thick fur does for large mammals. Endotherms’ habit of living beneath the snow is actually a type of avoidance response to cold.

Endothermy in mammals and birds

Endothermy is expensive energetically. An ectotherm, for instance, can stay for weeks without eating in cold environments. Whereas, an endotherm must always eat to have energy resources for its high metabolic rate. Small birds and mammals, for instance, may need a daily intake of food approaching their own body weight because of their intense metabolism. They require this daily consumption of food to maintain homeothermy.

Due to this, a few small birds and mammals have adopted ways to abandon homeothermy for some time periods. These periods range from few hours a day to several months. They allow their body temperature to fall until it equals the atmospheric temperature.

Some very small mammals like bats maintain high body temperature when they are active. Then when inactive or asleep, they allow their body temperature to drop. This is called daily torpor. Daily torpor is adaptive hypothermia that ensures enormous saving of energy to small endotherms. Also, hummingbirds may drop their body temperature too at night when the food supply is low.

Many small and medium-sized mammals in northern temperate regions solve the problem of winter, scarcity of food, and low temperature by entering hibernation. This is a prolonged and controlled state of dormancy. True hibernators prepare for hibernation by storing body fats. Such animals are marmot, ground squirrel, jumping mice, and woodchucks. Entering into hibernation is a gradual process. Firstly, they ‘test drops’ where the animal’s body temperature decreases a few degrees then returns to normal. After test drops, the animal cools to within a degree or less of the ambient temperature. Metabolism then slows down to a fraction of normal. During arousal, the hibernating animal shivers violently and applies nonshivering thermogenesis to generate heat.

Some mammals like bears, badgers, opossums, and raccoons enter a state of prolonged sleep in winter. They sleep without a decrease in their body temperature. Hence, they are not true hibernators. Their heart rate may decrease but their body temperature remains normal. Also, some invertebrates and vertebrates enter a state of dormancy during summer. This state is called estivation or summer sleep. In this state, their breathing and metabolic rates decrease when there’s food scarcity, temperatures are high and are threatened by dehydration. Animals that estivate are blue land crabs, lungfishes, land snails, desert tortoises, ground squirrels, and pigmy mice.

Facultative endothermy

Many insect species have the ability to use exercise to maintain a thoracic temperature above the air temperature. These organisms are known as exercise or facultative endotherms. A typical example is the honey bee that regulates its temperature by contracting antagonistic flight muscles without moving its wings. However, this kind of thermogenesis is only efficient above a certain temperature threshold. Thus, the honey bee goes back to ectothermy when the temperature is below about 9–14 °C. Also, facultative endothermy can be seen in multiple snake species. Some snakes make use of their metabolic heat to warm their eggs. The female snakes will surround their eggs and shiver to incubate them, as seen in python species like Python molurus and Morelia spilota.

Ectotherms vs Endotherms

The major difference between ectotherms and endotherms is how these animals regulate their body temperature. With an emphasis on whether or not it is dependent on the external environment.

However, ectothermic animals rely on their external environment for thermoregulation, whereas endotherms carry out thermoregulation via internal metabolic processes. Endothermic animals thereby, maintain a narrow range of internal temperatures and generate most of their heat from metabolism.

Many endotherms have a large number of mitochondria per cell than ectotherms. These mitochondria help them to produce heat by increasing their metabolism rate of fats and sugars. Ectothermic animals, on the other hand, possess a lesser number of mitochondria per cell.

Moreso, ectotherms have lower metabolic rates compared to endotherms at a given body mass. To sustain their high metabolism, endothermic animals must eat more food than ectothermic animals. This is because most of an endotherm daily caloric intake is required to produce heat, especially in cold weather.

Advantages of endothermy

The main advantage of endothermy is that it is not vulnerable to fluctuations in the environmental temperature as compared to ectothermy. Despite the environmental temperature, endothermy sustains a constant temperature for optimum enzyme activity.

Hence, endotherms can control body temperature by internal homeostatic mechanisms. Endothermic organisms can be optimally active at more times during the diurnal cycle and in places of great seasonal differences in temperature. Also, endothermy may be important during reproduction. For instance, since embryos are usually intolerant of thermal fluctuations, endothermy expands the range of temperature over which a species can reproduce.

Frequently Asked Questions

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Are birds endothermic?

Birds are both endothermic and homeothermic. They are homeothermic because they can maintain a constant body temperature. Also, they are endothermic because they can regulate their body temperature via metabolism. Birds generally have a temperature range between 40 degrees to 42 degrees Celcius. Endothermy allows an organism to sustain high activity levels at all times. This is why birds being endothermic can remain active throughout the day, year, and anywhere in the world. Endothermy in birds confers some advantages. When a bird is at a higher temperature, its nerve impulses travel faster. Also, its muscle strength is enhanced and they tend to physically endure better. These attributes and advantages are crucial for flight. However, being endotherms, birds have to eat at a far higher rate.

Are fish endotherms?

Fishes have no control over their body temperature and cannot generate and store internal metabolic heat. So fishes are ectotherms and not endotherms. A fish’s body temperature fluctuates and conforms to its environmental temperature. Although, how a fish functions at different temperatures, depends on the species of the fish. The enzymes and organs of the fish have to be able to function at a range of temperatures. Nevertheless, some fish species can be said to be endothermic. Fishes like the opah, tuna, lamnid sharks, and billfishes are endotherms.

How do endotherms and ectotherms differ?

An ectothermic animal’s body temperature is dependent on the environmental temperature. Their source of heat comes from the environment and not from within their body. Contrarily to ectotherms, endotherms can generate heat and maintain enough metabolic heat to improve their body temperature. The source of heat of endothermic organisms is internal and the temperature is increased to a high and stable level. Thereby endothermy is the opposite of ectothermy.

Are reptiles endotherms?

Since reptiles rely upon ambient temperature to regulate their internal temperatures, they are ectothermic and not endotherms. This is why it is common to see reptiles like lizards and crocodiles basking in sunlight to warm themselves up. Sometimes, they move to a shade to cool down their body temperature. This behavior is in striking contrast to endotherms like birds and mammals. Endotherms usually rely on metabolic heat production to maintain a constant internal temperature. Hence, reptiles are definitely not endotherms.

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